Sizing a home battery for backup power
Determining the right battery size for outages starts with identifying critical loads and the duration you want to cover. Battery capacity is given in kilowatt-hours (kWh), while power rating in kilowatts (kW) defines how much load the battery can serve at one time.
Steps to size a backup battery
- List critical circuits: Examples include refrigeration, lights, medical equipment, well pumps, communication devices and a few outlets. Prioritize what you must keep running.
- Estimate power draw: Sum the wattage of critical loads to get the continuous kW requirement. Add startup surge needs for motors and compressors.
- Determine desired backup duration: Decide how many hours or days you want to sustain those loads during an outage.
Sizing example
- Critical load: 1.5 kW continuous.
- Desired duration: 8 hours.
- Battery capacity needed: 1.5 kW * 8 hours = 12 kWh (plus margin for inefficiencies and depth of discharge).
Adjust for inefficiencies and safety margin
- Round-trip efficiency: Batteries lose energy when charging and discharging; assume 85–95% efficiency depending on chemistry.
- Depth of discharge: Warranty and battery life may limit usable capacity; a 10 kWh battery might have 9 kWh usable if rated for 90% DOD.
- Startup surges: Ensure the battery’s power rating can handle appliance startup currents for compressors or motors.
Other practical considerations
- Hybrid inverters and automatic transfer switches are needed to safely disconnect from the grid during outages and power critical circuits.
- Stacking or multiple batteries: Several smaller units can be used together for more capacity, but check compatibility and space.
- Budget and space: Larger capacity increases cost and requires physical space and ventilation considerations.
An installer can perform a load analysis and design a battery system that balances capacity, power rating and cost to meet your backup expectations while staying within warranty and safety limits.