Skip to content
Sign inGet Started

How Long Will This Tank Run the Generator? The Runtime and Days Left Formula

By Saeed Baatiah · Published:

Short answer

Runtime in hours equals the usable litres in the tank divided by the generator’s fuel use in litres per hour at its actual load. Days left equals usable litres divided by litres used per day. Example: 700 usable litres in a 136 kVA set burning 16 L/h at half load gives about 44 hours.

Key takeaways

  • Runtime (hours) = usable litres ÷ fuel use in L/h at the actual load; days left = usable litres ÷ litres used per day.
  • Fuel use depends on load, not on the nameplate: the Cummins C150 D5 data sheet lists 12.0 L/h at quarter load and 31.2 L/h at full prime load.
  • Charts are usually in kW; a nameplate in kVA converts to kW by multiplying by the power factor, and Cummins uses 0.8 (150 kVA standby = 120 kW).
  • Use chart figures to plan a new site, then switch to the daily litres your tank actually loses, because real load changes every hour.
  • ENTEK Fuel Tank shows the level in litres and the daily consumption of each tank; ENTEK Generator shows only whether the set is online and running, on a map.

01The formula in two lines

Every runtime question comes down to two divisions. First, runtime in hours equals usable litres divided by litres per hour at the load the generator is actually carrying. Second, days left equals usable litres divided by litres used per day, and litres per day is simply litres per hour multiplied by the hours the set runs each day. That is all the math. The hard part is getting the three inputs right. Usable litres is not the number on the tank label: it is what is in the tank now, minus the amount the engine cannot or should not draw. Litres per hour is not one number for a generator: it rises with load, so a set at a quarter load burns far less than the same set at full load. And daily running hours vary with grid outages, shifts and season. The sections below take each input in turn, then put them together in a worked example you can copy for your own site.

02Usable litres: what the engine can really draw

Start from the litres in the tank right now, not the capacity on the label. A tank that holds 1,000 L and reads 80% has about 800 L, and only if the level has been converted to volume using the real tank shape. A horizontal cylinder does not hold the same litres per centimetre at the bottom as in the middle, so a dipstick reading in centimetres needs a calibration table or a shape calculation before it means anything. Next, subtract a reserve. The fuel pickup sits above the floor of the tank, and the last litres often carry water and sediment that you do not want in the injectors. How large that reserve is depends on your tank and your own policy, so set it once and write it down. In the example below we keep 100 L back, which leaves 700 usable litres. If you plan refills by alert thresholds, you can also treat the litres above your Low level as the working stock.

03Litres per hour: load, kW vs kVA and power factor

Fuel use follows the real power the engine delivers, measured in kW. Many generators in Saudi Arabia are sold by kVA, the apparent power on the nameplate. To move from kVA to kW, multiply by the power factor. Cummins uses 0.8: the C150 D5 data sheet lists 150 kVA (120 kW) standby and 136 kVA (109 kW) prime at 50 Hz, and its full load current formula uses the same 0.8. So a 250 kVA set is about a 200 kW set when you read a chart in kW. Next, pick the right rating. The data sheet, following ISO 8528, defines prime power for a varying load with unlimited hours, and emergency standby power for the length of a grid outage. A site that runs every day should read the prime column. Then estimate load as a share of the rating: a set delivering 55 kW on a 109 kW prime rating is near half load. Finally, read the litres per hour for that load from the manufacturer’s table.

04A worked example with a real data sheet

Take a site with a Cummins C150 D5 on prime duty, 136 kVA or 109 kW, and a diesel tank that holds 800 L today. With a 100 L reserve, 700 litres are usable. The data sheet gives prime fuel use of 12.0 L/h at quarter load, 16.0 at half, 25.0 at three quarters and 31.2 at full load. Divide 700 by each: about 58 hours at quarter load, 44 hours at half load, 28 hours at three quarters and 22 hours at full load. Now days left. If the set runs 10 hours a day at half load, it burns 160 litres a day, so 700 litres last about 4.4 days. At three quarters load the same tank lasts 2.8 days. The gap between those two answers is the point: the same tank and the same generator give very different days left depending on load. The first table shows the full calculation. The second table gives approximate hours per 1,000 usable litres for other common sizes, from a public consumption chart.

05Days left from what the tank actually loses

A chart is a planning tool. It assumes a steady load, and the page that publishes the general chart says plainly that its figures are approximations. On a live site the load moves with air conditioning, shifts and outages, so the best input for days left is the litres the tank really loses per day. Take several days of history rather than one, divide usable litres by that daily average, and you have a days left figure that already contains your real load and your real running hours. Two checks keep the number honest. First, remove refills from the history, or the average will look too low. Second, compare the daily figure with the litres you actually had delivered over the same period: if the tank says you used far more than you bought, the sensor or the tank setup needs a look before anyone trusts the forecast. A sudden fall that does not match running hours is an unexplained drop to investigate, not consumption.

06Where ENTEK fits, and what it does not do

ENTEK Fuel Tank is the built-in part that feeds this formula. A capacitive probe in the tank reports the level, the platform converts it to litres from the tank shape and dimensions you enter, and each tank shows its level history and its hourly and daily consumption. Tanks turn Low below 35% and Critical below 20%, with thresholds you can change per tank, and a sudden drop raises an alert. Rules are checked every 30 seconds and messages reach the people you choose by email, SMS or WhatsApp, usually within about a minute. Days left is your own division of usable litres by the daily figure, and it is worth checking that figure against delivered litres as described above. ENTEK Generator is narrower: it shows whether each set is online and running, the asset list and a map. It does not log fuel or produce reports, so pair it with Fuel Tank for the genset’s tank. Probes, trackers and diesel deliveries are available through the ENTEK marketplace and RFQ at entek.ai.

Worked example: Cummins C150 D5, prime 136 kVA (109 kW), 700 usable litres, 10 running hours a day
LoadFuel use (L/h, data sheet)Runtime from 700 LLitres per day at 10 hDays left
25% (about 27 kW)12.058.3 h120 L5.8 days
50% (about 55 kW)16.043.8 h160 L4.4 days
75% (about 82 kW)25.028.0 h250 L2.8 days
100% (109 kW)31.222.4 h312 L2.2 days
Approximate fuel use and hours per 1,000 usable litres by size (chart in kW; kVA at power factor 0.8; 1 US gal = 3.79 L)
Generator size25% load50% load75% load100% load
60 kW (75 kVA)6.8 L/h, 147 h11.0 L/h, 91 h14.4 L/h, 69 h18.2 L/h, 55 h
100 kW (125 kVA)9.9 L/h, 101 h15.5 L/h, 64 h22.0 L/h, 45 h28.0 L/h, 36 h
200 kW (250 kVA)17.8 L/h, 56 h29.2 L/h, 34 h41.7 L/h, 24 h54.6 L/h, 18 h
500 kW (625 kVA)41.7 L/h, 24 h70.1 L/h, 14 h100.1 L/h, 10 h135.3 L/h, 7 h

Cummins C150 D5 prime fuel use by load

1/4 load12 L/h
1/2 load16 L/h
3/4 load25 L/h
Full load31.2 L/h

Cummins Power Generation, C150 D5 generator set data sheet, EMERD-5835a-EN (11/13)

Approximate full-load fuel use by generator size

60 kW (75 kVA)18.2 L/h
100 kW (125 kVA)28 L/h
200 kW (250 kVA)54.6 L/h
500 kW (625 kVA)135.3 L/h

Generator Source approximate diesel consumption chart, converted at 1 US gal = 3.79 L in this article's table

Figures and sources

Questions

How many hours will 1,000 litres of diesel run my generator?

Divide the usable litres by the fuel use at your load. On a public approximate chart, a 100 kW (125 kVA) set uses about 15.5 L/h at half load, so 1,000 usable litres last about 64 hours; at full load, about 28 L/h, they last about 36 hours. Check your own model’s data sheet.

My generator is rated in kVA. Which number do I use?

Convert to kW by multiplying by the power factor, usually 0.8 on the data sheet, then compare your load in kW with that rating. A 250 kVA set is about 200 kW. Fuel use follows kW, the real power the engine delivers.

Why does my tank run out faster than the chart says?

Usually because the real load is higher than you assumed, the set runs more hours than planned, or the usable litres were counted from capacity instead of the level. A fall that does not match running hours is an unexplained drop worth checking against delivery records.

Should I use the prime or the standby fuel figures?

Use prime if the generator runs regularly under a varying load, and standby only for a set that covers grid outages. The Cummins C150 D5 sheet, following ISO 8528, lists 31.2 L/h at full prime load and 36.0 L/h at full standby load.

Does ENTEK calculate days left for my generator tank?

ENTEK Fuel Tank shows the litres in each tank and its daily consumption, and alerts at Low (below 35%) and Critical (below 20%). You divide usable litres by the daily figure to get days left. ENTEK Generator shows only online and running status, assets and a map.

Sources

Related insights