When the grid goes down, the only number that matters is how long your battery keeps the important stuff alive. Product pages love to quote big kWh figures, but they rarely translate that into "your fridge runs for X hours." This tool does.
Pick a battery capacity and an appliance below, and it estimates real runtime with losses and duty cycles included, so the answer matches what you would actually get in a blackout.
Runtime calculator
Cycling appliance: it only draws power about 33 % of the time (the motor stops once it reaches temperature).
Estimated runtime
≈ 3d 21h
Estimate for 3600 Wh usable at ~85 % (inverter losses + real depth of discharge). Real numbers vary with temperature, battery age, and appliance efficiency.
How the runtime is calculated
The base math is straightforward:
Runtime (hours) = Usable capacity (Wh) / Appliance draw (W)
Two honest corrections keep it from lying to you:
- Usable capacity, not rated. You do not get 100% of a battery's rated kWh at the outlet. Between usable depth and inverter losses, plan for roughly 80 to 90% of the number on the box. The calculator uses the usable figure.
- Duty cycle. A fridge, a well pump, and an AC compressor all cycle on and off. Their average draw over hours is far below their peak wattage, so runtime is longer than a naive "peak watts" division suggests. The calculator accounts for this on cycling loads.
For a whole set of appliances at once, add their average draws together and divide the usable capacity by that total. Essentials together often average a few hundred watts, which is why a mid-size battery can carry them through most of a day.
Rough runtime by battery size
Ballpark figures for common essential loads, losses included.
| Load (average draw) | 1 kWh | 3.6 kWh | 10 kWh |
|---|---|---|---|
| Internet (router + modem, ~20W) | ~2 days | ~1 week | very long |
| Fridge (cycling, ~60W avg) | ~14 h | ~2 days | ~1 week |
| Essentials bundle (fridge + internet + lights + phones, ~180W avg) | ~5 h | ~17 h | ~2 days |
| Window AC (small, ~500W avg part-time) | ~1.7 h | ~6 h | ~17 h |
| Well pump (intermittent, ~300W avg) | ~3 h | ~10 h | ~28 h |
Two things jump out. Essentials alone stretch a long way, which is why most people need less battery than they fear. And anything with a compressor (AC, well pump) eats capacity fast, which is why whole-home backup gets expensive.
What the numbers usually tell people
- 1 kWh: covers internet, phones and a fridge for a while. Fine for a short outage or a renter's essentials.
- 3 to 5 kWh: the practical sweet spot for essentials through a full day without recharging.
- 10 kWh and up: longer autonomy or bigger loads, ideally paired with solar so it recharges each day rather than simply draining.
If the runtime you see is shorter than you want, the honest fix is usually solar recharging rather than a much bigger battery. A battery that refills each day effectively runs indefinitely in good weather. Size it properly with the home battery sizing guide, and if you are unsure whether to go fixed or portable, read home battery vs portable power station.
