kWh tells you how much energy is stored
Battery capacity in kilowatt-hours describes the amount of electrical energy available for later use. More usable kWh generally means longer backup duration for the same household load.
kW tells you how much can run at once
Continuous output power determines how many loads a battery can support simultaneously. A battery with plenty of stored energy can still be unable to run several high-power appliances at the same moment if its kW rating is too low.
Peak power matters for motors
Air conditioners, pumps and some other motor loads can require a short burst of power when starting. Battery specifications may publish short-duration peak or motor-start capability separately from continuous output.
Backup duration depends on the loads you choose
A refrigerator, lights, internet equipment and a few outlets use far less daily energy than whole-home HVAC, electric cooking and water heating. Estimate essential-load kWh per day before deciding how much storage is useful.
Solar can extend outage runtime
When a compatible solar-plus-storage system can recharge during an outage, daytime solar production can extend battery runtime. Weather, load behavior and system-control architecture determine how much extension is realistic.