What sets a generator's fuel burn
A generator burns fuel in proportion to the electricity it delivers, divided by how efficiently it converts one into the other. That is the whole model: take the kilowatts leaving the terminals, divide by the engine-alternator efficiency to get the fuel energy going in, then divide by the energy in one gallon or one therm to get a volume. Everything else — tank size, price, hours — is arithmetic on top.
The number that surprises people is the last one: cost per kilowatt-hour. It does not depend on the load at all. If a gallon of diesel holds 37.66 kWh and your set turns 33% of that into electricity, every gallon yields 12.43 kWh no matter how heavily you load it, so at $4.00 a gallon the electricity costs 32 cents a kilowatt-hour before you touch a single filter. Compare that against the delivered grid price you would put into the appliance energy cost calculator and the reason nobody runs a generator by choice becomes obvious.
Load percentage still matters for two other reasons. It sets the burn rate, and therefore the runtime on the fuel you have. And it sets the efficiency itself: an engine is a heat engine, and a heat engine running at a quarter of its design output is meaningfully worse at its job than the same engine near full load. This calculator holds efficiency at the figure you enter, so if you are modelling a lightly loaded set you must lower that figure yourself.
The formula, and the heating-value convention behind it
Start with the load. A set's nameplate rating is its continuous electrical output; multiply by the load fraction to get the kilowatts you are actually drawing. A 20 kW set at 50% load delivers 10 kW. If your set is rated in kVA, multiply by its power factor — usually 0.8 — to get kW, the same conversion the watts to amps calculator handles in the other direction.
Next, divide by efficiency. Electrical efficiency here means electricity at the terminals divided by fuel energy in, so it already contains the alternator and the engine's parasitic loads. At 33%, delivering 10 kW of electricity requires 30.3 kW of fuel energy flowing into the engine, and the other 20.3 kW leaves as exhaust heat, jacket water and radiation.
Then divide by the energy in a unit of fuel. This calculator uses lower heating value (LHV), which excludes the latent heat of the water vapour formed when hydrogen burns, because a reciprocating engine exhausts that vapour rather than condensing it. Using LHV is the engine industry's convention and is why a manufacturer's quoted efficiency is comparable with the figure you type in. Utility gas billing, by contrast, uses higher heating value — a therm is defined as 100,000 Btu HHV — so this calculator applies the methane LHV/HHV ratio of about 0.90 and treats a billed therm as 90,100 Btu, or 26.41 kWh, of usable energy.
Finally, runtime is the usable fuel divided by the burn rate, and cost per kilowatt-hour is the fuel price divided by the kilowatt-hours a unit of fuel yields. Piped natural gas has no runtime figure, because there is no tank; what limits a gas set is the capacity of the meter, the regulator and the piping, plus the service interval.
Worked example: a 20 kW diesel standby set at half load
A 20 kW diesel standby generator carries 50% load through a power cut. It has a 50-gallon base tank, and diesel costs $4.00 a gallon delivered. Assume 33% electrical efficiency, which is reasonable for a small diesel genset at half load.
- Electrical load. 20 kW × 50% = 10 kW.
- Energy in a gallon of diesel. 128,488 Btu (LHV) ÷ 3,412.14 Btu/kWh = 37.656 kWh per gallon.
- Electricity obtained per gallon. 0.33 × 37.656 = 12.426 kWh per gallon.
- Fuel burn. 10 kW ÷ 12.426 kWh/gal = 0.805 gallons per hour.
- Runtime. 50 gal ÷ 0.805 gal/h = 62.1 hours, a little over two and a half days.
- Cost per hour. 0.805 × $4.00 = $3.22 per hour.
- Cost per kilowatt-hour. $4.00 ÷ 12.426 = $0.322 per kWh. The same answer falls out of $3.22 per hour ÷ 10 kW, which is a useful cross-check.
- Fuel for a full day. 0.805 × 24 = 19.3 gallons, so a 50-gallon tank covers roughly two and a half days at this load.
Now double the load to 100%. The burn rate doubles to 1.61 gal/h and the runtime halves to 31.1 hours, but the cost per generated kilowatt-hour stays at $0.322, because it depends only on efficiency, energy content and price. That invariance is the single most useful property of this calculation — and the reason a lightly loaded set is expensive per kilowatt-hour only through its lower efficiency, not through the arithmetic.
Compare the result against the fuel-consumption table on your own set's spec sheet. If the manufacturer's figure at 50% load is materially higher than 0.805 gal/h, your engine's real efficiency is below 33%, and you should lower the efficiency input until the calculator matches the spec sheet. From then on it will predict the intermediate load points correctly.
How to read the result
Judge the burn rate against your fuel logistics, not against the engine. A 62-hour runtime sounds comfortable until you consider that a regional storm outage can run longer, that fuel deliveries stop when roads do, and that fire codes limit how much fuel you may keep on site before additional permitting and secondary containment apply — check the figure that governs where you are. If the runtime is under about 24 hours at your expected load, plan the refuelling before the outage, not during it.
Judge the cost per kilowatt-hour against your utility rate. Generated power from a small set lands well above the residential grid price — the worked example above costs $0.322 per kWh in fuel alone — which is why generators earn their keep on availability rather than on economics. The one case where the comparison flips is demand-charge shaving on a commercial tariff, where the value is in avoided peak demand charges rather than avoided energy charges, and the set may run only a handful of hours a month.
Watch the load band. Manufacturers publish ratings under ISO 8528-1 as standby, prime and continuous, each with its own permissible load profile and annual hours; a standby-rated set is not licensed for continuous duty at full output. At the other end, running a diesel below roughly 30% load lets unburnt fuel and soot collect in the exhaust — wet stacking — which fouls turbochargers and injectors. NFPA 110 requires emergency standby diesel systems to be exercised under load partly for this reason. If your real load is a small fraction of the set's rating, the fix is a load bank or a smaller set, not a longer idle.
Finally, treat the fuel cost as a floor. Oil and filter changes, coolant, valve adjustments, fuel polishing for stored diesel, and the amortised cost of the set itself all sit on top of the number this calculator gives you.
Fuel energy content and the cost of a generated kilowatt-hour
| Fuel | LHV per unit | kWh per unit | kWh generated per unit at 30% | Fuel used per kWh | Assumed price | Cost per kWh |
|---|---|---|---|---|---|---|
| Diesel, No. 2 | 128,488 Btu/gal | 37.66 | 11.30 | 0.0885 gal | $4.00/gal | $0.354 |
| Gasoline, E10 | 112,114 Btu/gal | 32.86 | 9.86 | 0.1015 gal | $3.50/gal | $0.355 |
| Propane (LPG) | 84,250 Btu/gal | 24.69 | 7.41 | 0.1350 gal | $3.00/gal | $0.405 |
| Natural gas, piped | 90,100 Btu/therm (LHV of a billed therm) | 26.41 | 7.92 | 0.1262 therm | $1.20/therm | $0.152 |
Liquid-fuel heating values are from the U.S. Department of Energy Alternative Fuels Data Center fuel properties comparison. The therm figure applies the methane LHV/HHV ratio of 0.901 to the 100,000 Btu HHV definition of a therm.
Ratings, exercising and the standards that govern them
ISO 8528-1 defines the rating classes a generating set is sold under — continuous operating power, prime power, limited-time running power and emergency standby power — each with its own permitted load factor and annual running hours. A set marked ESP may not be run continuously at its ESP figure, and reading a fuel table at 100% load for a standby-rated machine describes a duty the machine is not sold for. In the United States, NFPA 110 covers emergency and standby power systems and sets out the routine load testing that keeps a diesel set reliable, while NFPA 37 governs the installation of stationary combustion engines. On-site fuel storage brings the fire code and, above threshold quantities, secondary containment requirements into play. None of this is a substitute for the manufacturer's manual or a licensed installer.
Where fuel estimates go wrong
- Assuming efficiency is constant. It is not. A set at 25% load converts a smaller share of its fuel to electricity than the same set at 75%. Enter a lower efficiency for light loads or the burn rate will be optimistic.
- Mixing heating-value conventions. HHV figures run several percent above LHV for liquid fuels, and about 11% above for natural gas — the ratio this calculator applies when it converts a billed therm to usable energy. Quoting an LHV efficiency against an HHV energy content understates fuel use.
- Using the whole tank. Pickup tubes sit above the tank bottom, propane cylinders are filled to 80% of water capacity, and diesel below the suction point is not usable fuel. Enter what you can actually draw.
- Forgetting the no-load burn. This model scales fuel with output, so it reads zero at zero load. A real engine burns fuel simply to turn over, which matters when a set idles for long periods between load steps.
- Ignoring altitude and temperature derating. Naturally aspirated engines lose power with altitude and with intake temperature, so a set rated 20 kW at sea level may deliver appreciably less on a mountain site — which changes the load percentage you should be entering.
- Sizing gas piping from the electrical rating. A natural gas set needs a stated volumetric flow at a stated inlet pressure. Piping sized for a furnace will starve a generator and the set will fail to accept load.
- Quoting cost per kWh as the cost of running the generator. Fuel is the floor. Service intervals on a genset are measured in running hours, and those hours accumulate quickly during an outage.
Key terms
- Lower heating value (LHV)
- The heat released by burning a fuel with the product water left as vapour. It is the appropriate basis for engines, which exhaust that vapour rather than condensing it.
- Higher heating value (HHV)
- The heat released with the product water condensed back to liquid. Utility gas billing and the definition of a therm use HHV.
- Therm
- 100,000 Btu of gas on a higher-heating-value basis — the unit natural gas is billed in. One CCF (100 cubic feet) of pipeline gas is roughly 1.03 therms.
- Wet stacking
- The accumulation of unburnt fuel and soot in the exhaust of a diesel engine run too lightly loaded for its cylinder temperatures to burn fuel completely.
- Prime vs standby rating
- Under ISO 8528-1, prime power permits unlimited annual hours at a varying load, while emergency standby power permits a limited number of hours a year and no sustained overload.
Limits of this model, and what to reach for instead
This is a first-order model: fuel in proportion to electrical output, at a single efficiency you supply. It does not represent the no-load fuel burn that a Willans-line model would capture, it does not vary efficiency with load, and it does not model transient fuel use during motor starting or step loading. For a specific machine, the manufacturer's fuel-consumption table at 25%, 50%, 75% and 100% load is more accurate than any general formula, and this calculator's best use is then interpolating between those points and converting them into runtime and dollars.
Size the set before you cost it. A generator chosen from running watts alone will stall on the starting surge of a well pump or an air conditioner compressor, which is what the generator sizing calculator exists for. If the outage you are planning for is short and the load is small, batteries are usually cheaper, quieter and legal indoors — compare against the battery bank sizing calculator and the inverter sizing calculator. For extended off-grid supply, a hybrid arrangement in which a solar array carries the daytime load and the engine covers the shortfall cuts run hours sharply; start with the solar panel array sizing calculator.
One safety point that no calculator can soften: engine-driven generators emit carbon monoxide at levels that are lethal indoors within minutes. Run them outdoors, well away from doors, windows and vents, and never in a garage — open door or not.