Battery Runtime Calculator
How long a battery will run a given load — from its amp-hours and voltage, how deeply it can safely be discharged, and the inverter’s efficiency — with the usable energy and the load that would last a full day.
A battery’s energy is amp-hours × volts, in watt-hours.
How the battery runtime calculator works
A battery’s energy is amp-hours × volts, in watt-hours. Only part of it is usable: lithium batteries tolerate 80–90% depth of discharge, lead-acid about 50% if you want them to last. An inverter turning DC into mains AC loses about 10%. Divide the usable energy by the load for hours. Real runtimes are shorter when the battery is cold or old.
Formula: usable Wh = Ah × V × DoD × inverter efficiency; hours = usable Wh ÷ load W
Worked examples
| Inputs | Runtime | Note |
|---|---|---|
| 100 Ah, 12 V, 60 W load, lithium | 14.4 hours | 14.4 hours |
| Same battery, lead-acid at 50% | 9 hours | 9 hours |
| 10 kWh home battery on a 400 W night load | 21.9 hours | 21.9 hours |
FAQFrequently asked questions
Why not use the full capacity?
Discharging lead-acid below half its capacity shortens its life sharply, and even lithium cells last longer kept between 10% and 90%. The depth-of-discharge figure is the share you plan to use.
Does the load in watts include the inverter?
Enter the appliance’s own draw; the inverter’s loss is the efficiency field. Small inverters also draw a few watts idle, which matters for tiny loads over long periods.
What if my load varies?
Use the average — a fridge rated 150 W running a third of the time is a 50 W load. Watt-hours per day from an energy meter divided by 24 gives the average directly.
Why is the real runtime shorter?
Cold reduces capacity (lead-acid loses about a third at 0 °C), high discharge rates reduce it (Peukert’s effect on lead-acid), and capacity fades with age. Treat the result as an upper estimate.
Where these figures come from
- International Energy Agency — Energy Efficiency — appliance consumption and standby power context
- Ofgem — the UK energy regulator
Last checked: September 2026. Appliance power figures are typical ratings; check the label on your own equipment, which is authoritative for it.