What the calculator does
A generator's fuel bill depends far more on how heavily it is loaded than on how big it is, and the datasheet only tells you what it burns at 50, 75 and 100 % of its rating. The calculator fits the standard straight-line fuel model to two numbers you can read off that sheet, the full-load consumption and the no-load consumption, and evaluates it at the load your set actually carries. It reports the litres per hour, the litres per kilowatt-hour (the number that decides the cost), the cost per hour and per unit, what that comes to per day and per month at your running hours, the CO₂, and how the cost per unit compares with the grid tariff. A small table shows the same set at 25, 50, 75 and 100 % load so you can see how quickly a lightly loaded set becomes expensive.
It is the arithmetic behind a decision every business in a load-shedding area faces: whether to keep running the set for six hours a day, to move the daytime load onto solar, or to run the set fuller for fewer hours with a battery.
Formula
Full-load fuel Ffull = Prated × sfc (L/h) No-load fuel Fidle = Ffull × idle share
Load fraction x = Pload ÷ Prated
Fuel at the load F = Fidle + (Ffull − Fidle) × x (L/h)
Specific consumption at the load = F ÷ Pload (L/kWh)
Cost per hour = F × price Cost per kWh = cost per hour ÷ Pload
Per month: litres = F × hours × days kWh = Pload × hours × days CO2 = litres × 2.68 kg
where sfc is the specific fuel consumption at full load in litres per kWh from the datasheet, and the idle share is the no-load consumption as a fraction of the full-load figure. The straight line from no-load to full-load consumption is the Willans line, and it reproduces published 25/50/75/100 % tables for diesel sets to within a few percent. The CO2 factor of 2.68 kg per litre of diesel is the combustion factor published in the DEFRA greenhouse-gas conversion factors, derived from the IPCC guidelines; it is fixed in the calculator.
Worked example
A 40 kVA diesel set at a shop and office in Lahore, rated at 0.8 power factor, with a datasheet full-load consumption of 0.27 L/kWh and a no-load consumption of 20 % of that. It carries 20 kW on average for six hours of load-shedding a day, 30 days a month; diesel is PKR 280 a litre and the grid tariff is PKR 45 per unit.
- Rated output = 40 × 0.8 = 32 kW. Full-load fuel = 32 × 0.27 = 8.64 L/h; no-load fuel = 8.64 × 0.20 = 1.73 L/h.
- Load fraction = 20 ÷ 32 = 0.625 (62.5 %).
- Fuel at 20 kW = 1.73 + (8.64 − 1.73) × 0.625 = 1.73 + 4.32 = 6.05 L/h, which is 6.05 ÷ 20 = 0.302 L/kWh: 12 % more per unit than at full load, because the no-load share is spread over fewer units.
- Cost per hour = 6.05 × 280 = PKR 1 693; per unit = 1 693 ÷ 20 = PKR 84.67, which is 1.88 times the grid tariff, or PKR 39.67 more for every kWh.
- Per day: 36.3 L, PKR 10 161, 120 kWh. Per month (180 h): 1 089 L, PKR 304 819, 3 600 kWh, and 2 918 kg of CO2.
The same set at a quarter load (8 kW) would burn 3.46 L/h, or 0.432 L/kWh, and every unit would cost PKR 121: the fuel per unit rises by 60 % when the load falls by 60 %. That is the whole argument for loading a generator properly or running it for fewer hours.
The model against the datasheet pattern
| Load | Output (example) | Fuel, L/h (model) | L/kWh (model) | Typical published pattern |
|---|---|---|---|---|
| 25 % | 8 kW | 3.46 | 0.432 | ≈ 1.5–1.6 × the full-load L/kWh |
| 50 % | 16 kW | 5.18 | 0.324 | ≈ 1.15–1.25 × |
| 75 % | 24 kW | 6.91 | 0.288 | ≈ 1.05–1.1 × |
| 100 % | 32 kW | 8.64 | 0.270 | 1.0 × (the datasheet figure) |
Assumptions and limitations
- The model is a straight line. Real fuel curves bow slightly, and the no-load consumption differs between engines: a small naturally aspirated engine idles at a larger share of full load than a large turbocharged one. Take the full-load figure and, if the datasheet gives it, the 50 % figure, and set the no-load share so the model reproduces the 50 % point. Without a datasheet the defaults are a fair estimate for a 20–200 kVA set, not a measurement.
- The sfc input must come from the datasheet. Manufacturers declare consumption to ISO 3046-1 under reference conditions with a tolerance of +5 %; the figure applies to clean injectors, a clean air filter and fuel of standard density (0.83–0.85 kg/L). A worn or badly serviced engine burns more.
- Derating changes the rated kW. At altitude and in heat the engine cannot deliver its nameplate output, so the load fraction is higher than the calculator shows and the set may be nearer its limit than it looks. The generator sizing calculator applies the altitude and temperature derating.
- Diesel only. Natural-gas and LPG sets are rated in m³/h or kg/h of fuel at a stated calorific value, and their part-load curves differ; petrol sets of 1–5 kVA burn roughly 0.4–0.5 L/kWh at a good load and far more at a poor one. Convert the fuel and use the calculator for the shape of the answer, not the exact number.
- Fuel is not the whole running cost. Engine oil and filters at 250–500 hour intervals, coolant, belts, the starting battery every two or three years, injector service and, eventually, an overhaul, are all outside the number. A rough planning figure is that scheduled service costs 10–20 % of the fuel cost per hour for a set that is properly maintained. Engine life is consumed by the hour whether the load is 20 % or 80 %.
- No capital cost, and no losses from the tank. The purchase, the installation and the finance are outside the number, and so are fuel theft, evaporation and the sediment and water that accumulate in a tank filled from drums. Metered fuel-in against kWh-out over a month is the only honest check.
- The average load must be an average. A set that carries 30 kW for an hour and 10 kW for five is at 13.3 kW on average, and the fuel is a little more than the model gives for that average because the L/kWh curve is convex. The error is small unless the load swings between idle and full.
Frequently asked questions
Why does a bigger set cost more per kWh at the same load?
Because of the no-load term. A 40 kVA set carrying 20 kW burns 6.05 L/h in the example; a 100 kVA set with the same 0.27 L/kWh full-load figure and 20 % no-load share would idle at 4.3 L/h and burn about 8.6 L/h at 20 kW, 0.43 L/kWh, forty percent more per unit for the same electricity. The set that was bought "to be safe" for one motor start pays that penalty every hour it runs.
Is it cheaper to run two small sets than one large one?
Often, yes, if the load varies. Two 30 kVA sets with a load-sharing controller that stops one when the load falls below about 40 % of the other's rating keep the running engine in its efficient region, and you have redundancy for service. One 60 kVA set at a 12 kW night load is at 25 % and wet-stacking. The price is a synchronising panel, more service items and more floor space.
Why litres per kWh rather than litres per hour?
Litres per hour is what you buy; litres per kWh is what you get for it, and it is the only figure that can be compared with a grid tariff or a solar system. A set that burns 6 L/h sounds cheap until you find it is delivering 12 kW, at 0.5 L/kWh. Always divide by the output.
What about a small petrol generator?
Small 1–5 kVA petrol sets, the kind sold for a shop or a single house, typically burn 0.4–0.5 L/kWh at a reasonable load and much more at the low loads they usually see; they are also less efficient at part load than a diesel because a petrol engine is throttled. At PKR 280 a litre that is PKR 110–140 per unit before oil and wear, which is why a small inverter and a battery, charged from the grid when it is there, is cheaper for lights, fans and a router.
How does this compare with solar and a battery?
In the example the diesel unit costs PKR 85 against PKR 45 from the grid. A grid-tied or hybrid solar system in Pakistan produces electricity for a fraction of either once the array is paid for, and a battery charged from solar in the day carries the evening at a per-unit cost that depends mostly on the battery's cycle life. The solar system sizing calculator gives the array and battery for a daily load, and the solar sizing guide explains the losses. The usual outcome for a commercial site is solar for the daytime load, a battery for a few evening hours, and the generator kept for long outages and the summer peak, running fuller and for far fewer hours.
References
- ISO 8528-1:2018, Reciprocating internal combustion engine driven alternating current generating sets — Part 1: Application, ratings and performance — continuous, prime, limited-time and emergency standby ratings and the overload allowance
- ISO 3046-1:2002, Reciprocating internal combustion engines — Performance — Part 1: Declarations of power, fuel and lubricating oil consumptions, and test methods — reference conditions and the tolerance on declared fuel consumption
- Manufacturers' generator set data sheets (fuel consumption at 25, 50, 75 and 100 % of prime rating) — the source for the full-load and part-load figures; published for each model by the major engine and set makers
- UK Department for Energy Security and Net Zero / DEFRA, Greenhouse gas reporting: conversion factors — 2.68 kg CO₂ per litre of diesel, derived from the IPCC guidelines for national greenhouse gas inventories