In short. Generator runtime in hours is usable fuel in litres divided by consumption in litres per hour at the load the set will actually carry. Usable fuel is well below tank capacity once the fill limit, the heel below the suction pipe and the operating reserve are removed, often only 60 to 70 percent of nominal. Work from the real load profile and the consumption curve, and prove the result on a timed run.
Diesel generator calculator
Fuel use, runtime, tank size and cost
How this is calculated
Real power is kVA × power factor. Fuel use is estimated as L/h ≈ 0.04 × rated kW + 0.22 × load kW, which follows the usual shape of modern diesel gensets: about 0.26 L per kWh near full load, rising towards 0.38 L per kWh at a quarter load because the engine's fixed losses are spread over less output.
If you enter the full-load figure from your engine datasheet, the curve is scaled to match it. Runtime is usable fuel ÷ L/h. Tank size is L/h × hours, plus the reserve margin, divided by the usable fraction.
These are planning estimates, typically within 10 to 15 percent of a healthy engine. Actual use depends on the engine model, its condition, altitude, ambient temperature and how the load varies. For a figure you can rely on, use the manufacturer's fuel curve or a metered test.
Estimates only, worked out in your browser. For a checked selection, send us your load list and site details or call 098852 01608.
The runtime equation and what goes into it
Runtime (h) = Usable fuel (L) ÷ Consumption at expected load (L/h)
Usable fuel is the volume the engine can actually draw and the operator is willing to use, in litres; consumption is the fuel rate at the load expected during the outage, in litres per hour, taken from the set’s data sheet or from a measured run. Both inputs are where runtime estimates go wrong, so each deserves its own section.
Worked example (illustrative). A 250 kVA set has a 1,000 litre base tank. The tank is filled to 95 percent, so 950 litres go in. About 7 percent of the nominal volume, 70 litres, lies below the suction pipe and cannot be drawn. The site’s reserve policy keeps 25 percent of nominal, 250 litres, untouched for emergencies, leaving 630 litres for planning. The expected outage load is 120 kW, at which the data sheet gives 35 litres per hour. Planned runtime is 630 ÷ 35, or 18 hours, against the 28 hours that dividing the nominal tank by the same rate would have suggested.
Usable fuel is not tank capacity
A tank’s nominal capacity is a geometric volume. Four deductions separate it from the fuel available for an outage.
- Fill limit. Tanks are filled to 90 to 95 percent to leave space for thermal expansion and to avoid overflow through the vent during return-fuel heating.
- Unusable heel. The suction pipe ends above the tank floor to avoid drawing water and sludge, and a transfer pump loses prime before the tank is empty, so 5 to 10 percent of the volume stays behind.
- Operating reserve. A critical facility does not plan to run the tank dry; it keeps a margin for a late tanker, a blocked filter or an outage that outlasts the forecast.
- Fuel already consumed. A tank last filled three months ago, with weekly test runs since, is not full. Measure before relying on it.
The reserve is a policy decision that should be written down. The table gives the ranges commonly adopted; the right figure depends on the consequence of running out and on how quickly fuel can realistically arrive.
| Facility type | Typical planning reserve | Reasoning |
|---|---|---|
| Hospital, data centre, telecom core | 25 to 35 percent of nominal | Outage may extend, refuelling may be delayed, consequence is severe |
| Manufacturing with ordered shutdown | 15 to 25 percent | Process can be stopped safely if warned |
| Commercial building, warehouse | 10 to 20 percent | Loads can be shed, refuelling usually possible within hours |
| Construction and rental sites | Daily top-up, 10 percent minimum | Fuel delivered as part of site logistics |
Consumption at the load you will actually carry
Consumption follows the kW delivered, so a runtime estimate needs the load during the outage, not the generator rating. Outage load is usually lower than normal demand because non-essential circuits are shed, and it changes through the day. The correct method is to split the outage into periods, estimate the load in each, read the litres per hour at that load from the consumption curve, and sum the litres. A hotel might carry 180 kW through the evening, 90 kW overnight and 150 kW in the morning; using the evening rate for all 24 hours overstates fuel by a third, while using the overnight rate understates it dangerously. The generator fuel consumption guide explains how litres per hour varies with load and why light load costs more per kWh.
Two further corrections apply. Battery recharge for any UPS on the essential bus adds load for the first hour or two after transfer. And a set in a hot room or at altitude is derated, which does not change consumption at a given kW but may change which set carries which load.
Refuelling is part of the runtime plan
Runtime is only half the question; the other half is how fuel arrives before it runs out. In Hyderabad and the surrounding industrial belts a bowser can usually reach a site within a few hours during the day, but night deliveries, monsoon flooding of access roads, a city-wide outage when every site calls at once, and tanker access to a basement or rooftop tank all stretch that. A sound plan states who orders fuel, at what tank level, from which supplier and alternate, how the delivery reaches the fill point, and how the delivered quantity is checked. Where the site cannot guarantee deliveries, the tank must be large enough to cover the credible outage plus the delivery delay; the method is in fuel-tank sizing, and temporary capacity for a planned outage can come from rental fuel tanks.
Proving the estimate
An estimate becomes a plan only after a timed run. Fill to the mark, record the energy meter, run the essential bus at its real load for at least two hours, refill through a calibrated measure and compare. Check at the same time that the low-level alarm, the transfer pump and the day-tank float switches work, because a runtime figure assumes the fuel path is sound. Stored diesel degrades over 6 to 12 months and collects water through condensation, so a tank that has sat through a monsoon should be sampled from the bottom drain; cloudy fuel or a water layer means the runtime figure is fiction until the fuel is polished. The fuel tank and runtime guide gives a worksheet for the full calculation.
Common mistakes
- Dividing nominal tank capacity by consumption at rated load; the answer is wrong in both numerator and denominator, and sometimes the errors cancel by luck.
- Assuming the tank is full because it was full at commissioning; weekly test runs and leaks draw it down unnoticed.
- Planning to use every litre; the pump loses prime at the heel and the engine stops with fuel still showing on the gauge.
- Ignoring the delivery problem; a 12 hour runtime with a 14 hour tanker delay is a 12 hour facility.
- Trusting fuel that has stood for a year; water and microbial sludge block filters within the first hour of a real outage.
What to send us
Send the generating-set nameplate, the tank capacity and a photograph of the tank showing the fill, vent and suction arrangement, the expected load during an outage in kW or a load log, the longest outage the site must survive, and how fuel is currently delivered. For a rental or temporary installation, add the hours per day and the days on site. Send your application details and we will return a runtime plan with the reserve and refuelling trigger levels worked out.
Questions engineers ask
How long will a 500 kVA generator run on a 990 litre base tank?
At a typical outage load of 250 kW it burns about 65 to 70 litres per hour, and with a 95 percent fill, 7 percent heel and 20 percent reserve only about 670 litres are usable, so roughly 10 hours. At full load the same tank gives about 6 hours. The exact figures depend on the engine’s consumption curve.
Should runtime be calculated at rated load or expected load?
Plan at the expected outage load for fuel ordering and reserve, but check the worst case at the maximum credible load so that the reserve covers a heavier-than-expected night. Stating both figures avoids surprises.
How much reserve should a hospital keep in its generator tank?
Most hospital engineers keep a quarter to a third of nominal capacity untouched and set the refuelling trigger at about half tank, so that a delayed tanker does not consume the reserve. The applicable healthcare and fire codes may set a minimum storage duration as well.
Why did the generator stop with fuel still in the tank?
Usually the suction pipe uncovered because the tank is not level, the transfer pump lost prime, or water and sludge at the bottom blocked the primary filter. All three are reasons to keep a heel allowance and to drain the tank bottom regularly.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.