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The Cummins Onan QD 8000 diesel RV generator uses about 0.5 to 0.6 gallons of diesel an hour at half load and about 1 gallon an hour at its full 8,000 watts. With nothing plugged in, it idles along on roughly 0.13 to 0.21 gallons an hour, depending on the version.
Diesel gensets are thrifty. That’s a big part of why diesel pushers carry them.
Diesel use by load
| Load | RV model (8HDKAK) | Commercial model (8HDKAU) |
|---|---|---|
| No load | 0.13 gal/hr | 0.21 gal/hr |
| Half load (4,000 W) | 0.49 gal/hr | 0.61 gal/hr |
| Full load (8,000 W) | 1.02 gal/hr | 1.02 gal/hr |
Sources: Cummins Onan RV QD 7500/8000 specification sheet and the QD 6000/7500/8000 commercial specification sheet (A-1486). Ratings apply at up to 500 ft altitude and 85°F. Output drops about 3.5 percent per 1,000 ft of altitude, according to Cummins.
What an hour costs in diesel
At $4.00 a gallon for diesel, half load costs roughly $2 to $2.50 an hour. Full load costs about $4 an hour. Eight hours of air conditioning at a steady 0.7 gallons an hour comes to about 5.6 gallons, or around $22.
What a day of running costs
Using Cummins Onan’s half-load figure of about 0.5 to 0.6 gallon an hour, a 10-hour day comes to roughly 5 to 6 gallons of diesel. On a hot day with two air conditioners cycling, budget 0.6 to 0.8 gallon an hour, so about 6 to 8 gallons over the same 10 hours. Multiply by your local diesel price. For a cost estimate based on your own load, use the fuel run-time calculator with the diesel setting.
Planning fuel for a trip or an outage
An 8 kW generator in a motorhome often runs two roof air conditioners plus the usual 120 volt loads. On a hot afternoon that can sit around half to three-quarters load. Budget 0.6 to 0.8 gallons an hour for that kind of day, and about half a gallon on cooler days with lighter loads.
Like gasoline RV generators, a built-in diesel unit usually feeds from the coach’s main tank. The pickup tube stops it before the tank runs dry, so you keep enough fuel to drive. Over a long boondocking stretch, the generator can use more fuel than you expect, so watch the gauge.
The shared chassis tank
- The generator draws from the chassis tank through a pickup set higher than the engine’s. It stops at roughly a quarter tank, leaving fuel for driving.
- Keep fuel filters on schedule. Diesel contamination shows up first as hard starting or surging.
- In winter, use winter-blend diesel or an approved additive to prevent gelling.
Why diesel RV generators sip fuel at light load
Diesel engines don’t use a throttle plate the way gasoline engines do, so they waste less fuel at partial load. That’s one reason the QD 8000’s no-load burn of roughly 0.13 to 0.21 gallon an hour is so low. Running a big diesel at very light load for long stretches can cause “wet stacking”, though: unburned fuel and soot build up in the exhaust. Cummins Onan recommends regular exercise under a reasonable load. Running the air conditioners for a while during the monthly exercise run is a practical way to do that.
Keeping it efficient
- Fuel filters: follow the interval in the owner’s manual. A restricted filter starves the engine and can cause power loss or surging under load.
- Light loads: diesels don’t like long periods at very light load, because it leads to carbon build-up. Running a real load during the monthly exercise run helps.
- Altitude: above a few thousand feet the generator delivers less power, so it works harder for the same appliances.
How the totals compare with gasoline
For the same work, a diesel RV generator usually burns fewer gallons an hour than a gasoline one, especially at part load, for the reasons above. Diesel often costs more per gallon, though, so the difference in dollars is smaller than the difference in gallons. Run your own numbers with your generator’s spec sheet and local pump prices, or try the fuel run-time calculator.
Diesel use, in brief
Is the QD 8000 more efficient than a gasoline generator of the same size?
Generally yes, especially at partial load. Compare with the gasoline Onan 4000 figures.
How often should I change the oil?
Follow the QD manual’s hour and calendar intervals, and shorten them in dusty conditions.
To compare other sizes and fuels, try the fuel run-time calculator. The generator fuel guides cover diesel, propane and gasoline in more depth.
