12V 100Ah battery runtime by load
Hours from one 12 V 100 Ah battery with the load wired directly to it (DC, no inverter). LiFePO4 is counted to 90% usable, AGM to 50%.
| Load | Watts | LiFePO4 | AGM |
|---|---|---|---|
| 1 A | 12 W | 3 days 18 h | 2 days 2 h |
| 2 A | 24 W | 45 h | 25 h |
| 5 A | 60 W | 18 h | 10 h |
| 10 A | 120 W | 8 h 42 min | 4 h 30 min |
| 20 A | 240 W | 4 h 12 min | 2 h 2 min |
| 50 A | 600 W | 1 h 36 min | 42 min |
At 50 A the AGM battery is drained in well under an hour, much less than the simple “100 Ah ÷ 50 A × 50%” would suggest. That is Peukert’s law: lead-acid loses usable capacity as the current rises.
Working out runtime with amps
On a 12 V system it is often easier to think in amps, because battery capacity is already in amp-hours:
Runtime (h) = battery Ah × usable % ÷ load (A)
Example: a 100 Ah battery, 90% usable, running a 4 A load: 100 × 0.9 ÷ 4 = 22.5 hours. To convert a device rated in watts, divide by 12: a 60 W TV draws 5 A. Running AC appliances through an inverter adds about 10% to the current, so switch the calculator to AC via inverter for those.
Lithium or AGM for an RV or boat?
- Usable capacity: a 100 Ah LiFePO4 battery gives about 90 Ah you can use every day; a 100 Ah AGM about 50 Ah if you want it to last.
- Weight: a 100 Ah LiFePO4 battery typically weighs 11–14 kg against 27–32 kg for a 100 Ah AGM, so it is about a quarter of the weight for the same usable capacity.
- Cycle life: LiFePO4 typically lasts thousands of cycles; AGM hundreds when used to 50%.
- Cold: most LiFePO4 batteries must not be charged below 0 °C unless they have built-in heating. AGM charges in the cold but delivers less.
To size a bank for a weekend off-grid, use the battery size calculator. If you recharge from solar panels, the solar battery calculator works in daily energy and days of autonomy. The method behind the numbers is in how battery runtime is calculated.