Last updated: October 2026
How long will a 100Ah battery last?
Short answer: a 100Ah 12V LiFePO4 battery runs a 60W fridge for about 19 hours — (100 × 12 × 1.00 × 0.95) ÷ 60 = 19.0 hours. The same load on an AGM lead-acid battery lasts only about 8.5 hours, because only half its capacity is usable. The calculator above handles any battery size, voltage and chemistry — and reverses to size a battery for camping.
The battery runtime formula
Battery capacity in amp-hours means nothing on its own — multiply by voltage to get watt-hours, the real unit of stored energy. Then knock off what you can't actually use:
Usable Wh = Ah × V × usable share × efficiency (× 0.9 more if you're running an inverter). Runtime = usable Wh ÷ load watts.
So a 100Ah 12V battery holds 1,200 Wh in theory, but a lead-acid one only gives you about half of that. That's why chemistry matters more than the number on the label.
Chemistry cheat sheet
Different battery chemistries let you use different amounts of their rated capacity safely. Read about deep-cycle batteries on Wikipedia for the background.
How to use the calculator
1. In How long will it last? mode, enter your battery's amp-hours, voltage and chemistry, then your load in watts (or pick a preset like a mini fridge). 2. Tick AC load if the device plugs into an inverter — fridges, TVs and CPAP machines usually do; phone chargers and 12V lights don't. 3. Read your runtime as hours and minutes, plus the usable watt-hours.
Need a battery for camping or a power outage? Switch to What size do I need?, enter your load and how many hours you need, and the calculator tells you the minimum battery size in amp-hours — rounded up, so you never come up short.
Frequently asked questions
Why can't I use 100% of a lead-acid battery?
You can, but it shortens the battery's life dramatically. Lead-acid batteries (AGM and flooded) are typically limited to about 50% depth of discharge — draining them further causes sulfation and permanent capacity loss. That's why this calculator treats 50% as usable for AGM and flooded batteries. LiFePO4 batteries can safely use nearly 100% of their rated capacity without the same damage.
What does inverter efficiency do to runtime?
An inverter converts DC battery power to AC household power and loses energy as heat in the process. A typical inverter is about 90% efficient, so roughly 10% of your battery's energy is wasted before it reaches the appliance. Turn the AC-load toggle on whenever your device plugs into an inverter (fridges, TVs and CPAP machines usually do); leave it off for DC loads like phone chargers and 12V LED lights.
How accurate is this calculator?
It's a solid estimate, not a lab measurement. Real runtime varies with temperature (batteries hold less charge in the cold), battery age, wiring losses, and the Peukert effect — lead-acid batteries deliver noticeably less capacity when drained quickly by heavy loads. Use the result as a planning guide and add a 10–20% safety margin for critical loads like a CPAP machine.
What changes when I wire batteries in series vs parallel?
Wiring batteries in series doubles the voltage but keeps the amp-hours the same (two 12V 100Ah batteries in series = 24V 100Ah). Wiring in parallel doubles the amp-hours but keeps the voltage (12V 200Ah). Either way, total energy in watt-hours is the same — this calculator works on watt-hours, so just enter your system's total Ah and voltage.
Is my data private?
Completely. Every calculation runs locally in your browser with JavaScript — nothing you enter is sent to a server, stored, or tracked.
