CableHero

Battery Storage Sizing Calculator

Battery capacity, modules, inverter and cable currents for backup, daily cycling or off-grid, with the AS/NZS 5139 voltage class

Create a free account to save to a project Sign up to save

Battery capacity, module count, inverter size and cable currents for a home or small commercial battery, with the AS/NZS 5139:2019 + A1:2025 voltage class.

Sizing Method

h

How long the critical loads stay on

kW

Blank: critical daily energy ÷ 24

Energy Use

kWh

Whole site, per day

kWh

Backed-up part per day (blank: all of it)

Peak and Surge

kW

Largest load running at once

Default 1.0

kW

Motor starts, compressors

s

Battery Module

kWh
kWh

When given, sets the depth of discharge

kW

Per module

kW

Per module, short time; checked against the surge

s

How long the module holds its peak

V

Per module

V

End of discharge

V

End of charge

A

Per string, from the datasheet

1 for low-voltage modules in parallel

Datasheet stack limit

Datasheet stack limit

Design Factors

PV charges through the battery inverter

%

Default 90 %

%

Default 96 %

%

Default 95 %. Battery only (DC to DC), not the system AC figure

%

Default 100 %; from the datasheet curve

%

Default 70 %; warranty retention

%

Default 0 %

Inverter

kW
kVA

Blank: same as the kW rating

kVA
s
V

Inverter battery input window

V
A

When given, sizes the DC cable

V

Default 230 V

A

When given, sizes the AC cable

kVA

From the connection agreement

Cables and Protection

Subscription

Size the DC and AC cables in the cable calculator from these currents, with the battery's fault level and the 2 % inverter rise limit filled in, and keep them linked to this battery.

Create a free account

Battery System

Enter the energy use, battery module and inverter to size the battery.

Compliance: AS/NZS 5139:2019 + A1:2025 and AS/NZS 4777.1:2024

The voltage class and checks appear once the battery is sized.

How It Works

Sizing a Battery in Four Steps

  1. 1

    Choose what the battery is for

    Backup of critical loads for a number of hours, daily cycling of part of your use, or off-grid autonomy for a number of days.

  2. 2

    Enter the energy use

    Daily kWh, the kWh and days on a bill, a load list with hours of use, or 24 hourly values from interval meter data.

  3. 3

    Enter the battery module and inverter

    From the datasheets: module kWh, voltages and discharge power, and the inverter's kW, kVA, surge and battery voltage window.

  4. 4

    Read the system

    Usable and nominal kWh sized to end of life, modules in series and parallel, inverter checks, the DC and AC cable design currents and the voltage class.

Worked Example

10 Hours of Backup for the Essentials

A home uses 12 kWh a day on its critical circuits, an average of 0.5 kW, and wants 10 hours of backup from LFP modules of 5.12 kWh (51.2 V) through a 5 kW single-phase inverter, with a 4 kW peak load.

The loads need 0.5 kW × 10 h = 5.0 kWh. At 96 % inverter efficiency the battery delivers 5.21 kWh. Sized for 90 % depth of discharge and 70 % capacity at end of life, it needs 5.21 ÷ (0.90 × 0.70) = 8.27 kWh nominal.

Two modules (10.24 kWh) cover that, and their 5.12 kW of discharge covers the 4 kW peak (4.17 kW at the battery). At end of life they still give 6.45 kWh usable, about 12.4 hours of backup. The DC cable carries up to 5 kW ÷ 0.96 ÷ 44.8 V = 116 A, the AC cable 5 kVA ÷ 230 V = 21.7 A, and at 57.6 V maximum the battery is DVC-A.

Frequently Asked Questions

Battery Sizing Questions Answered

How big a battery do I need for backup?

Multiply the average power of the loads you want kept on by the backup hours, then divide by the inverter efficiency, the usable depth of discharge, the capacity left at low temperature and the capacity left at end of life. For example, 0.5 kW for 10 hours is 5 kWh at the loads, 5.2 kWh at the battery with a 96 % inverter, and about 8.3 kWh nominal for an LFP battery at 90 % depth of discharge sized to 70 % end-of-life capacity.

What is the difference between usable and nominal kWh?

Nominal kWh is the battery's rated capacity. Usable kWh is what you can take out of it within the depth of discharge the manufacturer allows. A 5.12 kWh module used to 90 % depth of discharge gives 4.6 kWh usable when new, and less as it ages.

What depth of discharge should I use for LFP and NMC batteries?

Use the usable kWh from the datasheet when you have it. Without it the calculator assumes 90 % for lithium iron phosphate (LFP) and 80 % for nickel manganese cobalt (NMC), which are typical of current products, not values from a standard.

Does AS/NZS 5139 apply to my battery?

AS/NZS 5139 covers battery systems used with power conversion equipment from 1 kWh up to 200 kWh per battery system. The calculator reports whether the sized system falls in that band. Amendment 1 (December 2025) is now part of the standard.

What are DVC-A, DVC-B and DVC-C?

The decisive voltage classification in AS/NZS 5139:2019 + A1:2025. A battery up to 60 V DC is DVC-A, up to 120 V DC is DVC-B and above that is DVC-C. Higher classes bring extra requirements for cable protection and labelling. The calculator classes the battery on its maximum string voltage.

Can a home battery go in a garage?

Often, yes. AS/NZS 5139 keeps batteries out of restricted locations and away from exits and from windows or vents that serve habitable rooms. Amendment 1 allows a battery within 600 mm of an opening wider than 900 mm, such as a garage door, if safe egress and clear space are kept. Check the full requirements in the standard and the manufacturer's manual.

How do I size the battery cable?

Size the DC cable for the most current the inverter can draw: its maximum DC input current, or its rated power divided by its efficiency and the battery's minimum voltage. The calculator gives that current, plus the AC current, for the cable size calculator.

Reviewed by Ahmed Tayeh, electrical engineer at CableHero. The calculator is a design aid: the designer remains responsible for checking the installation against the standards.

More free tools

Other calculators

Size the Battery Cables Too

Take the DC and AC design currents into the cable size calculator for current rating and voltage drop to AS/NZS 3008.1.1.

Open the Cable Size Calculator