Generators & Backup Power

Add the running watts, then account for the surge

Sizing backup power is mostly bookkeeping done in the right order. First, total the running (rated) watts of everything that will run at the same time — the loads you truly need during an outage, not every appliance in the house. Then add the starting (surge) watts of the single largest motor load, because a refrigerator compressor, well pump or air conditioner draws several times its running current for the first fraction of a second as the motor spins up. A generator must cover the running total continuously and absorb that one worst-case surge on top of it.

The Generator Size calculator does this addition for you and returns a recommended generator rating with headroom. Use the Appliance Wattage calculator, backed by the appliance wattage chart, to look up typical running and starting figures when you don't have a nameplate handy — though the nameplate on your own equipment always wins. Once you know the load, the Generator Fuel / Runtime calculator estimates runtime and fuel burn from the tank size and how hard the generator is working, since consumption scales with load rather than being a fixed number.

Want the full method with examples? Read what size generator do I need. Undersizing trips the generator or browns out sensitive electronics; oversizing wastes fuel and money. Treat the result as a planning estimate, size the transfer switch and any permanent wiring to code, and have a standby installation inspected by a licensed electrician.