How do I size a battery energy storage system?

Start from the job: for backup power the average load times the outage hours, for solar storage the energy used after sunset, for peak shaving the shave times the hours above the target times the shape of the peak. Divide that energy by the system efficiency to get what leaves the battery, add the sizing margin, then divide by the usable state-of-charge window (SoC max minus SoC min) and the end-of-life capacity to get the nominal kWh. The inverter is the highest load in the event with the same margin. This calculator runs those steps and shows each one.

What is the difference between nominal and usable battery capacity?

Nominal (or nameplate) capacity is the energy the battery holds from empty to full. Usable capacity is the part of it the system is allowed to use: the window between the minimum state of charge the BMS holds in reserve and the maximum it charges to. A 10 kWh battery run between 10% and 100% has 9 kWh usable as new, and less as it ages. The calculator sizes the nominal battery from the usable energy the job needs.

Is a BESS the same as a UPS?

No. A UPS keeps the load supplied through the transfer with no interruption, because its inverter is always in the circuit. A battery energy storage system behind a hybrid or battery inverter takes from a few cycles to a few seconds to establish its backup supply on loss of grid, and only the loads on the backup circuit are carried. A load that cannot ride that through, such as a server or a medical device, needs a UPS in front of it. This page sizes a BESS; the UPS & Battery Sizing page sizes a UPS battery string to IEEE 485.

Should I use round-trip efficiency or system efficiency to size a battery?

Use the one-way system efficiency from the battery terminals to the AC load, not the round-trip figure: charging losses change how much energy the array or the grid must put in, not how much the battery has to hold. The calculator applies it once, on the discharge, and starts at 92%, typical of an LFP battery behind a hybrid inverter at rated load; 88% to 95% is the usual range and part-load efficiency is lower. It flags anything above 97% as optimistic.

What usable state of charge or depth of discharge should I assume?

Assume the window the manufacturer allows; for LFP that is commonly 10% to 100%, a 90% depth of discharge, which is where the calculator starts. Some products hold 5% or 20% back, and some charge only to 95% to extend life. The usable window is SoC max minus SoC min, and the nominal battery is the usable energy divided by it, so a narrower window buys a larger battery. Below a 5% reserve the calculator warns that the BMS has almost nothing left to protect the cells with.

How much sizing margin should a preliminary BESS design carry?

10% to 20% is usual at a preliminary stage, for load growth and the unknowns of a first pass; the calculator starts at 15% and applies it to the energy and to the inverter power. It is separate from the end-of-life allowance: set end-of-life capacity to 80% (or the warranty figure) to hold the duration when the battery has faded, on top of the margin.

Why does the calculator flag motor starting?

A motor draws several times its running current for a second or two on starting. The inverter's continuous rating sized here does not cover that; its surge rating has to, for the largest motor starting with the rest of the load running. In backup power and solar storage the calculator flags a peak of twice the average or more as a possible motor start and names the checks: the inverter overload capability, the battery's continuous and peak discharge limits, and whether a soft starter or VSD brings the start current down.

How do I size the battery inverter in kW and kVA?

Size the inverter for the highest load during the event plus the sizing margin, then divide by the power factor for kVA: the default backup job, a 30 kW peak with a 15% margin, needs 34.5 kW, or 38.3 kVA at 0.9. For peak shaving the inverter is the shave, the site peak minus the demand target, with the same margin. That is the continuous rating; a motor start needs the surge rating, and the calculator also reports the discharge rate in C so the battery's own limit can be checked against it.

What does the discharge rate in C mean?

C-rate is the discharge power as a multiple of the battery's nominal capacity: a 100 kWh battery discharging at 50 kW runs at 0.5C. Most LFP modules are limited to 0.5C to 1C continuous, so a high C-rate means the battery, not the inverter, sets the power the system can deliver. The calculator reports the rate at the inverter rating with the inverter losses carried by the battery (inverter kW ÷ efficiency ÷ nominal kWh) and flags it above 0.5C and above 1C: the default backup job, a 34.5 kW inverter on a 115 kWh battery at 92% efficiency, draws 37.5 kW from the battery, 0.33C.

How much room does a battery energy storage system need?

It depends on the size: up to 45 kWh with an inverter up to 30 kW goes on one wall, about 3 to 5 m of it; up to 200 kWh it is a plant-room line-up, about 3.7 × 2.0 m internal for 115 kWh and 34.5 kW; beyond that a battery room, and above 2 MWh 20 ft enclosures on an outdoor pad. On a wall the calculator keeps 900 mm unimpeded in front with doors open (AS/NZS 5139 Cl 5.2.5; 600 mm for a DVC-A system or an integrated BESS, AS/NZS 5139 Cl 4.2.5), 600 mm clear to any exit, habitable-room window or vent, or appliance (AS/NZS 5139 Cl 4.2.2.2 / 5.2.2.2), and the habitable-room barrier 600 mm past each end and 900 mm above (AS/NZS 5139 Cl 4.2.4.2 / 5.2.4.2). A plant room lines up 600 × 1000 × 2200 mm LFP racks and a PCS cabinet with 900 mm in front measured from the face, each door leaf sweeping its own width inside it (AS/NZS 5139 Figure 6.3), 2.8 m clear height and about 14 kPa on the slab under each rack. A battery room follows the AS/NZS 5139 Section 6 layout with every facing pair of rows sharing one 900 mm aisle (AS/NZS 5139 Figure C.1), and an outdoor pad keeps 3 m clear all round. The plan is indicative: the manufacturer's drawings decide the room.

Does AS/NZS 5139 apply to my battery system?

AS/NZS 5139:2019 applies to battery systems from 1 kWh to 200 kWh per BESS, at 12 V to 1500 V d.c., connected to power conversion equipment (AS/NZS 5139 Cl 1.1.1). AS/NZS 3000:2018 Clause 7.8.2.15 requires a BESS within its definition to comply with it. AS/NZS 5139 does not apply to a UPS to AS 62040, to premises with critical power continuity such as acute-care hospitals, to telecommunications or to vehicles. So a home battery and most commercial systems fall inside it: AS/NZS 5139 Section 4 for a pre-assembled integrated BESS, Section 5 for pre-assembled battery systems, Section 6 for everything else. Each sets the restricted locations (not within 600 mm of an exit, of a habitable room's window or vent, or of an appliance, nor 900 mm below one; not in habitable rooms, ceiling spaces, wall cavities, under stairs or in an evacuation route), the non-combustible barrier where a habitable room is behind the wall (600 mm past the sides, 900 mm above), and the working-side clearance (900 mm with doors open, 600 mm for DVC-A systems or an integrated BESS). Above 200 kWh the installation is outside its scope, though Note 1 to the scope says its general requirements may be applied, and the manufacturer's installation manual, the fire authority and the DNSP's connection requirements govern; the calculator says which side of the line a system falls and draws the footprint to match.

How do I size a battery for peak shaving and demand charges?

The inverter is the shave, the site peak minus the demand target, plus the margin, and the battery holds the energy above the target each day: the shave times the hours the demand exceeds the target times a shape factor, the area of the shape over the full block: 1.0 for a flat block, 2/π (0.64) for a rounded peak drawn as a half sine, and 0.5 for a triangular one. That energy sizes the battery through the same efficiency, margin and window chain. Worked example: a 250 kW site peak held to 180 kW for 3 hours on a rounded peak is a 70 kW shave and 70 × 3 × 0.637 = 133.7 kWh a day above the target; at 92% efficiency, a 15% margin and a 90% window that is 185.7 kWh, so a 190 kWh battery with an 80.5 kW inverter, discharging at 0.46C. Verify the demand profile from interval data before relying on it, and the calculator says so when the shave is more than half the site peak: a peak that lasts longer than assumed empties the battery before the event ends.

Which Australian standards apply to a battery energy storage installation?

AS/NZS 5139:2019 covers the safety of battery systems used with power conversion equipment: location, clearances, ventilation and signage. AS/NZS 4777.1:2024 covers the grid connection of the inverter, including the voltage rise the export causes. AS/NZS 3000:2018 applies to the AC installation and the changeover of a backup circuit, and AS/NZS 5033 applies where a PV array charges the battery. The calculator sizes the battery and the inverter; the installation is designed to those standards afterwards.

Can the calculator export a report?

Yes. Copy summary puts the results, the working, the assumptions and the checks on the clipboard as plain text, and Export in the toolbar downloads a PDF calculation sheet once you have a free account: the recommended battery and inverter at the top with the duration verdict, the drawing of the day it is sized for, the inputs once, every step of the working with its formula and figures, the flagged checks, the footprint plan to scale with the AS/NZS 5139 clauses that set it, and the assumptions applied, under your firm's title block.

How long will a battery run during a power outage?

Runtime is the usable energy times the system efficiency divided by the average load: the default backup job, a 115 kWh battery with a 90% window and 92% efficiency feeding 20 kW, runs about 4.8 hours. The calculator reads the duration back from the battery it recommends, after rounding up to the next module step, not from the requirement, so the hours quoted are what that battery delivers; with end-of-life capacity below 100% it is the duration once the battery has faded.

Should I size a BESS as new or at end of life?

Size as new for a budget and a first conversation with a supplier; size at end of life when the duration is a requirement, such as a backup circuit that must run for a set number of hours throughout the warranty. Under Advanced, set end-of-life capacity to 80% or the warranty figure and the calculator buys enough nominal capacity that the faded battery still meets the duration.

What size home battery do I need?

Size a home battery on the energy you use after sunset, or on the circuits that must stay on in a blackout, grossed up for efficiency, margin and the usable window: 12 kWh a night at 92% efficiency, a 15% margin and a 90% window comes out at 17 kWh nominal. Use Solar storage with the evening energy from a bill and the peak from the largest appliance, or Backup power with the backup circuits' load and the outage hours. The result is preliminary: the product's usable capacity, window and backup rating decide it.

Does the calculator size the PV array?

No. For solar storage it reports the surplus the array has to leave each day, on top of the daytime load, to put back what the evening takes out of the battery (that energy ÷ the efficiency), so the array can be checked against it. A winter day that leaves less gives a shorter evening. The Solar & Battery ROI page models the yearly energy and the payback.

ScaleSet

BESS Sizing Calculator: Battery Energy Storage Sizing in kWh and kW for Australia

Preliminary sizing for a battery energy storage system (BESS): pick backup power, solar storage or peak shaving, enter the load, the energy or duration, the usable state-of-charge window, the system efficiency and a sizing margin, and read the recommended nominal capacity (kWh), the usable energy, the minimum inverter power (kW) and the operating duration, with the working shown line by line and a floor plan of the installation drawn to scale with its dimensions and AS/NZS 5139 clearances.

What the BESS sizing calculator does

A battery energy storage system is sized from the job it has to do, and the job is different for each application. Backup power is an outage: an average load for a number of hours, with a peak somewhere in it. Solar storage is an evening: the energy used after sunset, at an average load with a peak moment. Peak shaving is a demand curve clipped at a target: the shave times the hours above the target, shaped by how the peak rises and falls. This calculator takes those inputs one application at a time and runs one chain from them to the nominal battery in kWh and the inverter in kW, with every step of the working shown, the assumptions stated beside the result, and the checks that remain before a product is chosen listed under it.

Battery energy storage sizing at a glance

Nominal kWh = energy to the load ÷ system efficiency × (1 + margin) ÷ usable state-of-charge window ÷ end-of-life capacity, rounded up to the next module step; the inverter kW is the highest load in the event × (1 + margin).
Worked example, backup power: 20 kW for a 4 h outage is 80 kWh to the load; ÷ 0.92 efficiency × 1.15 margin ÷ 0.90 window = 111.1 kWh, so a 115 kWh battery, a 34.5 kW (38.3 kVA) inverter for the 30 kW peak, 0.33C at the battery and 4.8 h of runtime.
The usable window is SoC max minus SoC min: 10% to 100% is typical for LFP, so 90% of the nameplate is available as new, and less as the battery fades.
System efficiency is the path from the battery terminals to the AC load, applied once on the discharge; 92% is typical through a hybrid inverter, and the round-trip datasheet figure is not the one to use here.
The operating duration is read back from the battery actually recommended, so the figure quoted is what the system will deliver, not what was asked for.
A BESS is not a UPS: the inverter takes cycles to seconds to establish backup on loss of grid, and only the backup circuit is carried.
Motor starting, inverter overload capability, battery discharge limits and manufacturer verification are checks the sizing hands on; the result names them rather than pretending to have made them.
The footprint is planned from the recommended battery and inverter and drawn to scale with AS/NZS 5139:2019's own clearances: 900 mm unimpeded on the working side with doors open (AS/NZS 5139 Cl 5.2.5), 600 mm restricted zones to exits, habitable-room windows and appliances (AS/NZS 5139 Cl 4.2.2.2 / 5.2.2.2), the habitable-room barrier 600 past the sides and 900 above (AS/NZS 5139 Cl 4.2.4.2 / 5.2.4.2), and the AS/NZS 5139 Section 6 battery room layout (AS/NZS 5139 Cl 6.2.6.2, Figure 6.3) above the 200 kWh scope.

Who this page is for

Electrical engineers, solar and storage designers, energy consultants, contractors and facility managers sizing battery energy storage for Australian and New Zealand installations.

Relevant standards

AS/NZS 5139:2019
AS/NZS 4777.1:2024
AS/NZS 3000:2018
AS/NZS 5033

What this tool helps with

Three applications with their own inputs: backup power (load, peak and outage hours), solar storage (energy after sunset, evening load and peak) and peak shaving (site peak, demand target, hours above target and the shape of the excess).
One energy chain, shown line by line: energy to the load, energy out of the battery through the system efficiency, the sizing margin, the usable state-of-charge window and the end-of-life capacity, then the nominal kWh rounded up to the next module step.
Minimum inverter power from the highest load in the event with the same margin, in kW and in kVA at the power factor entered, and the discharge rate in C the battery has to carry.
The operating duration read back from the battery actually recommended, not from the requirement, so the figure quoted is what the system will deliver.
Every assumption stated beside the result, inline validation on every input, and an empty, error and sized state so a blank or contradictory input never produces a number.
Motor starting, inverter overload capability, battery discharge limits and manufacturer verification carried as checks before equipment selection; a BESS is never presented as UPS-grade uninterrupted supply.
A drawn battery with its usable window and the day the system is sized for (the outage, the evening, or the clipped peak), a copyable plain-text summary and a PDF calculation sheet with the working, the footprint plan, the checks and the assumptions.
A footprint plan drawn to scale with dimension chains in millimetres and the clauses on it: battery units and a hybrid inverter on one wall up to 45 kWh, a plant-room line-up of LFP racks and a PCS cabinet up to 200 kWh with the AS/NZS 5139:2019 900 mm working-side clearance measured from the face with the doors opening inside it (AS/NZS 5139 Cl 5.2.5, Figure 6.3), a battery room to the AS/NZS 5139 Section 6 layout beyond that with every facing pair of rows sharing one 900 mm aisle (AS/NZS 5139 Cl 6.2.6.2, Figure C.1), and 20 ft enclosures on an outdoor pad beyond 2 MWh, each with its internal dimensions, floor area, clear height, battery mass and the load a unit puts on the slab.

How to size a battery energy storage system

Pick the application: Choose Backup power, Solar storage or Peak shaving. The inputs below the selector change to the ones that application reads, and the sentence under it says what sets the capacity and what sets the inverter.
Enter the load: Backup: the average load in kW, the peak load in the outage and the hours to cover. Solar: the energy after sunset in kWh per day, the average evening load and its peak. Peak shaving: the site peak, the demand target, the hours above the target and the peak shape. Leave the peak blank to take it as the average; the assumption is then stated beside the result.
Check the assumptions: Under Advanced: the usable window (10% to 100%), the system efficiency (92%), the sizing margin (15%), the end-of-life capacity (100% as new; 80% holds the duration at the end of a typical warranty) and the power factor for the inverter kVA. Each one is applied to every result and listed under it.
Read the battery and the inverter: The result card shows the recommended nominal capacity, the usable energy, the minimum inverter power in kW and kVA, the operating duration against the target and the discharge rate in C. The drawing under it shows the usable window on the battery and the day it is sized for, and the footprint under that shows the installation to scale with its dimensions, floor area and clearances; click it to open the plan full screen.
Read the working and the checks: Every step from the load to the nominal battery is listed with its arithmetic. Below it are the assumptions applied and the checks that remain before equipment is selected: motor starting, inverter overload, battery discharge limits and manufacturer verification. Copy summary puts all of it on the clipboard; the PDF button in the toolbar produces the calculation sheet.

Sizing a battery energy storage system: from the job to the nominal battery

What the calculator actually does

The ScaleSet BESS sizing calculator is a preliminary sizing tool: it turns the job a battery has to do into a nominal battery capacity in kWh and a minimum inverter rating in kW. It runs one chain of arithmetic, in the same order for every application, and prints every step of it beside the result so the figure can be checked by hand. It does not select a product, and it says so: the checks that remain before equipment is chosen are listed under every result.

The application selector decides which inputs the page asks for. Backup power asks for the average load during the outage, the highest load in it and the hours it must be covered. Solar storage asks for the energy used after sunset, the average load while the battery is discharging and the peak moment in that evening. Peak shaving asks for the site peak demand, the demand target, the hours a day the demand sits above the target and the shape of the excess. Nothing else is shown, because nothing else is read.

The energy chain

The first figure is the energy the load takes in one event. For backup power it is the average load times the outage hours. For solar storage it is the energy after sunset, entered directly, with the average load setting how many hours that energy lasts. For peak shaving it is the shave (site peak minus target) times the hours above the target times a shape factor, because a real demand peak rises and falls and its area above the target is less than a flat block would be.

That energy is divided by the system efficiency to give what has to leave the battery, then multiplied by one plus the sizing margin. The result is the usable energy required. Dividing by the usable window (SoC max minus SoC min) gives the nominal capacity as new, and dividing again by the end-of-life capacity gives the nominal capacity that still meets the job when the battery has faded. The calculator rounds that up to the next half kWh below 20 kWh, the next whole kWh below 100, the next 5 kWh below 1000 and the next 10 kWh beyond, which is roughly the granularity batteries are sold in at each size.

The usable energy and the operating duration are then read back from the battery recommended, not from the requirement: recommended nominal × window × end-of-life capacity is the usable energy, and usable energy × efficiency ÷ average load is the duration. Rounding up always leaves the duration at or above the target, and the result says by how much.

The inverter, and the discharge rate

The inverter is sized from the highest load in the event, with the same sizing margin, as a continuous rating in kW; the kVA at the power factor entered is shown beside it because inverters are catalogued both ways. In peak shaving the highest load is the shave itself, since the grid carries the rest.

Dividing the inverter rating by the system efficiency gives the power the battery delivers at full output, since it supplies the inverter losses as well, and dividing that by the recommended nominal capacity gives the discharge rate in C. Most LFP modules are limited to 0.5C to 1C continuous, so a high rate means the battery, not the inverter, sets the power the system can deliver, and either the modules need a higher rating or the battery needs to be larger than the energy alone requires. The calculator reports the rate and flags it above 0.5C and above 1C.

What preliminary means

A preliminary size is enough to place a battery on a budget, allow a cabinet in a switchroom, size the section in a switchboard and open a conversation with a supplier. It is not enough to order from. The figures that decide the order are on the datasheet: the module's usable capacity and its window, the inverter's efficiency curve and its overload capability, the continuous and peak discharge limits, and the warranty's end-of-life capacity. The calculator names each of those as a check and leaves it to the selection stage.

Two things a BESS does not do are worth saying plainly. It does not provide uninterrupted power: on loss of grid a hybrid or battery inverter takes from a few cycles to a few seconds to establish its backup supply, and only the loads wired to the backup circuit are carried. A load that cannot ride that through needs a UPS, sized on the UPS & Battery page. And it does not start a motor on its continuous rating: a motor draws several times its running current for a second or two, and that has to fit inside the inverter's surge rating and the battery's peak discharge limit, or be brought down with a soft starter or a VSD.

The footprint

Once the battery and the inverter are known the calculator plans the installation in metres and draws it to scale the way an architectural plan is drawn: the walls pochéd outside internal dimensions, dimension chains in millimetres, the clearances hatched with the clause that sets each one, the door cut through the wall and swinging out, a scale bar, and a footer with the floor area, the unit sizes in three dimensions, the height to allow and the load each unit puts on the slab, so the room can be allowed for at the same stage the equipment is sized. The clearances are AS/NZS 5139:2019's own. Up to 45 kWh with an inverter up to 30 kW the installation goes on one wall: 650 × 300 mm battery units (wall-mounted or floor-standing towers) and a 500 mm hybrid inverter, 900 mm unimpeded in front with doors open (AS/NZS 5139 Cl 5.2.5(a); 600 mm for a DVC-A system with no more than 4 cal/cm², or an integrated BESS with no 230 V a.c. behind its panels, AS/NZS 5139 Cl 4.2.5), 600 mm kept clear at each end to any exit, habitable-room window or vent, or appliance and nothing 900 mm below one (AS/NZS 5139 Cl 4.2.2.2 / 5.2.2.2), and, where a habitable room is on the other side of the wall or the units stand within 300 mm of it, a suitably non-combustible barrier 600 mm past each side and 900 mm above the units (AS/NZS 5139 Cl 4.2.4.2 / 5.2.4.2). Up to 200 kWh it is a plant room: 600 × 1000 × 2200 mm LFP racks of up to 100 kWh and a PCS cabinet in one line-up with their backs to the wall and their fronts flush, 300 mm between the PCS and the first rack for its airflow and the DC cabling, the AC isolator and board on the wall at the head, and in front 900 mm unimpeded with the doors open (AS/NZS 5139 Cl 5.2.5(a)), measured from the face of the line-up to the wall the way AS/NZS 5139 Figure 6.3(c) draws it, each door leaf, no wider than 800 mm, sweeping its own width inside it and not added to it (a 600 mm rack door sweeps 600, an 800 mm PCS door 800), which is also the egress path, and a 900 mm door in the end wall that opens toward egress. The standard never measures the 900 from the tip of an open door: the one place a door swing widens an aisle is AS/NZS 5139 Figure 6.3(a), a hinged PCE door facing a hinged battery door, which wants 600 mm between the two open doors, and the line-up keeps the PCS beside the racks so that case never arises.

AS/NZS 5139:2019 stops at 200 kWh per BESS (AS/NZS 5139 Cl 1.1.1), but Note 1 to its scope says its general requirements may be applied to larger installations, so above it the calculator plans a battery room to the AS/NZS 5139 Section 6 layout and says the standard no longer governs: the same line-up runs along the head wall, the board, the PCS and the first racks, and the remaining racks stand in rows under those racks, never under the PCS, each pair back to back and every facing pair sharing one 900 mm aisle measured face to face with the doors open, the 900 mm unimpeded working side of AS/NZS 5139 Cl 6.2.6.2(a) drawn the way AS/NZS 5139 Figure 6.3 and the standard's own typical room in Figure C.1 draw it, with each door leaf, no wider than 800 mm, sweeping its own width inside it; a row that ends up facing the far wall gets 900 mm to it; the PCS never faces a rack, so the 600 mm between open hinged doors of AS/NZS 5139 Cl 6.2.6.2(1), Figure 6.3(a), does not arise; no aisle is under 600 mm (C), the racks stop at the 2.2 m the standard allows (H), the room is used for nothing else and its doors open toward egress (AS/NZS 5139 Cl 6.2.6.1), and the head row holds as many racks as make the smallest room no wider than two and a half times its depth, on a wall no longer than 12 m. Beyond 2 MWh it plans 20 ft enclosures of up to 3 MWh on an outdoor pad, a PCS and transformer skid in front of each, a metre between enclosures and 3 m clear to any other structure, the spacing NFPA 855 practice adopts where the Australian standard does not reach; the manufacturer's manual, the fire authority and the DNSP's connection requirements decide it. Every plan quotes its width, depth and floor area, the clear height to allow (the tallest unit plus 600 mm for the tray and the air above it), the mass of the batteries and the load a unit puts on the slab, a figure worth reading early: a 100 kWh rack is about 1.35 t on 0.6 m², over 20 kPa against an office floor's 3 to 5 kPa. Every plan carries the words not for construction.

The installation

The battery and the inverter are sized here; the installation is designed to the standards afterwards. AS/NZS 5139:2019 governs where a battery system may be placed, the clearances around it, ventilation and signage. AS/NZS 4777.1:2024 governs the inverter's grid connection, including the voltage rise its export causes, which the Voltage Rise calculator checks. AS/NZS 3000:2018 applies to the AC installation and the changeover arrangement for a backup circuit, and AS/NZS 5033 applies where a PV array charges the battery.

BESS sizing terms

BESS: Battery energy storage system: the battery modules, their management system, the inverter or power conversion system and the switchgear that connects them to the installation.
Nominal capacity: The energy the battery holds from empty to full, in kWh, as printed on the nameplate. Modules and cabinets are catalogued by it.
Usable energy: The part of the nominal capacity the system is allowed to use: nominal × (SoC max − SoC min), and × the end-of-life capacity when the sizing holds the duration at the end of the battery's life.
State of charge (SoC): How full the battery is, as a percentage of nominal. The BMS holds a minimum in reserve and charges to a maximum; the window between them is what the sizing uses.
System efficiency: The fraction of the energy leaving the battery that reaches the AC load, through the inverter and the cabling. Applied once on the discharge in this calculator.
Sizing margin: Headroom on the energy and on the inverter power for load growth and the unknowns of a preliminary stage; 10% to 20% is usual.
End-of-life capacity: The fraction of nameplate the battery keeps at the end of its warranted life, commonly 70% to 80%. Sizing against it holds the duration on the battery's last day rather than its first.
C-rate: Discharge power at the battery terminals as a multiple of nominal capacity: 50 kW from a 100 kWh battery is 0.5C. At full output the battery also supplies the inverter losses, so the rate is the inverter kW ÷ efficiency ÷ nominal kWh. Modules carry continuous and peak C-rate limits that cap what the inverter can draw.
Peak shaving: Discharging the battery when site demand would exceed a target, so the metered maximum demand, and the demand charge or supply limit it is measured against, stays under the target. The calculator works in kW; many Australian network tariffs bill demand in kVA, which is converted at the site power factor (kW = kVA × PF) first.
Shape factor: The fraction of shave × hours that is energy: the area of the shape over the full block, so 1.0 for a flat block, 2/π (0.64) for a rounded peak drawn as a half sine, and 0.5 for a triangular one.
Footprint: The floor the installation takes with its clearances: the wall run, the plant room, the battery room or the outdoor pad, planned in metres from the recommended battery and inverter and quoted as width × depth.
AS/NZS 5139:2019: The Australian and New Zealand standard for the safety of battery systems used with power conversion equipment, from 1 kWh to 200 kWh per BESS (AS/NZS 5139 Cl 1.1.1): restricted locations, the habitable-room barrier, working-side clearances, ventilation, signage and documentation. AS/NZS 5139 Section 4 covers a pre-assembled integrated BESS, Section 5 pre-assembled battery systems, Section 6 everything else. Above 200 kWh its general requirements may still be applied (Note 1) and the manufacturer and the fire authority govern.
Decisive voltage classification (DVC): The classification AS/NZS 5139 borrows from IEC 62109-1: DVC-A below 60 V d.c., DVC-B to 120 V, DVC-C above. A DVC-A battery with no more than 4 cal/cm² at its terminals may take 600 mm on the working side instead of 900 (AS/NZS 5139 Cl 5.2.5); a DVC-B or DVC-C battery is treated as an LV installation under AS/NZS 3000.

Methods reviewed by

Wisam Tozah (CPEng, NER, MIEAust) for ScaleSet Pty Ltd. The methods are tested against the standards' published tables and worked examples before every release. See how they are verified. ScaleSet is a design aid: your inputs and results are not reviewed or certified, and responsibility for the design stays with the engineer who issues it. LinkedIn. Updated 4 November 2026.

Battery storage sizing questions

How do I size a battery energy storage system?: Start from the job: for backup power the average load times the outage hours, for solar storage the energy used after sunset, for peak shaving the shave times the hours above the target times the shape of the peak. Divide that energy by the system efficiency to get what leaves the battery, add the sizing margin, then divide by the usable state-of-charge window (SoC max minus SoC min) and the end-of-life capacity to get the nominal kWh. The inverter is the highest load in the event with the same margin. This calculator runs those steps and shows each one.
What is the difference between nominal and usable battery capacity?: Nominal (or nameplate) capacity is the energy the battery holds from empty to full. Usable capacity is the part of it the system is allowed to use: the window between the minimum state of charge the BMS holds in reserve and the maximum it charges to. A 10 kWh battery run between 10% and 100% has 9 kWh usable as new, and less as it ages. The calculator sizes the nominal battery from the usable energy the job needs.
Is a BESS the same as a UPS?: No. A UPS keeps the load supplied through the transfer with no interruption, because its inverter is always in the circuit. A battery energy storage system behind a hybrid or battery inverter takes from a few cycles to a few seconds to establish its backup supply on loss of grid, and only the loads on the backup circuit are carried. A load that cannot ride that through, such as a server or a medical device, needs a UPS in front of it. This page sizes a BESS; the UPS & Battery Sizing page sizes a UPS battery string to IEEE 485.
Should I use round-trip efficiency or system efficiency to size a battery?: Use the one-way system efficiency from the battery terminals to the AC load, not the round-trip figure: charging losses change how much energy the array or the grid must put in, not how much the battery has to hold. The calculator applies it once, on the discharge, and starts at 92%, typical of an LFP battery behind a hybrid inverter at rated load; 88% to 95% is the usual range and part-load efficiency is lower. It flags anything above 97% as optimistic.
What usable state of charge or depth of discharge should I assume?: Assume the window the manufacturer allows; for LFP that is commonly 10% to 100%, a 90% depth of discharge, which is where the calculator starts. Some products hold 5% or 20% back, and some charge only to 95% to extend life. The usable window is SoC max minus SoC min, and the nominal battery is the usable energy divided by it, so a narrower window buys a larger battery. Below a 5% reserve the calculator warns that the BMS has almost nothing left to protect the cells with.
How much sizing margin should a preliminary BESS design carry?: 10% to 20% is usual at a preliminary stage, for load growth and the unknowns of a first pass; the calculator starts at 15% and applies it to the energy and to the inverter power. It is separate from the end-of-life allowance: set end-of-life capacity to 80% (or the warranty figure) to hold the duration when the battery has faded, on top of the margin.
Why does the calculator flag motor starting?: A motor draws several times its running current for a second or two on starting. The inverter's continuous rating sized here does not cover that; its surge rating has to, for the largest motor starting with the rest of the load running. In backup power and solar storage the calculator flags a peak of twice the average or more as a possible motor start and names the checks: the inverter overload capability, the battery's continuous and peak discharge limits, and whether a soft starter or VSD brings the start current down.
How do I size the battery inverter in kW and kVA?: Size the inverter for the highest load during the event plus the sizing margin, then divide by the power factor for kVA: the default backup job, a 30 kW peak with a 15% margin, needs 34.5 kW, or 38.3 kVA at 0.9. For peak shaving the inverter is the shave, the site peak minus the demand target, with the same margin. That is the continuous rating; a motor start needs the surge rating, and the calculator also reports the discharge rate in C so the battery's own limit can be checked against it.
What does the discharge rate in C mean?: C-rate is the discharge power as a multiple of the battery's nominal capacity: a 100 kWh battery discharging at 50 kW runs at 0.5C. Most LFP modules are limited to 0.5C to 1C continuous, so a high C-rate means the battery, not the inverter, sets the power the system can deliver. The calculator reports the rate at the inverter rating with the inverter losses carried by the battery (inverter kW ÷ efficiency ÷ nominal kWh) and flags it above 0.5C and above 1C: the default backup job, a 34.5 kW inverter on a 115 kWh battery at 92% efficiency, draws 37.5 kW from the battery, 0.33C.
How much room does a battery energy storage system need?: It depends on the size: up to 45 kWh with an inverter up to 30 kW goes on one wall, about 3 to 5 m of it; up to 200 kWh it is a plant-room line-up, about 3.7 × 2.0 m internal for 115 kWh and 34.5 kW; beyond that a battery room, and above 2 MWh 20 ft enclosures on an outdoor pad. On a wall the calculator keeps 900 mm unimpeded in front with doors open (AS/NZS 5139 Cl 5.2.5; 600 mm for a DVC-A system or an integrated BESS, AS/NZS 5139 Cl 4.2.5), 600 mm clear to any exit, habitable-room window or vent, or appliance (AS/NZS 5139 Cl 4.2.2.2 / 5.2.2.2), and the habitable-room barrier 600 mm past each end and 900 mm above (AS/NZS 5139 Cl 4.2.4.2 / 5.2.4.2). A plant room lines up 600 × 1000 × 2200 mm LFP racks and a PCS cabinet with 900 mm in front measured from the face, each door leaf sweeping its own width inside it (AS/NZS 5139 Figure 6.3), 2.8 m clear height and about 14 kPa on the slab under each rack. A battery room follows the AS/NZS 5139 Section 6 layout with every facing pair of rows sharing one 900 mm aisle (AS/NZS 5139 Figure C.1), and an outdoor pad keeps 3 m clear all round. The plan is indicative: the manufacturer's drawings decide the room.
Does AS/NZS 5139 apply to my battery system?: AS/NZS 5139:2019 applies to battery systems from 1 kWh to 200 kWh per BESS, at 12 V to 1500 V d.c., connected to power conversion equipment (AS/NZS 5139 Cl 1.1.1). AS/NZS 3000:2018 Clause 7.8.2.15 requires a BESS within its definition to comply with it. AS/NZS 5139 does not apply to a UPS to AS 62040, to premises with critical power continuity such as acute-care hospitals, to telecommunications or to vehicles. So a home battery and most commercial systems fall inside it: AS/NZS 5139 Section 4 for a pre-assembled integrated BESS, Section 5 for pre-assembled battery systems, Section 6 for everything else. Each sets the restricted locations (not within 600 mm of an exit, of a habitable room's window or vent, or of an appliance, nor 900 mm below one; not in habitable rooms, ceiling spaces, wall cavities, under stairs or in an evacuation route), the non-combustible barrier where a habitable room is behind the wall (600 mm past the sides, 900 mm above), and the working-side clearance (900 mm with doors open, 600 mm for DVC-A systems or an integrated BESS). Above 200 kWh the installation is outside its scope, though Note 1 to the scope says its general requirements may be applied, and the manufacturer's installation manual, the fire authority and the DNSP's connection requirements govern; the calculator says which side of the line a system falls and draws the footprint to match.
How do I size a battery for peak shaving and demand charges?: The inverter is the shave, the site peak minus the demand target, plus the margin, and the battery holds the energy above the target each day: the shave times the hours the demand exceeds the target times a shape factor, the area of the shape over the full block: 1.0 for a flat block, 2/π (0.64) for a rounded peak drawn as a half sine, and 0.5 for a triangular one. That energy sizes the battery through the same efficiency, margin and window chain. Worked example: a 250 kW site peak held to 180 kW for 3 hours on a rounded peak is a 70 kW shave and 70 × 3 × 0.637 = 133.7 kWh a day above the target; at 92% efficiency, a 15% margin and a 90% window that is 185.7 kWh, so a 190 kWh battery with an 80.5 kW inverter, discharging at 0.46C. Verify the demand profile from interval data before relying on it, and the calculator says so when the shave is more than half the site peak: a peak that lasts longer than assumed empties the battery before the event ends.
Which Australian standards apply to a battery energy storage installation?: AS/NZS 5139:2019 covers the safety of battery systems used with power conversion equipment: location, clearances, ventilation and signage. AS/NZS 4777.1:2024 covers the grid connection of the inverter, including the voltage rise the export causes. AS/NZS 3000:2018 applies to the AC installation and the changeover of a backup circuit, and AS/NZS 5033 applies where a PV array charges the battery. The calculator sizes the battery and the inverter; the installation is designed to those standards afterwards.
Can the calculator export a report?: Yes. Copy summary puts the results, the working, the assumptions and the checks on the clipboard as plain text, and Export in the toolbar downloads a PDF calculation sheet once you have a free account: the recommended battery and inverter at the top with the duration verdict, the drawing of the day it is sized for, the inputs once, every step of the working with its formula and figures, the flagged checks, the footprint plan to scale with the AS/NZS 5139 clauses that set it, and the assumptions applied, under your firm's title block.
How long will a battery run during a power outage?: Runtime is the usable energy times the system efficiency divided by the average load: the default backup job, a 115 kWh battery with a 90% window and 92% efficiency feeding 20 kW, runs about 4.8 hours. The calculator reads the duration back from the battery it recommends, after rounding up to the next module step, not from the requirement, so the hours quoted are what that battery delivers; with end-of-life capacity below 100% it is the duration once the battery has faded.
Should I size a BESS as new or at end of life?: Size as new for a budget and a first conversation with a supplier; size at end of life when the duration is a requirement, such as a backup circuit that must run for a set number of hours throughout the warranty. Under Advanced, set end-of-life capacity to 80% or the warranty figure and the calculator buys enough nominal capacity that the faded battery still meets the duration.
What size home battery do I need?: Size a home battery on the energy you use after sunset, or on the circuits that must stay on in a blackout, grossed up for efficiency, margin and the usable window: 12 kWh a night at 92% efficiency, a 15% margin and a 90% window comes out at 17 kWh nominal. Use Solar storage with the evening energy from a bill and the peak from the largest appliance, or Backup power with the backup circuits' load and the outage hours. The result is preliminary: the product's usable capacity, window and backup rating decide it.
Does the calculator size the PV array?: No. For solar storage it reports the surplus the array has to leave each day, on top of the daytime load, to put back what the evening takes out of the battery (that energy ÷ the efficiency), so the array can be checked against it. A winter day that leaves less gives a shorter evening. The Solar & Battery ROI page models the yearly energy and the payback.