Battery Capacity Calculator
Size a battery bank for a required runtime and load.
Please note: For planning and educational use only. Electrical installation is governed by wiring regulations and is dangerous when done incorrectly. Always have work designed, installed and inspected by a qualified electrician.
What the Battery Capacity Calculator does
Sizing a battery bank works backwards from the energy the load needs. That energy is inflated by inverter losses, then divided by the usable depth of discharge, because only part of a battery is safely available.
Formula
Energy at load = Load × RuntimeEnergy from battery = Energy at load ÷ Inverter efficiencyTotal capacity = Energy from battery ÷ (Depth of discharge × Temperature derating)Amp-hours = Total capacity ÷ System voltage
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Load power | W | Yes | Accepts more than 0. |
| Required runtime | hours | Yes | Accepts more than 0. |
| System voltage | V | Yes | Accepts more than 0. |
| Usable depth of discharge | % | Yes | — |
| Inverter efficiency | % | Optional | — |
| Capacity per battery | Ah | Optional | Accepts 0 or more. |
| Temperature derating | % | Optional | Cold reduces usable capacity. |
How to use it
- Enter Load power, Required runtime, System voltage and Usable depth of discharge.
- Optionally add Inverter efficiency, Capacity per battery and Temperature derating.
- Select Calculate.
Worked example
A 500 W load for 8 hours at 12 V, 50% depth of discharge, 90% inverter.
- Load
- 500 W
- Runtime
- 8 h
- Voltage
- 12 V
- DoD
- 50
- Inverter
- 90
4,000 Wh at the load → 4,444 Wh from battery → 8,889 Wh total → 740.7 Ah at 12 V.
Frequently asked questions
Why use 24 V or 48 V instead of 12 V?
The same energy needs half or a quarter of the amp-hours, so cables and connections carry far less current. Above about 1 kW, 12 V becomes impractical.
Method and sources
Method. Required capacity from load and desired runtime, grossed up for depth of discharge and conversion efficiency.
Applies to. None — arithmetic on manufacturer ratings
Assumptions
- The efficiency and depth-of-discharge figures entered match the chemistry and equipment.
Limitations
- Sizing to the nameplate leaves no margin for ageing. Capacity declines over the life of the bank, so a system sized exactly will fall short well before replacement.
- Temperature affects both available capacity and life, and cold conditions in particular reduce what a battery can deliver.
Sources
- The datasheet for the specific battery chemistry being sized — Varies by manufacturer. Usable depth of discharge, cycle life at that depth, and temperature derating.