A tank gauge tells you how much fuel is there. Run time depends on how fast the generator burns it, and that depends on load. Here is how to plan with real numbers.
Run time equals usable fuel divided by fuel consumption at your actual load. Both terms need care. The first is usable fuel, not tank capacity. The second is consumption at the load you expect, not the rating on the nameplate.
Start with the generator's specification sheet. Manufacturers list fuel consumption at several load levels, typically stated in gallons per hour. That table is the one reliable source for your unit. Online rules of thumb are a poor substitute because engines differ.
Two questions hide inside run time: how long you can run on what is in the tank, and how long you need to run. The second is a business decision about the outage you are planning for, whether hours, a day or several days, and it drives how much storage and how many deliveries your plan needs.
Here is a hypothetical, made up to show the arithmetic. Suppose a data sheet lists 10 gallons per hour at the load you expect, and your tank holds 300 gallons but you want to treat only 270 as usable after leaving a reserve and accounting for pickup height. Divide 270 by 10, and you plan for roughly 27 hours.
Replace the numbers with those from your own data sheet. Then check your answer against a second source, such as the generator service contractor's own estimate.
Round the answer down, not up. An estimate that errs short leaves you a cushion, while one that errs long can leave you out of fuel at the worst moment.
A generator running at a light load burns less fuel per hour than one at a heavy load, but the relationship is not simple. A lightly loaded unit uses fuel less efficiently per unit of power produced. Estimate your real load by adding up the equipment you would run in an outage, not the entire building's maximum demand.
Plan for the load you will actually have. During an extended outage, loads often shift, so build a margin into the estimate and recheck it as conditions change.
Motors and compressors draw more current when starting than when running, which can briefly push a generator harder. Base the fuel estimate on running load, and ask your electrician and the generator maker whether the unit can handle the starting demand.
Operating conditions can change engine performance as well, so ask the manufacturer whether the data sheet assumes standard conditions.
Diesel engines that run at very light load for long periods can fail to reach their intended operating temperature. Fuel then burns incompletely, and unburned fuel and carbon can collect in the exhaust system. This condition is called wet stacking and shows up as black smoke on engine runs.
Industry practice for emergency and standby power, reflected in NFPA 110, calls for exercising the unit at least monthly for 30 minutes. The load should be at least 30 percent of the nameplate kilowatt rating, or enough to maintain the exhaust temperature the manufacturer recommends. Your manufacturer and your authority having jurisdiction set the final requirements for your site.
Running the unit under a proper load also exercises the fuel system. It moves fresh fuel through the filters and lines, and it is a chance to spot leaks or other issues before an outage.
Follow the manufacturer's checklist for returning the unit to automatic standby after any test.
Diesel stored for long periods can degrade. Oxidation forms gum and sediment that clogs filters, and condensation can introduce water that feeds microbial growth. Temperature swings make it worse. Keeping the tank full reduces the air space where condensation forms.
Practical protection includes periodic fuel sampling and testing, draining water from the tank bottom, using stabilizer or biocide products that suit your fuel system, and replacing filters on schedule. Fuel polishing, which circulates fuel through filtration to remove water and particles, is a common service for tanks that sit for long periods. If testing shows problems, the fuel may need to be filtered or replaced.
Specific storage lives vary with fuel quality, temperature and tank condition, so base your schedule on test results and your manufacturer's guidance and not on a single date.
Keep a fuel log. Each time fuel is added, record the date, the gallons and the level before and after. Over a year the log shows how much your unit uses on your exercise schedule, and a level that drops between visits when the generator did not run may point to a leak.
A refueling plan turns the math into action.
Once a year, walk through the refueling plan as a drill: who calls, who authorizes, who meets the truck, where it enters the site and where the keys are. Gaps show up in a drill that you would otherwise find during an outage.
A supplier that comes to the tank takes the logistics off your plate. See generator refueling for how scheduled and event-driven deliveries work. For larger tanks, also review aboveground fuel storage tank rules. For cold climates, how diesel gels in winter explains how to keep fuel flowing.
Check the manufacturer's specification sheet, which lists consumption at several load levels. Your service contractor can also help.
Light or no-load running is a poor way to exercise a diesel unit. Follow the manufacturer's guidance, and see the wet stacking section above.
Follow your generator maker's recommendations and any codes that apply to your site. Many owners test on a regular schedule and after any sign of water or sludge.
It is a process that circulates stored fuel through filters to remove water and particulates. It can help when fuel has been stored for a long time.
Tell us what you run and where it sits, and we will come back with a delivery plan.