Generator Fuel Consumption and Runtime Calculator

How to get the right burn-rate input

Use the exact model's current manufacturer fuel-consumption table at the expected load, or a measured stabilized rate. Keep rate, tank amount and price in the same fuel unit. Do not convert diesel gallons to natural-gas volume here.

Diesel generator fuel consumption reference table

Fuel burn does not scale linearly from zero. A generator at no load still burns fuel to hold voltage and frequency, and at part load it is less efficient per kWh than it is near three-quarter load. A flat multiplying factor taken from the rated point therefore understates consumption badly at low load and can overstate it at full load.

How to use the tables: find the row nearest your rated output, pick the column nearest your expected load, and enter that figure as a known burn rate. It is a planning estimate for tank sizing and outage budgeting. It is not a substitute for the current fuel-consumption curve for the exact model you own, because emissions tier, alternator efficiency, cooling package and governor settings all move the number.

Reading a load curve and choosing your burn rate

Manufacturer fuel curves are published at discrete load points, commonly 25 %, 50 %, 75 % and 100 % of rated output. Two consequences follow directly from that.

• Between points, interpolate — never extrapolate. The calculator accepts a lower and an upper curve point and interpolates linearly between them. Outside the supplied pair it declines to answer rather than invent a value, because a straight line fitted to the published points diverges quickly away from them.
• Your load is probably not the load you named the generator for. A 200 kW set that normally carries 80 kW is running near 40 % of rating, where specific consumption is markedly worse than the data sheet headline. Enter the real expected load, not the generator rating.

Diesel has two further effects worth carrying into the fuel plan:

• The tank is not the fuel. Usable volume is reduced by the suction clearance, the fill neck, baffles and the heel you cannot draw. The default 85 % usable fraction is a reasonable allowance for a standard day tank; a deep tank with a low suction point may do better, a shallow one worse.
• Long running changes the fuel, not just the level. Stored diesel ages and can develop microbial growth; a figure derived from day-one consumption is optimistic for a long outage. Where the runtime requirement is long, treat the calculation as the fuel quantity and treat fuel condition as a separate maintenance item.

Common mistakes in generator fuel planning

• Sizing the tank from the nameplate flow. Quoting a single fuel figure for the generator rating rather than the connected load overstates consumption at light load and can lead to over-buying — or to under-buying when the set actually runs near rating.
• Counting the whole tank as usable. Filling to the brim and assuming all of it burns produces a runtime the site will never achieve. Apply the usable fraction and keep the low-level cut-out in mind.
• Mixing fuel units mid-calculation. Rate, tank quantity and price must share one unit. Diesel litres, LPG litres and natural-gas cubic metres are different fuels with different energy content and must not be converted into one another here.
• Forgetting the no-load floor. An idling set still consumes fuel. Where an outage includes long low-load periods, model them at the low curve point instead of assuming the average load applies throughout.
• Treating the runtime result as an autonomy guarantee. It is the fuel arithmetic only. It does not include fuel polishing, transfer time, cooldown, or the fact that a set running at very low load for hours may wet-stack.
• Using this page to size the machine. Fuel consumption is an output of the sizing decision, not an input. Size the alternator and engine first, then come back here for the tank.