Ask what a generator costs to run and the answer you get is litres per hour, which is the one number that cannot be compared with anything. The bill is decided by litres per kilowatt-hour, and that depends on how heavily the set is loaded far more than on what it is. A 40 kVA diesel carrying 20 kW makes electricity at about PKR 85 a unit at today's fuel price; the same set carrying 8 kW makes it at PKR 121. This guide explains where those numbers come from, how to read them off a datasheet, why an oversized set is punished every hour of its life, and what can be done about it short of buying another one.
Litres per hour is the wrong number: litres per kWh
Fuel is bought by the litre and electricity is used by the kilowatt-hour, so the only figure that connects the two is the specific fuel consumption: litres of fuel per kWh delivered at the terminals. Engine makers publish it as brake specific fuel consumption in grams per kWh at the flywheel; a modern turbocharged diesel of 20–200 kVA is in the region of 210–250 g/kWh at full load, which at a diesel density of 0.83–0.85 kg/L is 0.25–0.30 L/kWh. The generator set's own datasheet, which is what you want, gives litres per hour at 100, 75 and 50 % of the rating, and dividing the 100 % figure by the rated kW gives the same 0.25–0.30.
Litres per hour by itself hides the load. "It burns 6 litres an hour" describes a 40 kVA set at 20 kW (0.30 L/kWh, a fair figure) and a 100 kVA set at 12 kW (0.5 L/kWh, a poor one) equally well. Multiply by the pump price and divide by the output and you have rupees per unit, the number that can be set against the tariff on the electricity bill or the output of a solar array. Nothing else can be.
The fuel curve: why a lightly loaded set is expensive twice
Plot a diesel set's fuel consumption in litres per hour against its output in kW and, to a good approximation, you get a straight line that does not pass through the origin. That is the Willans line, and the fact that it misses the origin is the whole story. The intercept is the no-load consumption: what the engine burns just to turn itself, its fan, its water pump and the alternator over at 1 500 rpm with nothing connected, and for most sets it is 15–25 % of the full-load consumption. Every kilowatt added costs the same increment of fuel on top of that fixed amount.
Divide the line by the output and the litres per kWh come out as
where x is the load fraction and idle the no-load share. With a 20 % no-load share the second term is what hurts: at 75 % load the set uses 1.07 times its full-load L/kWh, at 50 % load 1.2 times, at 25 % load 1.6 times, and below that the curve heads for the sky. A set at 10 % load is burning 2.8 times as much fuel per unit as the datasheet figure.
That is the first way a lightly loaded set is expensive. The second is that a diesel engine at low load runs cold. The cylinder and exhaust temperatures are too low to burn the fuel completely, unburnt fuel and oil collect in the exhaust and on the injectors, the bores glaze, and the oil is diluted: the condition the trade calls wet stacking. Engine and set makers advise against running below about 30 % of the rating for long periods and recommend loading a lightly used set to 70 % or more for a few hours periodically, with a load bank if the site cannot provide it, to burn the deposits off. A set that idles all night at 15 % load pays in fuel per unit now and in injectors, oil changes and engine life later.
Reading the datasheet
A generator set datasheet carries a small table of fuel consumption in litres per hour at 100, 75, 50 and sometimes 25 % of the rating, usually for both the prime and the standby rating. Use the prime column for a set that runs through load-shedding: under ISO 8528-1 the prime rating (PRP) is for unlimited hours at variable load with 10 % overload allowed for one hour in twelve and an average of no more than 70 % over 24 hours, while the standby rating (ESP) is for emergency use for a limited number of hours a year with no overload at all. A set bought "as 50 kVA standby" is a 45 kVA prime set, and its fuel table says so.
If only the engine's figure is given, in g/kWh, convert it: 220 g/kWh ÷ 0.84 kg/L is 0.262 L/kWh at the flywheel, and the alternator, its fan and the radiator fan take another 5–10 %, so the set delivers electricity at about 0.28 L/kWh. Fuel consumption is declared to ISO 3046-1 at reference conditions with a tolerance of +5 %, for an engine in good order on fuel of standard density: Pakistani high-speed diesel is within the normal range, but a clogged air filter after a Punjab dust season, worn injectors or a set that has never had its valve clearances checked are not.
The two numbers the calculator needs are the full-load consumption divided by the rated kW, and the no-load share. If the datasheet gives the 50 % figure, set the no-load share so that the model reproduces it; if it gives 25 %, better still. With those two fixed the straight line will match the rest of the table within a few percent.
The price per unit against the grid and against solar
In the worked example a 40 kVA set carrying 20 kW burns 6.05 L/h, which at PKR 280 a litre is PKR 1 693 an hour and PKR 84.67 a unit: 1.88 times a grid tariff of PKR 45, or PKR 39.67 extra for every kWh. Six hours a day for a month is 3 600 units, 1 089 litres and about PKR 305 000, against PKR 162 000 for the same units from the grid, and that is before oil, filters and the engine's life. The gap is the price of the grid being away.
Solar changes the arithmetic in two ways. The obvious one is that an array carries the daytime load at a cost per unit that, once the array is paid for, is a fraction of either figure, so the generator no longer runs in the day at all. The less obvious one is what a hybrid inverter and a battery do to the hours the set runs: the battery carries the evening's lights, fans, fridge and router, so the set is only started for the long outage and the summer air-conditioning load, and when it does run it can run fuller, charging the battery at the same time, for fewer hours. The solar sizing guide and the solar system sizing calculator give the array and the battery; the generator becomes the backup to the backup, which is the right place for it.
Sizing and the cost interact
The commonest reason for a set that spends its life at 25 % load is one motor. A 15 kW compressor started direct-on-line needs a set several times larger than the running load to hold its voltage for the two seconds of starting, so a site with 20 kW of average load ends up with a 100 kVA set, and that set then burns 0.43 L/kWh for every one of its running hours instead of 0.30. The fuel penalty is 8.6 against 6.05 litres an hour, about PKR 130 000 a month at six hours a day, forever.
The fix is on the motor side, and it is cheaper than the fuel. A soft starter cuts the starting current from about six times the running current to about three; a variable-frequency drive to about one and a half; a star-delta starter to about two on a motor that can be started unloaded. Any of them lets the set be sized for the running load, which is where its fuel curve is good. The generator sizing guide covers the motor-starting step and the derating for heat and altitude, and the generator sizing calculator shows the loading at the running load, which is the number to look at before signing.
Where the load genuinely varies from a few kilowatts at night to a full shift in the day, two smaller sets with a synchronising controller that starts and stops the second one on demand keep whichever engine is running in its efficient region, and give you a set to run while the other is serviced.
Maintenance and the numbers nobody writes down
Fuel is the largest running cost but not the only one, and the others are counted by the hour, not by the kilowatt-hour, which makes them another reason to run the set for fewer hours rather than at a lower load. Engine oil and the oil filter are changed at 250–500 hours depending on the engine and the oil; a 40 kVA set at six hours a day reaches 250 hours every six weeks. Fuel filters go with the oil or sooner if the fuel is dirty; the air filter is the first casualty of a dusty site; coolant, belts and hoses on their own schedule; the starting battery every two or three years; injectors and the fuel pump when the smoke and the consumption say so. A rough planning figure for scheduled service on a properly maintained set is 10–20 % of the fuel cost per hour, and it is higher for a set that is wet-stacked or run on bad fuel.
The tank is its own account. Diesel in a warm tank grows microbes at the water interface, the sludge blocks filters, and water from condensation and from drums finds its way to the injectors. Fuel delivered is not fuel burned: evaporation is small, but leakage, spillage and theft are not, and the only honest measure of a set's cost is litres metered in against kilowatt-hours metered out over a month. Fit a kWh meter on the generator side of the changeover switch and keep a fuel log; the calculator's figure is what the log should show, and a gap between them is a service item or a security item.
The example in numbers
A 40 kVA set at a shop and office in Lahore, rated 0.8 power factor, datasheet full-load consumption 0.27 L/kWh, no-load 20 % of full load; 20 kW average for six hours a day, 30 days; diesel PKR 280 a litre, grid PKR 45 a unit.
- Rated output 40 × 0.8 = 32 kW; full-load fuel 32 × 0.27 = 8.64 L/h; no-load 8.64 × 0.2 = 1.73 L/h.
- Load fraction 20 ÷ 32 = 62.5 %; fuel 1.73 + (8.64 − 1.73) × 0.625 = 6.05 L/h; 6.05 ÷ 20 = 0.302 L/kWh.
- PKR 1 693 an hour; PKR 84.67 a unit, 1.88 times the grid.
- Per day 36.3 L, PKR 10 161, 120 kWh; per month 1 089 L, PKR 304 819, 3 600 kWh, 2 918 kg of CO2 at 2.68 kg per litre.
- At a quarter load the same set would burn 3.46 L/h, 0.432 L/kWh, PKR 121 a unit.
The generator fuel consumption calculator reproduces these figures and shows the 25/50/75/100 % table for any set; change the rating to 100 kVA and watch the price per unit at the same 20 kW.
Cutting the bill without buying a new set
- Run it fuller for fewer hours. A battery on a hybrid inverter lets the set charge the bank at 70–80 % load and then stop, while the battery carries the small evening load at a fraction of the fuel per unit. The saving is the difference between the L/kWh at 70 % and at 20 %, plus the hours the engine does not run at all.
- Schedule the load. Move what can move (pumping, charging, batch processes) into the hours the set is running anyway, so the same litres per hour deliver more units; and keep what can wait (geysers, non-urgent motors) off it altogether.
- Reduce the load itself. Inverter air conditioners in place of fixed-speed units, LED lighting and efficient fans cut the kilowatts. The set's L/kWh gets slightly worse as the load falls, but the litres, which are what you pay for, fall with the load.
- Stop it when the grid returns. An automatic transfer switch with a cool-down timer stops the set within minutes of the mains coming back; a set left running until someone notices burns its no-load fuel for nothing.
- Service it on hours, not on faith. Clean air and fuel filters, correct injectors and fresh oil are worth a few percent on the fuel figure; a load-bank run once a quarter clears the wet stacking from a set that is lightly loaded by necessity.
- Meter it. A kWh meter and a fuel log turn an argument into a number, and the number is what tells you whether the next rupee is better spent on solar, on a battery, or on a soft starter for the compressor. Ahmedonics designs and integrates the control and monitoring for exactly this: the changeover, the load management, the hybrid inverter and the metering, as one system rather than a set of boxes.
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; overload and average-load provisions
- 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 and engine performance curves (fuel consumption at 25, 50, 75 and 100 % of prime rating) — the source of the full-load and part-load figures for any particular set
- UK Department for Energy Security and Net Zero / DEFRA, Greenhouse gas reporting: conversion factors — 2.68 kg CO₂ per litre of diesel, from the IPCC guidelines for national greenhouse gas inventories