Home Battery Sizing Calculator
Tell us what you want to keep running and for how long. We size the battery for both energy (how long it lasts) and power (whether it can run your loads at all).
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How battery sizing works
The calculator sizes for two things and keeps the larger answer:
- Energy. Daily use of the loads you want backed up, times the length of the outage, divided by inverter efficiency, with your reserve held back.
- Power. The most your loads draw at one moment, compared with the continuous output of the battery units.
With solar recharge switched on, it simulates the outage hour by hour and finds the smallest number of units that makes it through in the weather you chose.
Getting better inputs
- Energy: add up your own loads in the Outage Simulator, or use a whole-home figure from your electric bill (monthly kWh divided by 30).
- Power: the simulator reports typical peak kW for the loads you select.
- Battery specs: use the usable kWh and continuous kW from the product's own data sheet. The defaults here are deliberately conservative.
Is a big battery bank really the right answer? Compare it with a generator.
Open the Generator vs Battery →Common questions
kWh is energy: how much is stored, and what your electric bill charges for. kW is power: how fast energy is being used or delivered at one moment. A battery rated 13.5 kWh and 5 kW can deliver at most 5 kW at once, and at that rate it would be empty in under three hours.
For essentials only, one typical home battery is often enough for a day. For a whole home with central air, two to four units is common for a single day, and multi-day whole-home backup on batteries alone can require more units than most people want to buy. That is when solar recharge or a generator becomes the practical answer.
The energy you can actually draw from the battery. Many products list usable capacity directly. Systems also hold back a reserve, often 10 to 20 percent, to protect the battery and keep the inverter alive, so plan around the usable figure minus your reserve.
Because a battery bank has to do two jobs. It must store enough energy for the hours you want, and it must deliver enough power for everything running at the same moment. If your loads draw more kW than the units can supply, you need more units, or a higher-output inverter, even though the energy would have been enough.