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.

What system efficiency should I use?

The efficiency here is the path from the battery terminals to the AC load, through the inverter and the cabling, applied once on the discharge. 92% is a typical figure for an LFP battery behind a hybrid inverter at rated load; 88% to 95% is the usual range, and part-load efficiency is lower. It is not the round-trip figure on a datasheet, which includes charging as well. Charging losses do not change how much energy the battery must hold, only how much the array or the grid must put in.

What usable state-of-charge window should I assume?

LFP batteries are commonly run between 10% and 100%, giving a 90% window. Some products hold 5% or 20% back, and some charge to 95% to extend life. The manufacturer's window governs; the calculator starts at 10% to 100% and lets you change either limit.

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 applies the margin 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. The calculator flags a peak more than twice the average 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.

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 and flags it above 0.5C and above 1C.

How much room does a battery energy storage system need?

The calculator plans the installation from the battery and the inverter it recommends and draws it to scale with its dimensions, the height to allow and the load on the slab, using the clearances AS/NZS 5139:2019 itself sets. Up to 45 kWh with an inverter up to 30 kW it goes on one wall: battery units and a hybrid inverter with 900 mm unimpeded in front with doors open (Cl 5.2.5; 600 mm for a DVC-A system or an integrated BESS, Cl 4.2.5), 600 mm kept clear to any exit, habitable-room window or vent, or appliance (Cl 4.2.2.2 / 5.2.2.2), and the habitable-room barrier 600 mm past each end and 900 mm above the units (Cl 4.2.4.2 / 5.2.4.2): about 3 to 5 m of wall. Up to 200 kWh it is a plant-room line-up of 600 x 1000 x 2200 mm LFP racks and a PCS cabinet with 900 mm unimpeded in front, measured from the face to the wall the way Figure 6.3 draws it and with the doors' 600 mm swing inside it, so a 115 kWh, 35 kW system takes a room about 3.7 x 2.0 m internal with 2.8 m clear height, and each rack puts about 14 kPa on the slab. Above 200 kWh it is a battery room to the Section 6 layout: the PCS in line with the racks along the head wall, never facing them, the remaining racks in rows under those racks with every facing pair sharing one 900 mm aisle the way the standard's own Figure C.1 draws it, and above 2 MWh it is 20 ft enclosures on an outdoor pad with a PCS and transformer skid in front of each and 3 m clear all round. The plan is indicative: unit sizes are typical of the band and 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 (Cl 1.1.1); it 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: 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 does peak shaving sizing work?

The shave is the site peak minus the demand target: that is the inverter power. The energy above the target each day is the shave times the hours the demand exceeds the target times a shape factor: 1.0 for a flat block, about 0.75 for a rounded commercial peak, 0.5 for a triangular one. That energy sizes the battery through the same efficiency, margin and window chain. Verify the demand profile from interval data before relying on it: 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 produces a branded PDF report with project details, the recommended battery and inverter, the drawing, the sizing breakdown, the assumptions applied, the flagged checks and the method and standards.

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.

Why does the recommended battery come out larger than the energy I need?

Because four things sit between the energy the load takes and the nameplate on the battery: the discharge path loses some of it (efficiency), the margin adds headroom, the usable window leaves a reserve at the bottom and sometimes headroom at the top, and the end-of-life allowance holds the duration as the battery fades. The breakdown shows each factor on its own line, so the reader can see which one is doing the most.

Can I use this for a home battery?

Yes. Solar storage with the evening energy from a bill and the peak from the largest appliance is the usual home case; backup power with the circuits that must stay on covers a blackout. The result is preliminary: a home battery is chosen from a short list of products, and 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 to refill the usable window, on top of the daytime load, 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.

ElecAS

BESS Sizing Calculator: Size a battery energy storage system for your load and operating needs

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).
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 (Cl 5.2.5), 600 mm restricted zones to exits, habitable-room windows and appliances (Cl 4.2.2.2 / 5.2.2.2), the habitable-room barrier 600 past the sides and 900 above (Cl 4.2.4.2 / 5.2.4.2), and the Section 6 battery room layout (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 branded PDF report with the working, the assumptions and the checks.
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 (Cl 5.2.5, Figure 6.3), a battery room to the Section 6 layout beyond that with every facing pair of rows sharing one 900 mm aisle (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: The usable window (10% to 100%), the system efficiency (92%), the sizing margin (15%) and, under Advanced, 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; Export in the toolbar produces the PDF report.

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

What the calculator actually does

The ElecAS 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 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 (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, 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 (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 (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 (Cl 5.2.5(a)), measured from the face of the line-up to the wall the way Figure 6.3(c) draws it, the doors' 600 mm swing inside it and not added to it, 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 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 (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 standard's own 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 Cl 6.2.6.2(a) drawn the way Figure 6.3 and the standard's own typical room in Figure C.1 draw it, with the 600 mm door swing 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 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 (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 as a multiple of nominal capacity: 50 kW from a 100 kWh battery is 0.5C. 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.
Shape factor: The fraction of shave × hours that is energy: 1.0 for a flat block, about 0.75 for a rounded peak, 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 (Cl 1.1.1): restricted locations, the habitable-room barrier, working-side clearances, ventilation, signage and documentation. 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 (Cl 5.2.5); a DVC-B or DVC-C battery is treated as an LV installation under AS/NZS 3000.

Reviewed by

Wisam Tozah: Associate Electrical Engineer. B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus). See how these calculations are verified. LinkedIn. Updated 17 September 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.
What system efficiency should I use?: The efficiency here is the path from the battery terminals to the AC load, through the inverter and the cabling, applied once on the discharge. 92% is a typical figure for an LFP battery behind a hybrid inverter at rated load; 88% to 95% is the usual range, and part-load efficiency is lower. It is not the round-trip figure on a datasheet, which includes charging as well. Charging losses do not change how much energy the battery must hold, only how much the array or the grid must put in.
What usable state-of-charge window should I assume?: LFP batteries are commonly run between 10% and 100%, giving a 90% window. Some products hold 5% or 20% back, and some charge to 95% to extend life. The manufacturer's window governs; the calculator starts at 10% to 100% and lets you change either limit.
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 applies the margin 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. The calculator flags a peak more than twice the average 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.
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 and flags it above 0.5C and above 1C.
How much room does a battery energy storage system need?: The calculator plans the installation from the battery and the inverter it recommends and draws it to scale with its dimensions, the height to allow and the load on the slab, using the clearances AS/NZS 5139:2019 itself sets. Up to 45 kWh with an inverter up to 30 kW it goes on one wall: battery units and a hybrid inverter with 900 mm unimpeded in front with doors open (Cl 5.2.5; 600 mm for a DVC-A system or an integrated BESS, Cl 4.2.5), 600 mm kept clear to any exit, habitable-room window or vent, or appliance (Cl 4.2.2.2 / 5.2.2.2), and the habitable-room barrier 600 mm past each end and 900 mm above the units (Cl 4.2.4.2 / 5.2.4.2): about 3 to 5 m of wall. Up to 200 kWh it is a plant-room line-up of 600 x 1000 x 2200 mm LFP racks and a PCS cabinet with 900 mm unimpeded in front, measured from the face to the wall the way Figure 6.3 draws it and with the doors' 600 mm swing inside it, so a 115 kWh, 35 kW system takes a room about 3.7 x 2.0 m internal with 2.8 m clear height, and each rack puts about 14 kPa on the slab. Above 200 kWh it is a battery room to the Section 6 layout: the PCS in line with the racks along the head wall, never facing them, the remaining racks in rows under those racks with every facing pair sharing one 900 mm aisle the way the standard's own Figure C.1 draws it, and above 2 MWh it is 20 ft enclosures on an outdoor pad with a PCS and transformer skid in front of each and 3 m clear all round. The plan is indicative: unit sizes are typical of the band and 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 (Cl 1.1.1); it 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: 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 does peak shaving sizing work?: The shave is the site peak minus the demand target: that is the inverter power. The energy above the target each day is the shave times the hours the demand exceeds the target times a shape factor: 1.0 for a flat block, about 0.75 for a rounded commercial peak, 0.5 for a triangular one. That energy sizes the battery through the same efficiency, margin and window chain. Verify the demand profile from interval data before relying on it: 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 produces a branded PDF report with project details, the recommended battery and inverter, the drawing, the sizing breakdown, the assumptions applied, the flagged checks and the method and standards.
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.
Why does the recommended battery come out larger than the energy I need?: Because four things sit between the energy the load takes and the nameplate on the battery: the discharge path loses some of it (efficiency), the margin adds headroom, the usable window leaves a reserve at the bottom and sometimes headroom at the top, and the end-of-life allowance holds the duration as the battery fades. The breakdown shows each factor on its own line, so the reader can see which one is doing the most.
Can I use this for a home battery?: Yes. Solar storage with the evening energy from a bill and the peak from the largest appliance is the usual home case; backup power with the circuits that must stay on covers a blackout. The result is preliminary: a home battery is chosen from a short list of products, and 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 to refill the usable window, on top of the daytime load, 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.