How it works
Runtime is the energy you can actually take out of the battery divided by how fast you use it. Start with the capacity in watt-hours (Wh), take off any reserve you keep back, then allow for conversion losses: running household appliances through the inverter (AC outlets) loses about 15%, while DC outputs such as USB and 12 V ports lose about 5%.
Add up the watts of what you plug in. Fridges and freezers cycle on and off, so use their average draw (yearly kWh ÷ 8.76) rather than the running watts on the label. The default list uses typical averages.
If you can recharge with solar, the calculator subtracts each day's solar harvest (panel watts × sun hours × 0.8) from your daily use. When solar covers the whole load, the battery can run indefinitely in good weather.
The formula
usable Wh = capacity Wh × (1 − reserve) × efficiency runtime = usable Wh ÷ load in watts solar Wh = panel watts × sun hours × 0.8 × efficiency days = usable Wh ÷ (load × 24 − solar Wh)
Worked example
A 1,000 Wh power station running a fridge (60 W average), router (15 W), four LED bulbs (40 W) and phone charging (10 W) from its AC outlets.
| Step | Working | Result |
|---|---|---|
| Load | 60 + 15 + 40 + 10 | 125 W |
| Usable energy | 1,000 Wh × 0.85 | 850 Wh |
| Runtime | 850 ÷ 125 | 6.8 hours |
A 13.5 kWh home battery such as a Powerwall is rated in usable AC energy, so pick the 95% setting. Keeping 20% in reserve leaves about 10.3 kWh, which runs a 500 W average load of essentials for about 20.5 hours.
Solar helps even when it cannot cover the whole load: 400 W of panels at 4.5 sun hours harvests about 1.22 kWh a day, stretching the 1,000 Wh station on the 125 W load from 6.8 to about 11.5 hours.
Frequently asked questions
How long will a 1,000 Wh power station last?
About 850 ÷ watts hours when running AC appliances: roughly 8.5 hours at 100 W, 17 hours at 50 W, or 14 hours for a typical fridge on its own.
How long will a Powerwall last in an outage?
A 13.5 kWh battery running essentials at 500 W lasts about a day; at 1 kW (fridge, lights, internet, a well pump cycling) about 10 to 13 hours. Solar recharging can extend that indefinitely in summer.
How long will a power station run a fridge?
A modern fridge averages 50 to 70 W across a day. A 1,000 Wh unit runs one for about 12 to 17 hours, and a 2,000 Wh unit for about a day and a half. See how much electricity a fridge uses.
Why is runtime shorter than capacity ÷ watts?
Because the inverter that turns battery DC into household AC loses energy as heat, and the inverter itself uses a few watts even with little plugged in. Home batteries may also keep a reserve you cannot use.
What size power station do I need?
Multiply your load in watts by the hours you want and divide by 0.85. For 125 W over 12 hours, that is about 1,800 Wh. Also check the continuous output rating covers your highest-wattage appliance.
Sources
- Inverter efficiency (about 85% from battery to AC outlet) and capacity figures from manufacturer specifications.
- Appliance averages: US Department of Energy, estimating appliance energy use, and ENERGY STAR ratings.
- Default sun hours: 4.5 for the US; change the country above for others.
Related guides
Estimates are for planning only and are not financial advice. Get written quotes before you buy.