This free battery runtime calculator shows how long a battery will last on any load — a portable power station or solar generator, a 12 V deep-cycle battery for an RV, van or boat, or a home backup battery. Enter the capacity in Wh, Ah or mAh and the watts of your fridge, CPAP, TV or laptop to see the runtime, or enter the hours you need to find what size battery or power station to buy. It works for any brand, including EcoFlow, Jackery, Bluetti and Anker.
How the runtime is calculated
Energy (Wh) = Ah × V (or mAh × V ÷ 1000)
Usable Wh = Energy × DoD × inverter efficiency
Runtime (hours) = Usable Wh ÷ load watts
Battery needed (Wh) = load watts × hours ÷ (DoD × inverter efficiency)
Worked example
A 1000 Wh power station running a 100 W load through its AC outlet: 1000 × 1.00 × 0.85 = 850 usable Wh, and 850 ÷ 100 = 8.5 hours. The same battery feeding a 12 V DC device at 100 W, with about 95% efficiency, lasts around 9.5 hours.
How long will a 12V battery last?
Runtime of common 12 V batteries through an 85% efficient inverter. Lithium (LiFePO4) is used to 90% of its capacity, lead-acid and AGM to 50% to protect battery life.
| Battery | 50 W load | 100 W load | 300 W load | 500 W load | 1,000 W load |
|---|---|---|---|---|---|
| 50 Ah lithium (LiFePO4) | 9 h 11 min | 4 h 35 min | 1 h 32 min | 55 min | 28 min |
| 100 Ah lead-acid / AGM | 10 h 12 min | 5 h 6 min | 1 h 42 min | 1 h 1 min | 31 min |
| 100 Ah lithium (LiFePO4) | 18 h 22 min | 9 h 11 min | 3 h 4 min | 1 h 50 min | 55 min |
| 200 Ah lithium (LiFePO4) | 1 d 12 h 43 min | 18 h 22 min | 6 h 7 min | 3 h 40 min | 1 h 50 min |
Power station runtime by size
How long portable power stations and solar generators of common sizes run everyday devices from the AC outlet (85% efficiency, full capacity usable). "Home essentials" is a fridge, Wi-Fi router, a few LED lights and phone charging.
| Capacity | CPAP (40 W) | Fridge (80 W avg.) | TV (100 W) | Home essentials (150 W) |
|---|---|---|---|---|
| 300 Wh | 6 h 23 min | 3 h 11 min | 2 h 33 min | 1 h 42 min |
| 500 Wh | 10 h 38 min | 5 h 19 min | 4 h 15 min | 2 h 50 min |
| 1,000 Wh | 21 h 15 min | 10 h 38 min | 8 h 30 min | 5 h 40 min |
| 2,000 Wh | 1 d 18 h 30 min | 21 h 15 min | 17 h 0 min | 11 h 20 min |
| 3,600 Wh | 3 d 4 h 30 min | 1 d 14 h 15 min | 1 d 6 h 36 min | 20 h 24 min |
How to choose the right size power station
- List what you want to run and find each device's watts on its label or with a plug-in meter.
- Add up the energy: watts × hours for each device. A fridge at 80 W for 24 hours is 1,920 Wh.
- Allow for losses and margin: divide by 0.85 for the inverter and add about 20%.
- Check the inverter rating: the continuous watts must cover everything running at once, and the surge rating must handle motors starting (fridges, pumps, power tools).
- Plan recharging: for multi-day outages or off-grid use, add solar panels to refill the battery each day.
Tips for getting the most runtime
- Use DC outputs when you can. Charging a laptop from USB-C instead of its AC brick can add 10–15% runtime.
- Measure real loads. Label ratings are maximums; a fridge rated 150 W may average 60 W.
- Mind the cold. Lithium batteries deliver noticeably less capacity below 0 °C (32 °F).
- Add solar. A 200 W panel in good sun can put 800–1000 Wh a day back in — see the solar panel calculator.
Related calculations
- Battery rated in mAh? Convert it with the mAh to Wh converter.
- Find the average watts of an appliance with the electricity cost calculator.
- Wiring a 12 V battery to an inverter? Size the cable with the wire size calculator.