Estimate only. An installer will check your loads, your panel and the battery's surge rating.
How to find your essential load
Add up the watts of everything you want to keep on at the same time: the refrigerator, some lights, the internet router, a medical device, a well or sump pump. The wattage is printed on each appliance's label or in its manual. Motors such as pumps and compressors draw more for a moment when they start, so leave margin.
How the calculator works
- Energy: essential load (W) ÷ 1,000 × hours of backup.
- Capacity with losses: energy ÷ round-trip efficiency. The default 85% is the representative value used by NREL's Annual Technology Baseline; applying it to the backup is conservative.
- Number of batteries: the larger of capacity ÷ capacity per battery and load ÷ continuous power per battery. A battery can run out of power before it runs out of energy.
The defaults of 12.5 kWh and 5 kW per battery are the representative residential system in the same NREL baseline. Use your battery's spec sheet if you have one.
What this does not include
Solar recharging during a long outage, the starting surge of large motors, and whole-home backup with central air conditioning, which usually needs a much larger system.