🔋 Battery Runtime Calculator
Enter your battery capacity, voltage, load, and efficiency to estimate usable watt-hours and how long the battery will power your load, in both hours and minutes.
🔋 Calculate Battery Runtime
What is a Battery Runtime Calculator?
A battery runtime calculator tells you how long a battery can keep a device or appliance running. It converts amp-hours and voltage into usable watt-hours — after a realistic efficiency derate — and divides by your load to give runtime in hours and minutes.
Use it to size a power station for a fridge, a UPS for your router, or an off-grid pack for a campsite. These are estimates for planning, not professional electrical advice, so always keep a margin for temperature and battery age.
❓ Frequently Asked Questions
How do I calculate battery runtime?
Convert the battery to watt-hours by multiplying amp-hours by voltage, apply an efficiency derate for inverter and discharge losses, then divide by your load in watts. A 100 Ah, 12 V battery at 90% efficiency holds about 1,080 Wh, so a 200 W load runs for roughly 5.4 hours.
Why include an efficiency percentage?
Stored energy is never delivered perfectly. Inverters, wiring, and the battery's own internal resistance lose some power as heat, especially when converting DC to AC. Derating by 85–90% gives a more realistic runtime than the raw watt-hour figure, which assumes a perfect, lossless system.
Does this work for any battery chemistry?
The watt-hour math is the same for lead-acid, AGM, and lithium, but usable capacity differs. Lithium (LiFePO4) can safely deliver most of its rated capacity, while lead-acid should only be drawn down partway, so adjust your amp-hour input or efficiency to reflect the safe usable portion of your pack.
Are these runtime numbers exact?
No — they are estimates for planning, not professional electrical advice. Actual runtime shifts with temperature, battery age, discharge rate (Peukert effect), and how deeply you discharge. Treat the result as a planning baseline and keep a safety margin.