See how many hours a home battery can keep your house running during an outage. Enter the battery's capacity, your average load and how charged it is when the power goes out; the tool allows for the reserve you keep and inverter losses.
Results update as you type. Without JavaScript you see the worked example below.
A battery backup runtime calculator divides the energy a battery can actually deliver by your average load. With the defaults, a full 13.5 kWh battery with a 10% reserve and 90% inverter efficiency delivers about 10.9 kWh, which runs an 800 W average load for about 13.7 hours, or 0.6 days, with no solar recharge.
You get the estimated runtime in hours, the same figure in days, and the energy delivered. A bar chart shows runtime at 300, 500, 800, 1,200, 2,000 and 3,500 W, and a runtime-by-load schedule (200 W to 5,000 W, hours and days) can be downloaded as CSV. The scenario table compares your inputs, twice the capacity, and the same battery starting half full.
Energy delivered (kWh) = capacity × (starting charge − reserve) × inverter efficiency
Runtime (hours) = energy delivered × 1,000 ÷ average load (W)
Runtime (days) = hours ÷ 24Assumptions that match the code:
Default inputs: 13.5 kWh, 800 W, starting at 100%, 10% reserve, 90% efficiency.
The default scenario table shows 13.7 hours for your inputs, 27.3 hours with twice the capacity (27 kWh) and 6.1 hours if the battery starts half full.
Second example, two batteries cycled daily: 27 kWh, a 1,200 W average load, 80% charge when the outage starts and a 20% reserve.
| Average load | Hours | Days |
|---|---|---|
| 200 W | 54.7 | 2.3 |
| 300 W | 36.5 | 1.5 |
| 500 W | 21.9 | 0.9 |
| 800 W | 13.7 | 0.6 |
| 1,000 W | 10.9 | 0.5 |
| 1,500 W | 7.3 | 0.3 |
| 2,000 W | 5.5 | 0.2 |
| 3,500 W | 3.1 | 0.1 |
| 5,000 W | 2.2 | 0.1 |
Load matters more than anything else. Cutting the average from 1,500 W to 500 W triples runtime.
If the runtime covers the outages you usually see, the battery fits your needs. If not, you can trim loads, add capacity, keep a higher charge before storms, or add solar recharge. Our home battery backup guide explains partial-home backup panels, which are the main way to keep the average load low.
To size a battery from an appliance list instead, use the solar battery size calculator. If outages are long and frequent, compare costs with the battery vs generator calculator.
This is an educational estimate. It uses a constant load and fixed efficiency, and it ignores temperature effects, battery aging and power limits on how much can run at once. Your battery's app and the manufacturer's spec sheet give the most accurate picture of real runtime.
With the defaults, a full 13.5 kWh battery with a 10 percent reserve and 90 percent inverter efficiency delivers about 10.9 kWh, enough for 13.7 hours at an average 800 W load. At 300 W of essentials it lasts about 36.5 hours; at 2,000 W, about 5.5 hours.
It depends on what is on backed-up circuits. A refrigerator, lights, internet and phone charging often average a few hundred watts, while running central air or an electric water heater can push the average past 2,000 W. Your battery app or a utility smart-meter history can show your real average.
Many home batteries let you set a backup reserve, a share of capacity kept for emergencies or to protect the battery. It is not available during the outage in this calculation. The default is 10 percent; set it to 0 if your battery lets you use the full capacity.
Runtime drops in proportion. If you use the battery every day for time-of-use savings, it may be half empty in the evening. With the defaults, starting at 50 percent cuts runtime from 13.7 to about 6.1 hours, which is why many owners raise the reserve before a forecast storm.
It can, if your system is designed to keep the panels running and recharge the battery while the grid is down. This calculator assumes no solar recharge, a safe worst case for nights and stormy days. Use the battery size calculator to model daily solar recharge over multi-day outages.
The inverter that turns stored DC power into household AC power loses some energy, about 10 percent at the default 90 percent efficiency. Combined with the 10 percent reserve, a 13.5 kWh battery delivers about 10.9 kWh to your appliances.
Yes, at the same load. The scenario table shows the default battery's 13.7 hours rising to about 27.3 hours with twice the capacity. Peak power is a separate limit, though; two units can also run more appliances at once.
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