Generator Fuel Consumption & Runtime — Real Numbers by Size and Load
Runtime & Fuel · Updated 2026-06-29
Generator Fuel Consumption & Runtime — Real Numbers by Size and Load
Short answer: A diesel generator burns roughly 0.5–0.6 gallons (about 2 liters) per 10 kW of capacity at 75% load — for example, a 60 kW set averages 3.6 gph (13.7 lph) at three-quarter load in our data. Runtime is simple division: runtime (hours) = usable tank gallons ÷ gph at your load. A 105‑gallon base tank on a 20 kW diesel running flat‑out (1.9 gph) lasts about 55 hours; back off to half load and it nearly doubles.
Every figure below is a median computed from our 71,681‑row fuel-consumption dataset (10,457 distinct generator models, gph and lph at 25/50/75/100% load) extracted from manufacturer spec sheets — not a rule of thumb. To pull the exact curve for one unit, open its model page and read the fuel panel: see exact figures for your model in the directory.
How much fuel does a diesel generator use by size and load?
One-line answer: Burn scales almost linearly with both size and load — a set at 100% draws roughly 2.5–3× what it draws at 25%.
The table below is the median diesel fuel burn for each size class at each load, in US gallons per hour (gph) and liters per hour (lph):
| Size (standby kW) | 25% load | 50% load | 75% load | 100% load |
|---|---|---|---|---|
| 20 kW | 0.6 gph · 2.3 lph | 1.0 gph · 3.8 lph | 1.5 gph · 5.4 lph | 1.9 gph · 7.2 lph |
| 60 kW | 1.5 gph · 5.7 lph | 2.6 gph · 9.7 lph | 3.6 gph · 13.7 lph | 4.7 gph · 17.8 lph |
| 125 kW | 3.3 gph · 11.4 lph | 5.1 gph · 19.3 lph | 7.1 gph · 26.9 lph | 9.1 gph · 34.4 lph |
| 250 kW | 5.4 gph · 20.5 lph | 9.7 gph · 36.4 lph | 14.0 gph · 52.9 lph | 17.7 gph · 66.8 lph |
| 500 kW | 10.5 gph · 39.7 lph | 17.2 gph · 65.3 lph | 24.0 gph · 91.3 lph | 33.0 gph · 125.0 lph |
Reading it: a 125 kW diesel at 75% load burns about 7.1 gph (26.9 lph), and a 250 kW unit at the same load burns roughly double — 14.0 gph. The 50‑kVA question (≈40 kW) people ask about lands between the 20 kW and 60 kW rows: expect ~2.4–3 gph at 75% load.
Real named models confirm the medians:
| Model | Standby kW | Load | Fuel burn |
|---|---|---|---|
| Kohler 20EKOZD | 20 kW | 75% | 1.39 gph · 5.2 lph |
| Kohler 60REOZK | 60 kW | 75% | 4.3 gph · 16.3 lph |
| Caterpillar C9 ATAAC | 200 kW | 75% | 12.3 gph · 46.6 lph |
| MTU 6R0150 DS250 | 250 kW | 75% | 17.0 gph · 64.2 lph |
| Kohler 400REOZJ | 400 kW | 75% | 22.1 gph · 83.8 lph |
How long will a generator run on 5 gallons?
One-line answer: A small portable (3–10 kW) runs 3.5 to 14 hours on 5 gallons, depending on size and how hard you load it — runtime roughly doubles between full load and half load.
Five gallons is portable-gasoline territory, so these are real gasoline models from our data with runtime computed as 5 ÷ gph:
| Portable model | kW | At 50% load | At 100% load |
|---|---|---|---|
| Generac QuietPact 40G | 3.6 kW | 0.35 gph → 14.3 h | 0.55 gph → 9.1 h |
| Kohler 6.5RMY | 6.5 kW | 0.72 gph → 6.9 h | 1.27 gph → 3.9 h |
| Kohler 7ER | 7.0 kW | 0.63 gph → 7.9 h | 0.94 gph → 5.3 h |
| Kohler 10ERG | 10.0 kW | 0.87 gph → 5.7 h | 1.4 gph → 3.6 h |
The lesson: don't run a generator at 100% to stretch fuel. Half load gives you the most kWh per gallon and the longest run between refuels.
What is the diesel fuel-consumption formula?
One-line answer: A modern diesel genset consumes about 0.07 US gallons (0.27 L) of fuel per kWh it actually produces.
That single constant is what falls out of our dataset. Take any row — a 60 kW set at 100% load burns 4.68 gph while delivering 60 kWh, so 4.68 ÷ 60 = 0.078 gal/kWh; a 250 kW set at full load gives 17.7 ÷ 250 = 0.071 gal/kWh. It holds across the catalog. So:
gph ≈ 0.07 × (load fraction) × (rated kW)
For a 50 kVA (≈40 kW) genset at 75% load: 0.07 × 0.75 × 40 ≈ 2.1 gph (~8 lph) — matching the spec sheets. Stated per kVA per hour, plan on roughly 0.05–0.06 gph (0.2 L) per kVA at three-quarter load for a diesel. Gaseous and gasoline units burn more volume per kWh (see below), so this formula is diesel-specific.
How do I turn that into real runtime?
One-line answer: Runtime = usable tank gallons ÷ gph at your load. Use the unit's real base-tank capacity, not a guess.
Of the 7,750 models in our corpus that publish a tank size, here are real diesel sets with the runtime that math produces at 100% load (the worst case — light it lighter and you get more):
| Model | Standby kW | Tank | Full-load burn | Runtime @ 100% |
|---|---|---|---|---|
| Generac MLG 8 | 7.3 kW | 30 gal | 0.63 gph | ~48 h |
| Kohler 20REOZK-C | 20 kW | 105 gal | 1.9 gph | ~55 h |
| Kohler 300REOZD | 300 kW | 550 gal | 13.7 gph | ~40 h |
| Generac MD 500 | 500 kW | 703 gal | 31.3 gph | ~22 h |
| Kohler 500REOZT | 504 kW | 468 gal | 31.9 gph | ~15 h |
Two things jump out: a bigger generator does not mean longer runtime — the Kohler 500REOZT empties its 468‑gallon tank in ~15 hours at full load, while the 20 kW Kohler on a smaller-but-proportionally-larger 105‑gallon tank runs ~55 hours. And these are 100% numbers; at typical 50–75% standby loading, expect 1.3–1.8× longer between fills.
Does propane or natural gas use more fuel than diesel?
One-line answer: Yes by volume — propane/LPG carries less energy per gallon, so an LP set burns roughly 1.5–1.7× the gallons a comparable diesel would for the same load.
Our anchor proves it. The Kohler 70REZGT (62 kW standby, LP fuel) burns 6.2 gph (23.5 lph) at 75% load in its spec sheet. A comparable ~60 kW diesel at 75% load averages just 3.6 gph in the same dataset — about 1.7× less volume. That tracks energy density: diesel holds ~138,000 BTU/gal versus ~91,500 for propane. Gasoline portables sit in between but with smaller tanks. (Natural-gas units are metered in ft³/hr or therms rather than gallons, so they don't get a gph here — check the model's fuel panel for the gaseous figure.)
So if fuel logistics and runtime matter, diesel wins on gallons-per-hour; propane wins on storage life and clean starts. Pick the model with the fuel curve that fits your runtime target.
Key takeaways
- Diesel rule of thumb: ~0.07 gal/kWh produced, or ~0.5–0.6 gph per 10 kW at 75% load.
- Runtime = tank gallons ÷ gph. Always use the published tank size and your real load.
- Half load is the sweet spot — most kWh per gallon and longest run between refuels.
- Bigger ≠ longer runtime; tank-to-burn ratio is what counts.
- LPG/propane burns ~1.5–1.7× the gallons of diesel for the same kW (lower energy density).
- Every model page lists the exact gph/lph curve at 25/50/75/100% — start there for your unit.