How this generator fuel calculator works
Generator datasheets usually list fuel use at 25%, 50%, 75% and 100% load, and the points sit almost exactly on a straight line that does not pass through zero. The line has two parts: a fixed idle burn that scales with engine size, and a marginal burn that scales with the electricity you actually draw. The calculator uses this linear fuel curve (the same model as HOMER and most hybrid-system design tools) for diesel, petrol, LPG and natural-gas generators.
From that curve you get hourly consumption at your average load, runtime per tank, energy per litre (or per m³ of gas) and the true cost of each kWh. That cost adds maintenance and engine wear to the fuel, with wear meaning the purchase price spread over the engine’s life in hours. It is then set against grid electricity and solar plus battery, alongside the CO₂ per kWh.
The formula
kWh per litre = P_out ÷ F Runtime per tank = tank ÷ F Electrical efficiency = P_out ÷ (F × fuel energy content)
Cost per kWh = fuel price ÷ (kWh per litre) + (purchase ÷ engine life hours + maintenance per hour) ÷ P_out
Default coefficients: diesel a = 0.08, b = 0.25 L/kWh; petrol a = 0.12, b = 0.33; LPG a = 0.15, b = 0.45 (litres of liquid); natural gas a = 0.10, b = 0.30 m³/kWh. Energy contents: diesel 10.0 kWh/L, petrol 9.1, LPG 7.08 kWh/L, natural gas 10.55 kWh/m³.
Worked example
The defaults describe a 10 kW diesel generator at 50% load (5 kW), running 8 hours a day for 30 days, with a 50 L tank and diesel at €1.60/L.
Fuel flow is 0.08 × 10 + 0.25 × 5 = 2.05 L/h. Each litre yields 5 ÷ 2.05 = 2.44 kWh (about 24% fuel-to-electric efficiency), and the 50 L tank lasts about 24 hours. Fuel alone costs 1.60 ÷ 2.44 = €0.66 per kWh.
Engine wear adds €6,000 ÷ 15,000 h = €0.40 per hour and maintenance another €0.25. Spread over 5 kW that is €0.13/kWh, bringing the total to €0.79/kWh. In a month the generator runs 240 hours, burns 492 L of diesel and produces 1,200 kWh. Fuel and maintenance come to about €847. The same energy would cost €300 from the grid at €0.25/kWh, or €216 from solar plus battery at €0.18/kWh. CO₂ is about 1.1 kg per kWh, or 1.3 tonnes a month.
The load table shows why sizing matters. At 20% load the same machine makes only 1.54 kWh per litre (€1.04/kWh in fuel); at full load it reaches 3.03 kWh/L (€0.53/kWh).
How to read the results
- Fuel consumption is the headline: litres (or m³) per hour at your average load.
- Energy per litre tells you best how well the generator suits the load. A diesel below about 2 kWh/L is probably oversized.
- Total cost per kWh is the number to compare with the grid and solar. Fuel-only cost understates it.
- The fuel-curve chart plots litres per hour (rising) and kWh per litre (falling at low load). The sweet spot is usually 60–85% load.
- The verdict warns you when load drops below 30%, where diesels waste fuel and start wet-stacking.
Typical values
| Generator | Load | Typical fuel use | kWh per unit of fuel |
|---|---|---|---|
| Portable petrol 3 kW | 50% | 0.8–1.0 L/h | 1.5–2.0 kWh/L |
| Inverter petrol 2 kW | 25% | 0.3–0.5 L/h | 1.0–1.7 kWh/L |
| Diesel 10 kW | 50% | 1.8–2.2 L/h | 2.3–2.8 kWh/L |
| Diesel 10 kW | 100% | 3.0–3.5 L/h | 2.9–3.3 kWh/L |
| Diesel 100 kVA (80 kW) | 75% | 16–19 L/h | 3.2–3.7 kWh/L |
| Natural-gas standby 20 kW | 50% | 3.5–4.5 m³/h | 2.2–2.9 kWh/m³ |
Tips to cut generator fuel use
- Right-size the generator. Running a large set at 20% load can use 50–60% more fuel per kWh than a smaller one at 70%.
- Add a battery. Let the generator charge a battery at high load for a few hours, then shut it down. This “cycle charging” approach is standard in off-grid design. Size it with the solar battery calculator.
- Add solar. Even a modest PV array can cut generator runtime by 50–80% in sunny months. Model its output with the solar panel calculator.
- Use an inverter generator for light loads. Variable engine speed saves 20–40% below half load.
- Keep it serviced. Clean air filters, the right oil and healthy injectors keep fuel use close to the datasheet.
Limitations
The linear fuel curve is an approximation, so enter your datasheet’s coefficients for precise results. Real consumption also depends on altitude, temperature, power factor and how often loads start and stop. Engine life and maintenance costs vary widely between portable consumer units and industrial prime-power sets. Fuel prices are held constant and delivery to remote sites isn’t included. CO₂ covers combustion only, not fuel production.
Method & assumptions
Fuel flow = a × rated kW + b × output kW (litres or m³ per hour). Electrical efficiency = output ÷ (fuel flow × energy content). Cost per kWh = fuel price ÷ kWh per unit + (purchase price ÷ engine life hours + maintenance per hour) ÷ output kW. CO₂: diesel 2.68, petrol 2.31, LPG 1.51 kg/L, natural gas 1.93 kg/m³.
Frequently asked questions
How much diesel does a generator use per hour?
How long will a generator run on a full tank?
Why is a generator so inefficient at low load?
What does electricity from a generator cost per kWh?
Is a generator or solar plus battery cheaper for off-grid power?
How much CO₂ does a diesel generator emit?
Sources & references
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