How this calculator works
Battery capacity in watt-hours (Wh) is the energy stored. Not all of it can be used: lithium iron phosphate batteries are usually run to about 90% of capacity, other lithium types around 85%, and lead-acid batteries only to about 50% if you want them to last. Converting DC battery power to AC loses another 5–15% in the inverter. Divide the usable energy by your average load in watts to get hours.
Assumptions and tips
- Average load, not peak. A fridge cycles on and off, so its average is far below its running wattage.
- Cold batteries deliver less capacity.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Find | Runtime of my battery |
| Battery capacity | 2,000 Wh |
| Average load | 150 W |
| Battery type | Lithium (LiFePO₄) — about 90% usable |
| Inverter efficiency | 90 % |
| Result | Value |
|---|---|
| Runtime | 10.8 hours |
| As hours and minutes | 10 h 48 min |
| Usable energy | 1,620 Wh |
| Days at this load | 0.45 days |
Step by step:
- Usable energy = 2,000 Wh × 90% × 90% = 1,620 Wh
- Runtime = 1,620 Wh ÷ 150 W = 10.8 h
Runtime of a 2,000 Wh LiFePO₄ power station
90% usable, 90% inverter efficiency — calculated with this tool.
| Average load | Runtime |
|---|---|
| 10 W — Wi-Fi router | 162 hours |
| 60 W — Laptop and lights | 27 hours |
| 150 W — Refrigerator (average) | 10.8 hours |
| 300 W — Fridge + freezer | 5.4 hours |
| 500 W — Sump pump (average) | 3.2 hours |
| 1000 W — Space heater on low | 1.6 hours |
Common mistakes to avoid
- Dividing capacity by a device’s peak wattage instead of its average.
- Draining lead-acid batteries below 50%, which shortens their life sharply.
Frequently asked questions
How long will a 2,000 Wh power station run a fridge?
At a 150 W average, about 10.8 hours with a LiFePO₄ battery and 90% inverter efficiency.
How do I convert amp-hours to watt-hours?
Multiply by the battery voltage: a 100 Ah 12 V battery stores 1,200 Wh.
How big a battery do I need for 12 hours?
For a 150 W average load, about 2,222 Wh with LiFePO₄ — or about 4,000 Wh of lead-acid.
Built and reviewed by Anirban Kundu, Creator & Editor · M.Sc Physics, 20 years teaching science