The calculator turns your generator rating, load factor and engine type into the fuel numbers that matter for planning and budgeting: liters per hour, liters per kWh, and running cost at your local diesel price.
The rated capacity of your diesel generator set, combined with power factor to determine real kW output.
The percentage of rated power you actually run at — the single biggest driver of fuel consumption.
Hourly, daily and monthly fuel cost at your local diesel price, refined by engine-specific efficiency.
The calculator applies the specific fuel oil consumption (SFOC) of your selected engine brand — Cummins, Perkins, Weichai or Yuchai — combined with the load factor to estimate L/h and L/kWh. Add your diesel price to see the cost per hour, per day and per month at your planned runtime.
Premium efficiency, 205–230 g/kWh range. Ideal for prime power where fuel cost dominates.
Reliable performance, 210–240 g/kWh. Widely used in standby and prime applications.
Cost-effective industrial engine, 220–250 g/kWh. Dependable for continuous duty.
Value-oriented performance, 215–245 g/kWh. Good balance of cost and efficiency.
Choose Standard Mode for a quick estimate based on rating, load and engine brand, or Advanced Mode to model a variable load profile across a typical day.
Enter your generator rating, operating load and local diesel price.
Enter the hours you run at each load level for a full daily fuel estimate.
A quick reference for common generator sizes at three load levels, based on the standard planning estimate of ~0.25 L/kWh. Use this table to check whether your calculator result falls within a realistic range.
| Generator Size | kW (0.8 PF) | 50% Load | 75% Load | 100% Load |
|---|---|---|---|---|
| 20 kVA | 16 kW | 2.0 L/h | 3.0 L/h | 4.0 L/h |
| 50 kVA | 40 kW | 5.0 L/h | 7.5 L/h | 10.0 L/h |
| 100 kVA | 80 kW | 10.0 L/h | 15.0 L/h | 20.0 L/h |
| 150 kVA | 120 kW | 15.0 L/h | 22.5 L/h | 30.0 L/h |
| 200 kVA | 160 kW | 20.0 L/h | 30.0 L/h | 40.0 L/h |
| 250 kVA | 200 kW | 25.0 L/h | 37.5 L/h | 50.0 L/h |
| 300 kVA | 240 kW | 30.0 L/h | 45.0 L/h | 60.0 L/h |
| 500 kVA | 400 kW | 50.0 L/h | 75.0 L/h | 100.0 L/h |
| 800 kVA | 640 kW | 80.0 L/h | 120.0 L/h | 160.0 L/h |
| 1,000 kVA | 800 kW | 100.0 L/h | 150.0 L/h | 200.0 L/h |
Understanding the relationship between generator rating, load and fuel consumption helps you budget accurately and compare engine options before you buy.
The most widely used planning estimate: L/h ≈ 0.25 × kW × load factor. For a 100 kVA set at 75% load: 80 kW × 0.75 × 0.25 = 15 L/h. Accurate to within 5–10% for modern engines.
L/h = (kW × SFOC) ÷ (density × 1,000) where SFOC is specific fuel oil consumption in g/kWh and diesel density is ~0.85 kg/L. A 210 g/kWh engine yields about 0.247 × kW at full load.
Modern diesel generators range from 195–260 g/kWh. Larger engines are generally more efficient. Cummins and Perkins typically sit at the efficient end; Weichai and Yuchai offer dependable performance at a lower price point.
Hourly cost = L/h × diesel price. At 15 L/h and US$1.10/L, running cost is US$16.50/hour. Scale by daily and monthly runtime to build your operating budget.
Fuel consumption is not linear. A generator at 50% load uses about 60–65% of full-load fuel, not 50%. The calculator accounts for this non-linear relationship using engine-specific data.
A 100 kVA set with 0.8 PF delivers 80 kW. At 75% load, actual output is 60 kW. Using the 0.25 L/kWh estimate: 15 L/h. At US$1.10/L, that's US$16.50/hour, US$132/day (8 hours), and roughly US$2,900/month at 22 operating days — a significant operating cost that justifies engine selection.
Beyond the basic formula, several factors shift real-world fuel consumption. Understanding these helps you select the right engine and avoid costly surprises.
Premium engines such as Cummins and Perkins typically have lower SFOC (205–230 g/kWh) than value engines (220–250 g/kWh). Emission tier also affects efficiency — newer Tier 4 engines may trade some efficiency for lower emissions.
Efficiency peaks at 70–80% load. Below 30% load, fuel per kWh rises sharply and wet stacking becomes a risk. Sizing for the actual load profile is critical to fuel economy.
Engine output decreases with altitude. Above 1,000 m, derate approximately 1% per 100 m. At 3,000 m, a sea-level rated engine delivers roughly 20% less power for the same fuel input.
High temperature reduces cooling effectiveness and usable output. Above 40 °C, additional derating may be required. Hot climates and poor ventilation increase fuel consumption per kWh.
Dirty air filters, clogged fuel filters and worn injectors all raise fuel consumption. Regular maintenance keeps the engine at its rated efficiency and prevents cost creep over time.
Water, contaminants and poor cetane rating reduce combustion efficiency and accelerate injector wear. Using clean, high-quality diesel and proper fuel storage is a low-cost way to protect fuel economy.
These recurring errors lead to inaccurate fuel budgets, oversized or undersized tanks, and unexpected operating costs. Avoiding them is straightforward.
A generator at 75% load does not burn 75% of full-load fuel — it burns about 65–70% of it. Assuming linear consumption overestimates fuel use and distorts operating budgets.
A 5% difference in SFOC is not a rounding error on a machine that runs thousands of hours a year. Two generators of the same kVA can have meaningfully different fuel bills over their lifetime.
Fuel cost scales with runtime. A generator that runs 24/7 burns three times the fuel of one running 8 hours a day — and the lifetime fuel cost can exceed the purchase price of the machine.
A generator at high altitude or in extreme heat delivers less power for the same fuel. Derating means you may need a larger set to deliver the required load, which changes the fuel consumption curve.
An oversized generator running at low load burns more fuel per kWh than a properly sized set, and risks wet stacking. Right-sizing saves fuel and extends engine life.
Final fuel consumption figures should be confirmed against the engine datasheet for your specific generator set. Our engineers provide verified figures as part of the quotation process — free of charge.
Our engineers will confirm fuel consumption against the actual engine datasheet for your selected generator set and provide a tailored quotation — free of charge, with no obligation.
A: A 100 kVA generator at 75% load uses about 15 L/h; a 500 kVA unit at the same load uses about 75 L/h. Use the formula L/h ≈ 0.25 × kW × load factor or the calculator above for an engine-specific figure.
A: At 0.8 power factor, a 100 kVA unit is 80 kW. It burns about 10 L/h at 50% load, 15 L/h at 75% load, and 20 L/h at full load — before engine-specific adjustment.
A: 70–80% of rated load is the efficiency sweet spot, at roughly 0.25–0.26 L/kWh. Below about 30% load, fuel per kWh rises sharply and the engine risks wet stacking.
A: SFOC is the amount of diesel an engine burns per kWh of output, measured in grams per kWh. Modern diesel generators range from 195–260 g/kWh, with larger engines generally more efficient.
A: Multiply consumption by your diesel price. A 100 kVA unit at 75% load (15 L/h) at US$1.10/L costs about US$16.50 per hour. Larger units scale accordingly — see the calculator for your specific case.
A: Premium engines such as Cummins and Perkins typically sit at the efficient end of the SFOC range (205–230 g/kWh). Weichai and Yuchai offer dependable performance at a lower purchase price (220–250 g/kWh). The right choice depends on your duty cycle.
A: At low load, the engine’s fixed losses — oil and water pumps, friction, cooling — become a larger share of output, so it burns more fuel per useful kWh. Below 25% load, efficiency is poor and wet stacking becomes a risk.
A: Right-size the generator for your load profile, operate in the 70–80% band, follow maintenance schedules, use quality fuel, avoid extended idling, and account for site conditions such as altitude and temperature.
