How Much Backup Power Do I Need?
A practical method for estimating backup power by watts, surge demand, watt-hours and outage duration.
Power versus energy
Watts describe how much power a load needs at a moment. Watt-hours describe energy used over time. Backup planning needs both.
Add running loads
List the devices you may run at the same time and add their running watts. Do not assume every nameplate load will operate continuously.
Account for starting surge
Motors and compressors can briefly need more power when starting. Your inverter or generator must tolerate the relevant surge.
Estimate energy
Multiply average watts by hours of use to estimate watt-hours. Then add margin for conversion losses, battery reserve and uncertainty.
Prioritize
If the required system becomes too large, reduce simultaneous loads or shorten runtimes for nonessential devices before simply buying more capacity.