Generator size for 10500 running watts with a 1½ HP motor on propane

Generator size

20,000watts

10,500 W running with a 1.5 HP motor on propane at 8,000 ft

Running wattseverything on at once	10,500W
Starting surgerunning load plus the 1.5 HP motor's inrush	12,738W
After the 80% rulea generator should not run continuously above 80% of its rating	13,125W
Propane deratestores indefinitely; about 10% less output than petrol	−10%
Altitude derate8,000 ft at 3% per 1,000 ft	−24%
Rated size needed	19,189W
Nearest size sold4% headroom left	20,000W

Running watts: 10500 W
Largest motor: 1½ HP
Fuel: propane
Altitude: 8000 ft

Read this before the numbers

Carbon monoxide kills people every storm season. 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.

Never back-feed. Plugging a generator into a wall socket energises the utility line outside and can kill a lineman restoring your power. A transfer switch or interlock kit is the only correct connection, and it is an electrician's job.

Two numbers, not one

10,500 W running — everything on at once. 12,738 W starting — the running load plus the inrush of the single largest motor.

Only one motor's surge is counted, and that is deliberate. A motor draws about three times its running watts for a second or two on start; adding every motor's surge together, which is what most sizing tables do, buys a generator twice the size you need.

Then the derates

The 80% rule first: a generator should not run continuously above 80% of its rating, because heat is what ends them. Then fuel — propane costs 10% of output against gasoline on the same engine. Then altitude, about 3% per 1,000 ft.

That comes to 19,189 W of rated capacity, so the size to buy is 20,000 W, leaving 4% headroom.

Inverter or conventional

Inverter generators 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, an inverter unit is usually the better buy. For a well pump and central air, it usually is not.

Fuel choice is a storage decision

Gasoline makes the most power and stores the worst — it needs stabilizer and rotation. Propane stores indefinitely and costs about 10% of output. Natural gas never runs out but costs 20% and needs a gas line. Diesel lasts longest and starts worst in cold.

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 1.5 HP induction motor draws about 3,357 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 13,125 W of rated capacity for 10,500 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.

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