Capacity answers how much energy is stored
Battery capacity is usually expressed in kilowatt-hours (kWh). It helps estimate how long selected loads may run, but usable energy can differ from headline capacity because of reserve settings, operating limits and conversion losses.
Power answers how much can run at once
Power is expressed in kilowatts (kW). Continuous output limits the sustained load the system can support. Surge capability matters for short starting loads such as pumps, compressors and some motors.
The inverter can become the bottleneck
A large battery bank does not automatically increase AC output. The inverter, transfer equipment, battery current limits and system topology can constrain available power.
Size backup from loads, not just battery size
List essential circuits, estimate simultaneous demand and identify starting loads. Then compare expected runtime under realistic energy use rather than assuming the full house will operate normally during an outage.
Try the battery runtime calculator and review essential-loads vs whole-home backup.