How long will a 100Ah battery last? It depends on the battery type, the voltage and what you run. The quick answer for a 12V lithium battery: about 1,200 usable watt-hours, so roughly 12 hours at 100W. Here’s the math and real examples.
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The Formula
Runtime (hours) = battery Wh × usable fraction × efficiency ÷ load watts
- Battery Wh = Ah × volts. 100Ah × 12.8V (LiFePO4) = 1,280Wh
- Usable fraction: about 90–100% for LiFePO4, about 50% for lead-acid to protect its life
- Efficiency: about 85–90% through an inverter for AC loads, close to 100% for direct DC loads
How Long Will a 100Ah Battery Last? Examples
Example math, LiFePO4 12.8V 100Ah, 90% usable, 88% inverter efficiency for AC loads (about 1,010Wh net AC, 1,150Wh DC):
| Load | Watts (typical) | Runtime |
|---|---|---|
| 12V compressor fridge (DC, ~35% duty) | ~45W running, ~16W avg | ~70 hours |
| CPAP without humidifier (DC adapter) | ~30W | ~38 hours |
| LED lights (DC) | 20W | ~57 hours |
| Laptop (AC) | 60W | ~17 hours |
| 50″ LED TV (AC) | 100W | ~10 hours |
| Starlink Mini (DC) | ~25–40W | ~29–46 hours |
| Microwave (AC) | 1,500W | ~40 minutes |
Wattages are estimates. Check your appliance’s label or measure with a plug-in meter.
Lead-Acid vs Lithium
A 100Ah AGM battery at 50% usable gives about 600Wh, roughly half the runtime of a lithium battery of the same Ah rating. Heavy loads also cut lead-acid capacity further (the Peukert effect).
Things That Shorten Runtime
- Inverter idle draw: 10–30W on many units, even with nothing plugged in
- Cold: lithium can’t be charged below freezing without a heater, and all batteries lose some capacity
- Age: capacity fades over the cycles
- High current: check the BMS limit. Many 100Ah batteries are limited to 100A (about 1,280W)
Runtime Chart: 12V 100Ah Battery
| Constant load | LiFePO4 (about 1,280Wh usable) | AGM at 50% DoD (about 600Wh) |
|---|---|---|
| 10W | about 128 hours | about 60 hours |
| 50W | about 25.6 hours | about 12 hours |
| 100W | about 12.8 hours | about 6 hours |
| 300W | about 4.3 hours | about 2 hours |
| 500W | about 2.6 hours | about 1.2 hours |
Calculated as usable Wh ÷ load W, before inverter losses. Through an inverter, divide again by its efficiency. Lead-acid capacity also shrinks at high discharge rates (the Peukert effect), so AGM runtimes at heavy loads will be shorter than shown. See depth of discharge and cycle life.
Worked Example: A Weekend Van Trip
Illustrative daily loads for a 12V LiFePO4 100Ah battery:
| Load | Watts | Hours a day | Wh a day |
|---|---|---|---|
| 12V compressor fridge (cycling) | 45W while running | about 12 hours of run time | 540Wh |
| LED lights | 20W | 4 | 80Wh |
| Roof fan | 15W | 8 | 120Wh |
| Phone and laptop charging (via inverter at 90%) | 60W | 3 | 200Wh |
| Total | 940Wh |
- Usable energy: about 1,280Wh.
- Daily use: about 940Wh.
- Runtime with no charging: 1,280 ÷ 940 ≈ 1.4 days.
- To cover a full weekend, add solar or alternator charging, or a second battery.
Turning that into a solar size is covered in how many solar panels do I need. Appliance numbers come from labels; see how to read an appliance label.
100Ah at Other Voltages
| Battery | Nominal energy |
|---|---|
| 12.8V 100Ah LiFePO4 | 1,280Wh |
| 25.6V 100Ah LiFePO4 | 2,560Wh |
| 51.2V 100Ah LiFePO4 | 5,120Wh |
The same “100Ah” holds four times as much energy at 48V. See amp-hours vs watt-hours.
Getting the Most Runtime
- Run DC loads directly on 12V instead of through an inverter where possible.
- Switch the inverter off, or use its power-save mode, when it’s not needed. See inverter efficiency and idle draw.
- Keep the battery in its rated temperature range. LiFePO4 datasheets usually block charging below freezing unless the battery has built-in heating.
- Measure real use with a battery monitor (shunt), which counts amp-hours in and out.
FAQ
How long will a 100Ah battery run a 1,000W load?
About 1 hour on lithium after inverter losses, and only if the BMS allows about 90A continuous.
Is 100Ah enough for a van?
For a fridge, lights, fans and phones, often yes, with solar topping it up. Add capacity for induction cooking or AC.
How do I calculate runtime for my loads?
Add up each load’s watts × hours per day, then compare with the battery’s usable Wh.
How long will a 100Ah battery run a fridge?
Using the example above (about 540Wh a day for a 12V compressor fridge), a 12V LiFePO4 100Ah battery runs it for about two days on its own. Check your fridge’s actual draw.
How long does it take to charge a 100Ah battery?
Roughly Ah ÷ charger amps, plus some absorption time. A 20A charger takes a little over 5 hours from empty.
Sources: U.S. Department of Energy, “Estimating Appliance and Home Electronic Energy Use” and “How Does Electricity Work?”; ENERGY STAR product data. Accessed Oct 2026.
Related: LiTime 12V 50Ah LiFePO4 Battery Review, XZNY 12V 18Ah LiFePO4 Battery Review