Battery Runtime Calculator
How long a battery, power bank or UPS runs a load, from its capacity, depth of discharge and inverter efficiency, and how long it takes to charge back.
Result…
How it's calculated
Energy (Wh) = capacity (Ah) × voltage (V) Usable energy = energy × depth of discharge × efficiency Runtime (h) = usable energy ÷ load (W) Recharge time (h) = energy taken out ÷ (charger power × charging efficiency)
A battery stores energy, which is its amp-hours times its voltage. Only part of it should be used (the depth of discharge), and an inverter or DC converter loses some on the way to the load, so the usable energy is the capacity multiplied by both. Dividing that by the load in watts gives the hours it lasts; this is how a UPS runtime is worked out too. Charging puts back what was taken out, plus the charging losses. The calculation assumes the load stays steady and ignores the Peukert effect, where a battery gives less than its rated capacity when drained fast.
Your numbers, step by step
About the Battery Runtime calculator
This battery runtime calculator tells you how long a battery will keep a load running, and how long a charger takes to fill it back up. Enter the capacity in amp-hours with the battery voltage, or straight in watt-hours, then how deep you let it discharge and how efficient the inverter or DC converter is. The load can be in watts or in amps at a given voltage. It suits a UPS behind a computer or router, a leisure or camper battery, a solar storage bank, a power station or a power bank. You enter every figure from your own labels and datasheets, so the answer fits your equipment. The steps show the watt-hours at each stage, and a chart shows how the runtime changes if the load is lighter or heavier.
How to use it
- Under “Capacity given in”, choose amp-hours or watt-hours and enter the “Battery capacity” (with the “Battery voltage” for Ah).
- Set the “Usable depth of discharge” from the battery maker and the “Inverter or converter efficiency” from the datasheet.
- Enter the “Load” in watts, or switch “Load given in” to amps and give the “Load current” and “Load voltage”.
- To see the charge time, pick “Charger amps” or “Charger watts” and set the “Charging efficiency”.
- Read the runtime, the usable energy, the current from the battery and the time to recharge.
Worked example
A 100 Ah, 12 V battery holds 100 × 12 = 1,200 Wh. Using 80% of it through a 90% efficient inverter leaves 1,200 × 0.8 × 0.9 = 864 Wh, so a 100 W load runs for 864 ÷ 100 = 8.64 h, about 8 h 38 min, drawing 9.26 A from the battery. Putting the 960 Wh back with a 10 A charger at 85% efficiency takes 960 ÷ (120 × 0.85) = 9.41 h. A small UPS with two 12 V 9 Ah batteries in series (24 V, 216 Wh) at 85% efficiency runs a 150 W computer for 1.224 h, about 1 h 13 min.
Updated
Questions people ask
How do I calculate how long a battery will last?
Multiply amp-hours by voltage to get watt-hours, multiply by the usable depth of discharge and the inverter efficiency, then divide by the load in watts. The answer is in hours.
How long will my UPS run my computer?
Add up the watts of what is plugged in, take the battery watt-hours from the UPS's specifications (or its battery labels), and use the UPS efficiency from its datasheet. Many UPS makers also publish a runtime chart for their model.
What depth of discharge should I use?
Use the figure the battery maker gives for the cycle life you want. Draining a battery deeper gives more runtime now but usually shortens its life, and lead-acid types are hurt by it more than lithium ones.
How do I convert amp-hours to watt-hours?
Multiply by the nominal voltage: a 50 Ah, 12 V battery stores 600 Wh. The electrical power calculator links watts, volts and amps for the load side.
Why does my battery run out sooner than the calculator says?
Capacity drops at high discharge rates, in the cold and as a battery ages, and loads such as fridges and pumps draw more power when they start. Treat the answer as a best case and leave a margin.