Diesel Generator Run Time: How to Calculate Yours
A four-step calculation that works for any diesel generator size, with typical consumption ranges from 5 kW to 30 kW and the derating you should apply for real-world conditions.
When to use this guide
What you'll need: your generator's wattage rating, your expected load percentage, and tank capacity in gallons.
Time: about 5 minutes.
Quick steps
- Find your generator's wattage rating. Check the nameplate or the owner's manual.
- Estimate your load. Typically 25 to 75 percent of rated output.
- Look up typical diesel consumption at that load (see the reference table below).
- Divide tank size by gallons per hour to get run time in hours.
- Subtract 10 to 25 percent for real-world conditions to get a planning number.
What you'll need
- Generator nameplate showing rated watts (5,000 W = 5 kW, etc.).
- Tank size in gallons. Typical ranges: 7 to 10 gallons for portable diesel units, 50 to 500 gallons for residential standby generators with a sub-base or external tank.
- Load estimate. Sum the running watts for the appliances you plan to power. A working number is fine; you can refine it later.
Identify your generator's wattage tier
Diesel generators are sized by kilowatts. Three common tiers cover most home and small-commercial use:
- Portable diesel: 5 to 12 kW. Trailerable or two-person carry, 7 to 12 gallon tanks, designed for jobsites and short-duration backup.
- Residential standby: 10 to 25 kW. Permanently installed, sub-base tank or external above-ground tank, runs essential or whole-home loads.
- Commercial or agricultural: 30 kW and up. External tanks (often 500+ gallons), sized to run buildings, well pumps, milking parlors, or shop equipment.
Estimate your load
Load is the percentage of rated output your generator is supplying at any moment. A 10 kW generator powering 5,000 W of appliances is at 50 percent load. Real-world reference points:
- Lights, fridge, furnace blower, well pump: 4 to 7 kW combined.
- Add central HVAC: 8 to 15 kW total.
- Off-grid cabin baseline (lights, fridge, small electronics): 2 to 4 kW.
Most homeowners run portable units at 25 to 50 percent and standby units at 50 to 75 percent. If you don't know your load, start with 50 percent. It is a defensible planning number for almost any setup.
Apply the formula
Once you have tank size and consumption rate, the math is one line:
Worked example
A 7.5 kW diesel generator at 50 percent load with a 7 gallon tank:
- Consumption from the table below: 0.5 gal/hr.
- Run time: 7 ÷ 0.5 = 14 hours.
- Real-world planning number after 20 percent derating: about 11 hours.
Use the planning number, not the spec-sheet number, for storm-prep math. Cold weather, aging fuel, and load swings all shave runtime in directions that compound, not cancel.
Typical diesel consumption reference
Manufacturer specs at 50 percent load, averaged across Cummins Onan, Kohler, Generac diesel, and Yanmar units. Individual model variation is roughly plus or minus 15 percent.
| Generator size | Consumption (gal/hr at 50% load) | Run time on 30 gal | Run time on 100 gal | Run time on 500 gal |
|---|---|---|---|---|
| 5 kW | 0.4 | 75 hr | 250 hr | 1,250 hr |
| 7.5 kW | 0.5 | 60 hr | 200 hr | 1,000 hr |
| 10 kW | 0.7 | 43 hr | 143 hr | 714 hr |
| 15 kW | 0.95 | 32 hr | 105 hr | 526 hr |
| 20 kW | 1.3 | 23 hr | 77 hr | 385 hr |
| 22 kW | 1.4 | 21 hr | 71 hr | 357 hr |
| 30 kW | 1.9 | 16 hr | 53 hr | 263 hr |
Real-world consumption typically runs 10 to 25 percent higher under variable loads, cold weather, or aging fuel. Treat these numbers as a planning ceiling, not a guarantee.
Skip the math: plug your tank size and consumption rate into the Generator Fuel Runtime Calculator.
When calculations underestimate
Five conditions consistently push real-world consumption above the spec-sheet numbers. Plan for whichever apply to you.
- Cold weather (below 20°F): diesel viscosity rises and combustion efficiency drops 5 to 10 percent. In severe cold, gelling can stop the engine entirely without a winter additive.
- High altitude (above 3,000 ft): derate output by roughly 3 percent per 1,000 ft of elevation. Naturally aspirated engines feel it first; turbocharged units less so.
- Aging fuel: diesel stored 12+ months without biocide can lose 5 to 15 percent efficiency to microbial growth and water absorption.
- Water-contaminated fuel: water in the tank cuts efficiency dramatically and can damage injectors. A water-separating filter on the supply line is cheap insurance.
- Wet stacking: running diesel at under 30 percent load for long stretches causes incomplete combustion and unburned fuel buildup in the exhaust. Either size down, or run a load bank or higher loads periodically to burn the residue out.
Common scenarios
Three setups that cover most household and small-commercial diesel use, with realistic planning numbers.
- Portable jobsite, 7 kW with a 7 gallon tank at 50 percent load: 14 hours theoretical, 11 hours after derating. Plan a refill every shift.
- Whole-home standby, 20 kW with a 100 gallon sub-base tank at 60 percent load: roughly 67 hours theoretical, 50 to 55 hours real-world. Covers a 2 day outage with margin; 3+ day events need a refill plan.
- Off-grid cabin, 10 kW with a 500 gallon external tank at 30 percent load: over 1,000 hours theoretical at light load, but expect more wet-stacking risk. Consider a smaller unit if you rarely exceed 3 kW.
FAQ
Why is diesel more efficient than gasoline or propane?
Energy density. A gallon of diesel stores about 139,000 BTU vs 114,000 for gasoline and 91,500 for propane, roughly 22 percent more energy than gasoline and about 50 percent more than propane. That is why a diesel unit at the same kilowatt rating burns fewer gallons per hour.
Can I run my diesel generator on biodiesel?
Most modern diesel generators handle B5 to B20 (5 to 20 percent biodiesel) without modification. Check the manufacturer spec sheet first. Pure biodiesel (B100) can shorten injector life and is not recommended for long-term storage because it gels at higher temperatures and degrades faster than petroleum diesel.
How long does diesel last in storage?
6 to 12 months untreated in a sealed tank, and 1 to 2 years with biocide and stabilizer added. The failure mode is biological growth (microbes feeding at the water-diesel boundary in the bottom of the tank), not evaporation. Inspect quarterly, keep the tank full to limit condensation, and add a biocide if you store more than a few months.
What is specific fuel consumption (SFC), and should I use it instead of gallons per hour?
SFC measures fuel per unit of electricity produced (grams per kilowatt-hour). It is the engineering-grade metric: a tighter SFC number means a more efficient engine. Quality diesel generators typically run 200 to 280 g/kWh. For everyday planning, gallons per hour at your expected load (the table above) is good enough.
Should I size my diesel generator for peak load or average load?
Average. Diesel engines run cleanest and most efficiently at 50 to 75 percent load. Sizing for peak means running at 25 to 35 percent most of the time, which causes wet stacking (unburned fuel buildup in the exhaust) and shortens engine life. Pick a unit that puts your typical load near the middle of its rated output.
Bottom line
Diesel beats gasoline and propane on raw run time per gallon, but the actual hours you get depend on load and tank size, not fuel chemistry. Calculate once for your worst-case load, store enough for a 72 hour outage, and rotate stored diesel every 12 months with a biocide treatment. The math is simple; the discipline of fuel storage is what runs out first.