Rule of thumb: a diesel generator burns roughly 0.25–0.30 litres per kW per hour at full load. So a 20 kVA set (≈16 kW) working hard uses about 4–5 litres an hour; at half load, closer to 2.5–3. That single line answers the question for most buyers — but the number that actually decides your fuel bill is load percentage, not the machine on the datasheet. Here’s how to estimate properly before you commit.
The quick table (50 Hz, 0.8 power factor)
| Set size | ≈ kW | At 50% load | At 75% load | At 100% load |
|---|---|---|---|---|
| 5 kVA | 4 kW | ~0.7 L/h | ~1.0 L/h | ~1.2 L/h |
| 10 kVA | 8 kW | ~1.3 L/h | ~1.9 L/h | ~2.4 L/h |
| 20 kVA | 16 kW | ~2.6 L/h | ~3.8 L/h | ~4.8 L/h |
| 30 kVA | 24 kW | ~3.9 L/h | ~5.6 L/h | ~7.2 L/h |
| 60 kVA | 48 kW | ~7.8 L/h | ~11 L/h | ~14 L/h |
Quoted at 50 Hz. A 60 Hz build of the same set runs at 3,600 rpm and produces more output — consumption per hour rises roughly in step, so the litres per kWh stay in the same band. Compare machines on L/kWh, not L/h.
These are planning figures for small-to-medium sets, derived from the 0.25–0.30 L/kWh band. Every engine publishes its own specific fuel consumption (often as g/kW·h) — use the manufacturer’s figure when you have it, and treat the table as a sanity check.
Reading the real spec: g/kW·h
Engine datasheets state fuel consumption in grams per kilowatt-hour. To convert to litres: divide by roughly 840 (the approximate density of diesel in g/L), then multiply by the kW you’re actually drawing.
Example: an engine rated 245 g/kW·h running at 16 kW → 245 × 16 = 3,920 g/h ÷ 840 ≈ 4.7 L/h.
That is the honest way to compare two machines. A supplier who can’t give you a g/kW·h figure is either selling something they don’t understand, or hoping you won’t ask.
Why load percentage decides your fuel bill
A diesel engine is most efficient in the 70–85% load band. Below that, efficiency falls off — and the fuel you waste is only part of the damage:
- At 25–30% load, consumption per unit of useful power rises sharply, and the engine starts to suffer wet-stacking: unburned fuel and soot fouling the exhaust, injectors, and valves.
- At 100%+ load, consumption climbs steeply too, and there’s no headroom for a motor start.
This is the practical argument against “buy two sizes up to be safe”: an oversized generator spends its life in its worst efficiency zone, burning extra fuel to produce power you’re not using — and shortening its own service life while doing it. Size against the corrected load with 10–20% reserve; the method is in how to size a diesel generator, and the class-by-class picture is in the kVA size chart.
Calculating your real running cost
Four numbers give you a defensible figure:
- Average load (kW you actually draw, not the nameplate)
- Consumption at that load (from the table or the g/kW·h spec)
- Hours per day of operation
- Local diesel price
A workshop drawing 12 kW for 8 hours a day: ~3.3 L/h × 8 = ~26 L/day. Multiply by your fuel price for the daily cost, then by operating days for the month.
Run this before you buy, not after. For sites running long hours, fuel cost over a year routinely exceeds the purchase price of the generator — which is why the efficiency band matters more than a small difference in sticker price.
Five things that quietly raise consumption
- Clogged air filter — the engine works harder for the same output; it’s the cheapest fix on this list. Intervals are in the maintenance schedule.
- Worn or fouled injectors — poor atomization burns fuel incompletely (and smokes).
- Bad fuel — water and sediment degrade combustion and damage the injection system; see can bad diesel fuel damage your engine.
- Altitude — thin air means less complete combustion; output drops and specific consumption worsens (altitude derating).
- Chronic light loading — the wet-stacking cycle above, which then makes the engine even less efficient.
What to ask a supplier
- What is the specific fuel consumption (g/kW·h) at 75% and 100% load?
- At what load percentage is the quoted figure measured?
- What tank capacity does the set have, and how many hours does that give at my load?
Those three answers turn a brochure into a running-cost calculation. HTX publishes engine specifications on every product page — browse the diesel generator range or the emergency power solutions page for standby sizing.
Want the fuel maths for your actual duty cycle? Send us your load, daily hours, and site altitude — or message us on WhatsApp — and our engineers will come back with consumption and tank-run-time figures for the right set within 24 business hours.