Generator runtime:
tank size, fuel burn and how load changes both

The basic division and why it is not enough

A tank holds a certain number of litres, and the engine burns a certain number of litres every hour. Divide one by the other and you have the runtime. If a generator burns 2 litres an hour and the tank holds 20, it runs for 10 hours.

The catch is the hourly figure. Datasheets often quote consumption at full load, or at 75%, and your home or shop almost never draws that. A set that burns 2.5 L/h at full load does not burn half that at half load: the engine still has to turn at constant speed to hold the frequency, so a large part of the fuel goes just to keep it running.

That is why the same generator can last much longer than the full-load number suggests when lightly loaded, and why the runtime on the sticker is a lower bound, not an average.

Fuel in the tank is also not always the whole story. Some portable sets have a small tank sized for four to eight hours, and the practical runtime is then the time between refills, which you must do with the engine stopped and cooled. A larger external tank or a day tank connected by a hose is the usual fix for longer outages.

Runtime formula and load share

The last column is the telling one. At 25% load the set burns about 0.9 litres to make each kilowatt-hour, and at full load about 0.63. Light loads are expensive per unit of energy, even though the hourly burn is small.

Worked example: a 25 litre tank at 2.4 kW

At an assumed diesel price of ₹90 a litre, 1.6 L/h costs about ₹144 per hour of running. Use the current local price, and remember that the figure excludes servicing and oil.

What a unit of electricity costs from your own set

The runtime figure also gives you the cost of the power you generate. Divide the fuel burned per hour by the energy delivered per hour and multiply by the fuel price.

That is why generators are for backup rather than routine use, and why it pays to shed non-essential load during an outage. Turning off a 1 kW heater cuts the fuel burn noticeably, and the saving is larger than the heater's share of the load suggests because of the low-load penalty at the bottom of the curve.

Why running too light is its own problem

A diesel engine running at a very low share of its rating, below roughly 30%, tends to operate cooler than it should. Unburnt fuel and soot build up in the exhaust and on the injectors, a condition often called wet stacking. It shortens the engine's life and wastes fuel.

Manufacturers generally recommend a sustained load of at least 30% to 40%, and some suggest an occasional period at higher load to burn off deposits. If your normal load is far below the rating, the practical answer is to choose a smaller set or to run a load bank for a short while from time to time.

• Do not size a generator by the largest appliance alone. Add up the loads that run together and allow for starting surges.
• Motors can need several times their running current for a moment at start, so the set must handle the peak, not only the average.
• A set loaded above about 80% for long hours will burn more and run hotter. Leave some headroom.

Diesel, petrol and inverter sets

Small petrol generators are cheap and light, but burn more fuel per kWh and are noisier. Diesel sets cost more and last longer, and are the usual choice for shops, clinics and housing societies. Inverter generators vary engine speed to match the load, so they burn much less at light loads than the fixed-speed sets in the curve above, and produce a cleaner output suited to electronics.

If your usual load is small, a modest inverter generator may be a better fit than a big diesel set that spends its life at 20% load. If you run heavy motors, the opposite is true. The runtime formula is identical in each case; only the fuel curve changes, so you must obtain the curve for the machine you actually have.

Planning for a power cut of a given length

Work backwards when you know how long the outage will last. If a 10-hour night-time cut is common and the set burns 1.6 L/h, you need 16 litres, plus a reserve and some margin for a longer cut. A 25-litre tank then gives comfortable cover.

1. 1List the appliances that must run during an outage and add their running power in watts.
2. 2Convert to kW and divide by the generator's rated kW to get the load share.
3. 3Read the fuel burn from the datasheet curve at that share.
4. 4Multiply by the outage length, add 10 to 20% reserve, and compare with the tank.
5. 5Keep spare fuel in an approved container, stored safely and away from ignition sources.

Finally, test the set. Run it on a typical load for an hour and note the fuel used, by measuring the drop in the tank. That single measured litres-per-hour figure is better than any datasheet, and it also tells you whether the engine is in good health.