How to size battery capacity for your home

Battery capacity is usually listed two ways: nominal capacity and usable capacity, both measured in kilowatt-hours (kWh). Usable capacity is the number that matters — it's the energy you can actually draw before the system reserves the rest to protect battery life.

To estimate what you need, add up the wattage of the circuits you want to keep running during an outage, multiply by the hours you want to run them, and divide by 1,000 to convert to kWh. A refrigerator, a few lights, and networking equipment typically add up to a few kWh per day — a whole home with HVAC and appliances can run well into double digits.

Continuous power output (kW) is a separate number: it's how much you can draw at one instant, regardless of total capacity. A system with plenty of capacity but low continuous output may still trip if you run too many appliances at once.

If you're not sure, a licensed electrician can do a load calculation for your home before you buy.