Gas Use of a 4,000-Watt Onan Generator: Gallons Per Hour and Cost

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The Cummins Onan 4000 RV generator (sold as the MicroQuiet 4000 and later the QG 4000) burns about 0.5 gallons of gasoline an hour at half load. Cummins’ spec sheet lists 0.3 gal/hr with no load and 0.7 gal/hr at its full 4,000 watts. Those figures are for the 60 Hz gasoline model.

That is a lot less than the 1 to 1.5 gallons an hour you sometimes see quoted. A 4 kW engine simply cannot burn that much at its rated output.

Burn rate by load

Source: Cummins Onan QG 4000 specification sheet (A-1399). Figures are averages; real use varies with altitude, temperature and the condition of the unit.

Cost per hour

Multiply the burn rate by your local pump price. At $3.50 a gallon, half load works out to roughly $1.75 an hour, or about $14 for an eight-hour evening and night. At full load it’s closer to $2.50 an hour.

Fuel for a weekend

Most RV owners don’t run the generator at full load for long. The roof air conditioner is the big draw. A 13,500 BTU unit pulls roughly 1,500 watts once it’s running, which puts a 4,000 watt generator a bit under half load. With the fridge, a few lights and a phone charger on top, figure on about half a gallon an hour.

Built-in RV generators usually draw from the vehicle’s main fuel tank. Most rigs are set up so the generator shuts off while there’s still a quarter tank or so left, so you’re not stranded. Check where your pickup sits before you plan a long dry-camping stretch.

A two-day boondocking budget

Two days with the generator running 6 hours a day at about half load (one AC plus small loads) is 12 hours at about 0.5 gallon an hour, so roughly 6 gallons. With the RV’s chassis tank feeding the generator, that’s well within the quarter-tank reserve the generator leaves for driving. At full load, the same 12 hours would use about 8.4 gallons.

Ways to use less fuel

• Don’t stack big loads. Running the microwave and the AC together pushes the unit toward full load and its highest burn rate.
• Keep up the basics. A clogged air filter or a worn spark plug makes the engine work harder. The service intervals are in the owner’s manual.
• Exercise it. Cummins recommends running the generator regularly under load. It keeps the carburetor from varnishing, and a gummed-up carburetor wastes fuel and causes surging.

A few more habits stretch each gallon:

• Use propane for the fridge and water heater while on generator power. That keeps several hundred watts off the 4000.
• Charge batteries efficiently. A modern multi-stage converter finishes the job faster, so the generator runs fewer hours.
• Reduce AC load. Park in shade, close blinds and use reflective window covers. The AC cycles less, so average load drops.
• Keep the generator maintained. A dirty air filter or old spark plug raises fuel use and makes starting harder.

Maintenance that affects fuel use

Cummins Onan’s QG 4000 manual lists oil changes at set hour and calendar intervals (commonly every 150 hours or yearly), plus air filter and spark plug service. It also recommends exercising the generator regularly under load, commonly monthly. Fuel left sitting in the carburetor causes the classic surge-and-stall problem. Regular exercise prevents most of that.

Onan 4000 fuel use in brief

Why does my 4000 seem to burn more than the spec sheet says?

Common reasons are higher loads than you think (the water heater on electric, battery charging), an old air filter or a carburetor problem, and altitude.

Compare diesel: Onan QD 8000 fuel use.

Is an Onan 4000 enough for a 13,500 BTU AC plus a microwave?

Running both together often reaches or exceeds the generator’s 4,000 watt rating, especially on hot days. Stagger them.

For other generator sizes, my fuel run-time calculator gives a quick estimate. The RV and camping guides cover sizing and hookups. Also useful: the cost of running an RV generator.