Every fuel call that turns into an emergency starts with the same bad number: one gallons-per-hour figure someone wrote on a clipboard years ago. It is almost never the right number, because a diesel generator’s burn rate is a function of how hard it is working, and a job site’s load changes hour to hour.
The math itself is simple. Getting the two inputs right is the whole job.
What is the actual formula?
Runtime, in hours, equals usable tank gallons ÷ gallons per hour at your load.
Two words there do the work. Usable, because the number stamped on a tank is rarely the number you can burn — manufacturers publish a usable capacity, and no operator should plan on drawing a tank to the last inch anyway. And at your load, because the spec sheet gives you four different burn rates, not one.
How much diesel does a generator burn per hour?
Here are published standby figures straight off manufacturer spec sheets. Every row is from a dated document, and the sources are listed at the end.
| Generator set | Rating | 25% load | 50% load | 75% load | 100% load |
|---|---|---|---|---|---|
| Generac SD020T | 20 kW standby | 0.9 gal/hr | 1.1 | 1.3 | 1.7 |
| Generac SD050 | 50 kW standby | 1.3 gal/hr | 2.3 | 3.3 | 4.3 |
| Multiquip DCA70SSIU4F (towable) | 62 kW standby / 56 kW prime | 1.7 gal/hr | 2.6 | 3.5 | 4.4 |
| Generac SD100 | 100 kW standby | 2.2 gal/hr | 4.2 | 5.9 | 7.3 |
| Cummins C150D6R | 150 kW standby | 3.6 gal/hr | 6.6 | 9.1 | 11.8 |
| Generac SD200 | 200 kW standby | 4.4 gal/hr | 8.3 | 11.9 | 14.8 |
| Generac SD250 | 250 kW standby | 5.5 gal/hr | 10.4 | 14.8 | 18.5 |
Look at the spread. The SD100 burns 2.2 gal/hr at a quarter load and 7.3 gal/hr flat out — more than triple. Any single “gallons per hour” number you carry around is wrong at three of those four columns.
Use these as a sanity check, not as your set’s numbers. Pull the spec sheet for your exact model. Engine, emissions tier, and alternator all move the figure, and a 2015 revision of a sheet does not describe a 2024 unit.
Why doesn’t a “24-hour tank” last 24 hours?
Because the hour figure on a tank option is calculated at the rated load, and almost nothing runs at rated load.
Generac lists the SD100’s tank options by runtime: 12 hours at 90 usable gallons, 30 hours at 220, 48 hours at 350. Divide 90 by the full-load rate of 7.3 gal/hr and you get 12.3. Divide 350 by 7.3 and you get 47.9. The runtime column is simply capacity divided by the 100 percent burn rate.
That cuts both ways, and mostly in your favor:
| Load on a 100 kW SD100 | Burn rate | 90-gal tank | 350-gal tank |
|---|---|---|---|
| 100% (100 kW) | 7.3 gal/hr | 12.3 hr | 47.9 hr |
| 75% (75 kW) | 5.9 gal/hr | 15.3 hr | 59.3 hr |
| 50% (50 kW) | 4.2 gal/hr | 21.4 hr | 83.3 hr |
| 25% (25 kW) | 2.2 gal/hr | 40.9 hr | 159.1 hr |
Runtime calculated from published burn rates and Generac’s listed usable capacities. Real hours land lower — block heaters, cold starts, and any reserve you refuse to burn all come off the top.
The trap runs the other direction too. If your building’s emergency system is rated under NFPA 110, the standard’s Class is the minimum time the system is designed to run at rated load without being refueled. Class 2 means two hours. A tank that satisfies the code is not the same thing as a tank that gets you through a Front Range outage, and plenty of compliant buildings are two hours from needing a fuel truck.
What load is the machine actually carrying?
This is the input nobody has, and it is the one that matters most.
- Read the controller. Most modern gensets display live kW on the front panel. Check it during a real run, not at idle.
- Pull the last test report. An annual load-bank or exercise report gives you the number in writing.
- Clamp it. An electrician can read actual amps at the transfer switch in a few minutes.
- On a job site, count the machines. Tower lights, a welder, and a trailer office are a very different load than a concrete pump, even on the same towable set.
Once you know the load, the runtime question stops being a guess and turns into arithmetic you can put in a schedule.
Does Denver’s altitude change the math?
Not the formula — the rating it applies to.
Every OEM publishes deration data because air density falls with altitude and rises with heat. Generac’s SD250 sheet states plainly that operational characteristics assume maximum ambient conditions and that derate factors may apply under atypical site conditions, then points to a separate deration bulletin rather than printing one universal factor. Cummins publishes a deration table on its data sheets with altitude on one axis and ambient temperature on the other.
The practical consequence for a set running at 5,280 feet in August: the “50% load” column means 50 percent of the derated output, not 50 percent of the sea-level nameplate on the sticker. A building that draws 60 kW from a nominal 100 kW set is not necessarily at 60 percent here. Get the deration table for your model and your elevation before you size a tank or a delivery schedule around it.
When should you call for a refuel?
At about a third of a tank. Not at the low-fuel alarm.
Three reasons, in order of how much they cost you. Ordering at a third leaves real hours of runtime while a truck is dispatched and driven, which is the difference between a scheduled delivery and an emergency one. It keeps your draw well above the water and sediment that settle in the bottom of any tank that has been sitting. And it avoids running a fuel system dry, which on most sets means bleeding and priming before it will restart — in the dark, in the rain, with the building down.
For anything that has to stay running — a generator carrying a data room, a pump on a night pour, broadcast trucks on a live event — we would rather be on a schedule you set than a phone call you make at 2 a.m. That is what our delivery and fleet programs and our 24/7 emergency service exist to cover, across the Denver metro and Front Range.
Work out your number first. Usable gallons, divided by the burn rate at the load you actually carry, minus the reserve you will not touch. Then tell us the hours you need to hold and we will build the schedule backward from it.
