Battery Backup & Solar Sizing
Convert your load list into watt-hours, size a battery power station and see how long solar takes to refill it.
Buy a unit rated for at least 11.18 kWh of capacity and at least 860 watts of continuous output. Capacity decides how long; output decides whether it runs at all. 400 watts of panel in 4 peak sun hours realistically returns about 1,280 Wh per day.
Outage and solar assumptions
Peak sun hours vary from about 3 in the winter northeast to 6 in the southwest summer, so check a solar irradiance figure for your own state and season rather than using a national average.
What has to stay on
Duty cycle is applied automatically, so a refrigerator counts at about a third of its running watts across the hour.
How this is calculated
Each appliance contributes running watts × duty cycle × hours, so a refrigerator rated at 700 watts counts at roughly 245 watts across an hour because its compressor only runs about a third of the time. The total is increased by 15 percent for inverter and conversion losses. Solar production applies an 80 percent derate to the panel rating, which is a realistic allowance for temperature, angle, wiring and controller losses.
Check both numbers on any unit you consider: capacity in watt-hours decides how long it lasts, continuous output in watts decides whether it can run the load at all, and the surge rating decides whether a motor will start.