Battery Runtime Calculator
Calculate how long a battery will power a load, allowing for depth of discharge.
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 Runtime Calculator does
Battery runtime is usable energy divided by the load. Usable energy is capacity in amp-hours times voltage times the depth of discharge you allow, because draining a battery fully shortens its life severely.
Formula
Total energy (Wh) = Capacity (Ah) × Voltage × BatteriesUsable energy = Total energy × Depth of dischargeBattery draw = Load ÷ Inverter efficiencyRuntime = Usable energy ÷ Battery draw
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Battery capacity | Ah | Yes | Accepts more than 0. |
| Battery voltage | V | Yes | Accepts more than 0. |
| Load specified as | one of 2 options | Yes | — |
| Load power | W | In some modes | Accepts more than 0. Shown Load specified as is Watts. |
| Load current | A | In some modes | Accepts more than 0. Shown Load specified as is Amps. |
| Usable depth of discharge | % | Yes | Lead-acid 50%, lithium 80–90%. |
| Inverter efficiency | % | Optional | Leave at 100 for DC loads. |
| Number of batteries | number | Yes | Accepts 1 or more. |
How to use it
- Choose Load specified as.
- Enter Battery capacity, Battery voltage, Usable depth of discharge and Number of batteries.
- Fill in the remaining inputs the form shows for your choice.
- Optionally add Inverter efficiency.
- Select Calculate.
Worked example
A 100 Ah 12 V battery powering a 200 W load at 50% depth of discharge through a 90% efficient inverter.
- Capacity
- 100 Ah
- Voltage
- 12 V
- Load
- 200 W
- DoD
- 50
- Inverter
- 90
Total 1,200 Wh, usable 600 Wh. Draw 222.2 W → runtime 2.7 hours.
Frequently asked questions
Why not use the full battery capacity?
Deep discharge damages most chemistries. A lead-acid battery cycled to 50% may last 1,200 cycles; cycled to 80% it may manage only 400.
Why is my real runtime shorter?
The Peukert effect: capacity falls as discharge current rises. A 100 Ah battery drained in one hour may deliver only 60 Ah.
Method and sources
Method. Usable energy from capacity, voltage and depth of discharge, divided by the load, with an inverter efficiency factor where applicable.
Applies to. None — arithmetic on manufacturer ratings
Assumptions
- The load is constant, and the battery delivers its rated capacity.
Limitations
- Rated capacity is quoted at a specified discharge rate. Drawing faster yields less — markedly so for lead-acid, where the Peukert effect can cut usable capacity substantially at high loads.
- Capacity falls with age, cycle count and temperature; a battery several years old will not meet its nameplate.
- Discharging below the recommended depth shortens life considerably, which is a cost this runtime figure does not show.
Sources
- The datasheet for the specific battery in use — Varies by manufacturer. Rated capacity at a stated discharge rate, recommended depth of discharge, and temperature derating.