ArrayBudgetPanels, batteries and inverters sized to the load you actually have.

Backup Generator Sizing

Running watts, starting surge and the derates that decide the rated size — plus the two safety rules that matter more than any of the arithmetic.

Read this before any number on these pages.

Carbon monoxide kills people every storm season. Run a generator outdoors only, at least 20 feet from any door, window or vent, exhaust pointed away from the house. Not in a garage — not even with the door open. Fit CO alarms indoors and treat a headache during an outage as CO until proven otherwise.

And never back-feed. Plugging a generator into a wall socket energises the utility line outside and can kill a lineman restoring your power. It is illegal almost everywhere for exactly that reason. A transfer switch or interlock kit is the only correct connection, and it is an electrician's job.

Two numbers, not one

Running watts — everything on at once. Starting surge — the running load plus the inrush of the single largest motor, which draws about three times its running watts for a second or two.

Only one motor's surge is counted, and that is deliberate. Adding every motor's surge together, which is what most sizing tables do, buys a generator twice the size you need and burns fuel accordingly.

Then the derates

A generator should not run continuously above 80% of its rating, because heat is what ends them. Fuel costs output: propane about 10% against gasoline on the same engine, natural gas about 20%. Altitude costs about 3% per 1,000 feet.

Fuel choice is a storage decision

Gasoline makes the most power and stores worst — stabilizer and rotation. Propane stores indefinitely. Natural gas never runs out but needs a gas line. Diesel lasts longest and starts worst in cold.

Inverter or conventional

Inverter units are quieter, produce clean power for electronics and throttle down under light load. They cost more per watt and top out lower. For a fridge, a furnace fan and lights they are the better buy; for a well pump and central air they are not.

1 article

Generator size for 8000 running watts with a 1 HP motor on propane

Generator size
12,000watts
8,000 W running with a 1 HP motor on propane at 2,000 ft
Three numbers, not one8,000 Wrunningeverything on9,492 Wstarting surgelargest motor12,000 Wgeneratorrated sizeOnly the largest motor’s surge is counted — adding them all doubles the machine
Running wattseverything on at once8,000W
Starting surgerunning load plus the 1 HP motor's inrush9,492W
After the 80% rulea generator should not run continuously above 80% of its rating10,000W
Propane deratestores indefinitely; about 10% less output than petrol−10%
Altitude derate2,000 ft at 3% per 1,000 ft−6%
Rated size needed11,820W
Nearest size sold1% headroom left12,000W

Notes

  • Carbon monoxide kills people every storm season. Run it outdoors only, at least 20 feet from any door, window or vent, with the exhaust pointed away from the house. Not in a garage — not even with the door open. Fit CO alarms indoors, and treat a headache during a power cut as CO until proven otherwise.
  • Never back-feed. Plugging a generator into a wall socket to power the house energises the utility line outside and can kill a lineman working to restore your power. It is illegal almost everywhere for that reason. A transfer switch or an interlock kit is the only correct connection, and it is an electrician's job.
  • Only the largest motor's inrush is counted, and that is deliberate. A 1 HP induction motor draws about 2,238 W for a second or two on start — roughly three times its running draw. Adding every motor's surge together, which is what most sizing tables do, buys you a generator twice the size you need and burns fuel accordingly.
  • Propane: stores indefinitely; about 10% less output than petrol. The 10% derate is real — the same engine makes less power on this fuel, and it is already in the number above.
  • The 80% rule is not caution. A generator held at its nameplate rating runs hot, and heat is what ends them. Sizing to 10,000 W of rated capacity for 8,000 W of load is what the machine is designed around.
  • Inverter generators change the arithmetic. They are quieter, produce clean power for electronics, and throttle down under light load — but they cost more per watt and top out lower. For a fridge, a furnace fan and some lights, an inverter unit is usually the better buy; for a well pump and central air, it usually is not.

Backup generator size

232 calculations · browse all