Size a backup generator from total running watts, add the largest motor’s startup surge, then build in 20–25% headroom.
For the full breakdown, see our best Backup Generator For House guide.
When the power dies at midnight, the first question isn’t brand or fuel — it’s size. Knowing how to size a backup generator for a house does not start with square footage; it starts with the loads you would run during the outage. The whole method is arithmetic: total the running watts of those loads, add the starting surge of the largest motor, then apply a 20–25% safety margin. A few wiring rules follow that math.
The process below applies to U.S. 120/240 V single-phase service, whether you are shopping for a portable unit or a permanently installed standby. Run it with real numbers and you will buy with confidence instead of crossing your fingers.
Build A Load List First
Load-based sizing starts on paper, not on house plans.
Grab a notepad and walk through the house, listing every appliance you genuinely want during an outage. Read the nameplate on each one and apply these rules while you build the list:
- Record running watts. If the label lists amps instead, convert with watts = amps × volts — 120 V for standard circuits, 240 V for two-pole circuits like dryers and AC.
- Only include appliances that can actually run at the same time. The refrigerator cycling while the sump pump runs means both go into the running total.
- Flag every motor-driven load — central AC, heat pump, well pump, sump pump, furnace blower. These draw far more at startup than they do while running, and their surge demand drives the next step.
Now decide the coverage level. Essential loads only (refrigerator, freezer, lights, furnace, well or sump pump) support a much smaller unit than whole-home backup that also carries the kitchen, electric water heater, and central AC. Write the coverage decision down before you total anything.
How Many Watts Does A House Generator Need?
A house needs the sum of its running watts plus the single largest starting surge in the list, multiplied by about 1.2 to 1.25 for margin. That final number is the generator size you shop against.
Electric motors can draw 3 to