Generator size for 10500 running watts with a 5 HP motor on diesel

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. 17,960 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 — diesel costs 0% of output against gasoline on the same engine. Then altitude, about 3% per 1,000 ft.

That comes to 17,960 W of rated capacity, so the size to buy is 18,000 W, leaving 0% 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.