Enter your battery size and your device's power draw, and this calculator gives you an estimated runtime in hours. It uses standard planning numbers — lead-acid batteries give you about half their rated capacity in practice, LiFePO4 about 90% — and it shows its math so you can sanity-check the result.
How the runtime estimate works
The math is two steps. First, convert battery capacity to usable watt-hours:
Usable Wh = capacity (Ah) × 12 V × usable depth of discharge
Then divide by your load, allowing for inverter losses:
Runtime (hours) = usable Wh × inverter efficiency ÷ load (W)
The one judgment call is depth of discharge. Running a lead-acid battery flat kills it fast, so the standard planning figure is 50%. LiFePO4 cells tolerate deep discharge, so 90% is the usual planning figure.
Worked examples
| Battery | Load | Usable energy | Est. runtime (90% inverter) |
|---|---|---|---|
| 100 Ah lead-acid | 50 W | 600 Wh | ≈ 10.8 hours |
| 100 Ah lead-acid | 100 W | 600 Wh | ≈ 5.4 hours |
| 100 Ah LiFePO4 | 50 W | 1,080 Wh | ≈ 19.4 hours |
| 200 Ah LiFePO4 | 200 W | 2,160 Wh | ≈ 9.7 hours |
Why the real number can be lower
- Cold weather. Battery capacity drops noticeably below freezing; lead-acid suffers more than lithium.
- Battery age. An older lead-acid bank may hold 70–80% of its original capacity. Size with margin.
- High draw on lead-acid. Pulling a big load from a small lead-acid battery delivers less than the arithmetic suggests (the Peukert effect). If your inverter runs near its limit, derate the estimate.
- Inverter idle draw. An inverter left on with no load still burns 5–20 W. Overnight that's real energy.
How to keep the battery charged
If your runtime math shows the battery draining faster than your solar can refill it, the fix is on the charging side: a correctly sized solar charge controller setup for a 12V battery bank, or a panel matched to your daily watt-hours. Browse our solar battery chargers, or ask us to size a panel for your battery and location.
Frequently asked questions
How long will a 100Ah battery run a 12V fridge?
Most 12V camping fridges draw 40–60 W while the compressor runs, and they cycle on and off. Budget 30–50 W average and a 100 Ah lead-acid battery gives you roughly 12–18 hours; a 100 Ah LiFePO4 roughly a day and a half.
Can I run a 1000W inverter off a 100Ah battery?
Technically yes, briefly. At 1,000 W the same 100 Ah lead-acid battery is empty in about 30 minutes of real use — less once the high-draw penalty applies. For sustained loads that size you need a much larger bank.
Is LiFePO4 worth it for a camper or boat?
On usable energy per kilogram and per cycle, yes — you get almost double the usable watt-hours from the same Ah rating, and it holds voltage flat until nearly empty. Upfront cost is higher; whether the math works depends on how often you cycle it.
What inverter efficiency should I assume?
90% is a fair planning figure for a decent pure sine wave inverter. Cheap or heavily loaded inverters run closer to 80–85%.
Does battery age change runtime?
Yes. Lead-acid banks lose capacity steadily with cycles and time; lithium fades much more slowly. If your setup used to last a weekend and now lasts a day, age is the first suspect.
Get panels matched to your battery
Tell us your battery size and what you're running. We'll reply with a panel and charge controller setup sized for your actual load — no guesswork.
Ask for a sizing quoteNeed something custom? See custom flexible solar panels or custom solar panels.