Battery Runtime Calculator
How long will your battery last per charge? Our free battery runtime calculator estimates hours of runtime from your battery's capacity, voltage and chemistry plus your load — handy for RV trips, solar backup systems, camping, and power outages.
Enter your numbers
Usable-capacity factor — an assumption, not a spec. Deeper discharge shortens lead-acid battery life.
Your results
Planning estimate. Assumes a steady load, a healthy battery at room temperature, and the usable-capacity factor shown above.
| Total stored energy | — |
| Usable energy (85% factor) | — |
| Load drawn from battery | — |
For example: a 100Ah 12V lead-acid battery powering a 60W load gives about 17 hours of runtime — (100 × 12 × 0.85) ÷ 60.
How to use this calculator
Start with your battery: enter its rated capacity in amp-hours (Ah) and its nominal voltage — 12V is the most common for RV, marine and solar backup setups. Then pick your battery chemistry: the calculator adjusts how much of the rated capacity it counts as usable.
Next, enter your total load in watts — the device or devices you want to run, added together. If they run on AC power through an inverter, tick the AC option and set your inverter's efficiency (90% is a typical default); the tool will count the extra draw.
Everything updates instantly as you type, so you can compare setups side by side: what happens if you switch from lead-acid to lithium, drop the load from 60W to 30W, or add a second battery (just double the Ah).
How it works
The math is straightforward energy bookkeeping. First the calculator converts your battery's rating to watt-hours of stored energy: total Wh = capacity (Ah) × voltage (V). Then it applies a usable-capacity factor based on chemistry — usable Wh = total Wh × factor (85% lead-acid, 90% AGM, 95% lithium) — because draining a battery to the very bottom isn't practical or, for lead-acid, healthy.
If your load runs through an inverter, the battery has to supply more than the load itself: effective load = load (W) ÷ inverter efficiency. Finally, runtime (hours) = usable Wh ÷ effective load (W).
Assumptions, stated honestly: the usable-capacity factors are assumptions, not manufacturer specs — battery makers publish their own depth-of-discharge limits, and regularly discharging a lead-acid battery deeper shortens its lifespan. The calculator also assumes a steady load and a healthy battery at room temperature; real capacity falls in the cold and with age. All results are planning estimates, not guarantees.
Frequently asked questions
How long will a 100Ah battery last?
It depends on the load. A 12V 100Ah lead-acid battery holds about 1,020 usable watt-hours at the 85% usable-capacity factor this calculator assumes. Divide that by your load: a 60W load runs about 17 hours (1,020 ÷ 60), while a 10W LED lamp could run for days. Enter your actual battery and load numbers above for your estimate.
Why does battery chemistry change the runtime?
Different chemistries tolerate different depths of discharge. This calculator assumes you can safely use about 85% of a lead-acid battery's rated capacity, 90% of AGM, and 95% of lithium (LiFePO4). Regularly discharging a lead-acid battery deeper than recommended shortens its life — the usable-capacity factors here are assumptions, and checking your battery manufacturer's depth-of-discharge guidance is wise.
What does inverter efficiency do to runtime?
If your load runs on AC power through an inverter, roughly 10% of the energy is lost as heat. A 60W AC load on a 90%-efficient inverter actually draws about 67W from the battery, shrinking runtime by the same proportion. Check the AC-load option above to apply the efficiency factor, or leave it off if your load runs on DC power.
Why is my real-world runtime shorter than the calculator says?
This calculator assumes a steady load and a healthy, room-temperature battery. Real-world losses include: discharge gets less efficient at high currents, capacity drops in cold weather, old batteries lose capacity, and many loads like fridges cycle on and off. Treat the result as a planning estimate, not a guarantee.
How can I make my battery last longer?
Reduce the load (LEDs instead of incandescent bulbs), run on DC instead of through an inverter, keep the battery warm and fully charged between uses, and don't discharge lead-acid batteries too deeply if you want long cycle life — or switch to lithium for a deeper usable range.