Generator Fuel Consumption Calculator: Runtime & Tank Sizing
Generator fuel consumption depends on size and load: a 100 kW diesel generator burns about 7.4 gallons per hour at full load. Get your burn rate and runtime, then the answer spec sheets skip: is the tank big enough for your requirement?
Every spec sheet answers gallons per hour. The question that matters during an outage is whether the tank on the pad covers your required runtime. A 250 kW unit at 75% load burns about 328 gallons a day. A 72 hour requirement therefore means roughly 1,000 gallons of usable fuel, and a 500 gallon base tank quietly fails the plan.
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Diesel generator fuel consumption chart (gal/hr)
Burn rates below are the median of three published diesel consumption tables at standby rating (Generac, Kohler, and Generator Source), not straight-line scaling. Notice the quarter-load column: a lightly loaded engine burns proportionally more per kilowatt delivered, which is one reason oversizing a generator wastes fuel every hour it runs.
| Generator size | 25% load | 50% load | 75% load | 100% load | Gal per day at 75% |
|---|---|---|---|---|---|
| 20 kW | 0.6 | 1.1 | 1.3 | 1.7 | 31 |
| 50 kW | 1.4 | 2.4 | 3.5 | 4.4 | 84 |
| 100 kW | 2.5 | 4.2 | 5.9 | 7.4 | 142 |
| 250 kW | 5.5 | 9.5 | 13.6 | 18.0 | 326 |
| 500 kW | 11.0 | 18.5 | 26.4 | 35.5 | 634 |
| 1000 kW | 21.6 | 38.2 | 55.1 | 71.1 | 1,322 |
Runtime requirements: what the rules actually demand
The runtime target is rarely yours to pick. NEC Article 700 requires legally mandated emergency systems to carry at least 2 hours of fuel on site, the floor for egress lighting and life safety loads.
The code minimum and the survivable minimum are different numbers.
A facility that meets NEC's 2 hours still goes dark on hour three of a regional outage. Most operators without a regulator pick 48 to 72 hours based on how fast their fuel vendor can realistically deliver during a storm, when every other generator in the county is calling the same dispatcher. One counterpoint: sites with a solid priority-delivery contract can rationally size smaller and buy runtime by contract instead of by steel.
The stored fuel problem nobody budgets for
A tank sized for 96 hours holds fuel that mostly sits for years. The generator that only runs in emergencies is exactly the machine that cannot tolerate clogged filters mid-outage. Check your storage against the fuel shelf life calculator, stabilize on delivery, and sample the tank bottom annually. Bigger tanks make this worse, not better: the same fuel turns over more slowly. Sites with fleet operations can dodge the whole problem by cycling generator fuel through the fleet fuel system so nothing sits longer than a season.
How the math works, and where it bends
Burn rate comes from the median of published consumption tables for 20, 50, 100, 250, 500, and 1,000 kW standby sets at 25, 50, 75, and 100 percent load, interpolated between sizes. Runtime is tank gallons divided by burn rate. Sizing mode multiplies burn rate by required hours and adds a 10% reserve before rounding up to a standard tank size, and the enough-or-undersized verdict applies the same reserve, because pickup tubes and low-fuel cutoffs make the bottom of every tank unusable.
Published tables differ by manufacturer: at 20 kW and full load they run from 1.6 to 2.1 gallons per hour. Older mechanical-injection units burn more, and altitude derating changes the picture. For a specific unit, the manufacturer's consumption table beats this tool; use ours when you are comparing scenarios, sizing tanks, or the spec sheet is long gone. Natural gas and propane units follow entirely different math and are out of scope here.
Frequently Asked Questions
How much fuel does a 20kW generator use?
Roughly 1.7 gallons per hour at full load, about 1.3 at 75%, and near 1.1 at half load, the median of published Generac, Kohler, and Generator Source consumption tables. Over a 24 hour outage at typical standby loads, plan on about 26 to 31 gallons. A 20 kW unit usually serves a large home or small commercial building, and the sobering math is the week-long outage: about 220 gallons at 75% load, which is why a 250 or 500 gallon tank shows up next to rural units. Propane and natural gas units burn differently and are outside this tool.
How long will a generator run on 500 gallons of diesel?
About 85 hours for a 100 kW unit at 75% load (5.9 gal/hr), or close to three weeks for a 20 kW at half load. Size and load dominate everything, which is why the same 500 gallon tank is generous behind a small building and marginal behind a data center. Two practical deductions apply before trusting any runtime figure. Usable fuel is less than rated capacity, since pickup tubes stop an inch or two off the bottom. And fuel that sat untreated for a year may not be dependable fuel at all.
What size fuel tank do I need for a backup generator?
Work backward from required runtime, never forward from what fits the budget. Multiply your generator's burn rate at realistic load by the hours your operation must survive, add about 10% for unusable volume, then round up to a standard tank size. A hospital planning to a 96 hour horizon with a 250 kW unit needs roughly 1,440 gallons including the reserve, so a 2,000 gallon tank is the honest answer. For a homeowner with a 20 kW unit and a 3 day target, a 275 works. Local fire codes limit aboveground fuel storage at some occupancies, so large tanks sometimes become underground tanks with the permitting that follows.
How does load change generator fuel consumption?
Burn rate climbs with load, but not in step with it. The calculator's burn rates, the median of three published consumption tables, put a 100 kW diesel set at about 2.5 gallons per hour at 25% load, 4.2 at 50%, 5.9 at 75% and 7.4 at full load, so a quarter of the load still takes about a third of the full-load fuel. Run the numbers at the load your building actually draws, not the nameplate kW, or the runtime estimate will be off in one direction or the other.
Estimates use a generic diesel genset consumption model, not your unit's test data. Verify against the manufacturer's fuel consumption table for procurement or compliance decisions, and confirm tank sizing against local fire code storage limits before ordering steel. Sizing decisions for emergency or life-safety power systems, including healthcare and code-required standby power, must be made by a licensed professional engineer.
Burn rates are the median of the Generac and Kohler (Rehlko) industrial diesel spec sheets and the Generator Source diesel consumption chart, standby rating. Last verified September 2026.
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