A generator is cheap to buy and expensive to feed. Over a year, fuel usually dwarfs the purchase price — so before you rely on one, it's worth knowing exactly what it costs to run. That number is also the clearest way to see how much a solar-and-battery system could save you.
How the cost adds up
Start with how much fuel the generator burns per hour: the load in kilowatts times the fuel rate in litres per kWh. Multiply by the fuel price for the cost of one hour, then by your daily run hours and by 30 for the monthly bill. A 5 kW load at 0.3 L/kWh burns 1.5 litres an hour; at 0.9 per litre that's 1.35 an hour, about 10.80 a day over 8 hours, and roughly 324 a month.
Why light loads waste fuel
Generators are most efficient at around 70–80% of their rated output. Run one lightly loaded — a big generator powering a few small loads — and it burns almost as much fuel as it would fully loaded, so the cost per useful kWh shoots up. Right-sizing the generator to the load is one of the easiest ways to cut fuel waste; the generator sizing calculator helps you match them.
Where solar changes the maths
The single biggest lever on this bill is run hours — and that's exactly what solar attacks. With panels and a battery carrying the load through the day and into the evening, the generator drops to a backup role for the occasional cloudy stretch, and fuel use falls sharply. Use the solar bill savings calculator to compare, and the off-grid system wizard to design a system that keeps the generator off most of the time.
Related tools
Size the generator itself with the generator sizing calculator, work out your loads with the daily load calculator, and see the payback of replacing fuel with solar using the solar bill savings calculator.
Frequently asked questions
How much fuel does a generator use per hour?
Roughly load (kW) × fuel rate (L/kWh). Diesel runs about 0.25–0.35 L/kWh at a sensible load, so a 5 kW load uses about 1.5 L/h. Light loading pushes fuel-per-kWh higher.
How do I calculate generator running cost?
Litres per hour = load × fuel rate; cost per hour = that × fuel price; then × run hours for daily and × 30 for monthly. This calculator returns all three.
How can I reduce my generator fuel costs?
Cut run hours — that's what solar and a battery do, dropping the generator to backup only. Right-sizing and properly loading it also lowers fuel per kWh.