Here is how to work out how long your set can really run, and what to plan before the next long outage.
The rating sets the rules
Generator ratings follow ISO 8528 categories. A standby rating is meant for emergency use with a limited number of hours a year and a variable load. A prime rating allows unlimited hours at variable load. A continuous rating allows unlimited hours at constant load. A standby set can run through a multi-day outage, but running it at its full standby rating for long periods is outside what it was designed for and shortens its life.
| Rating | Intended use | Long-run guidance |
|---|---|---|
| Emergency standby | Outages only, limited hours per year | Fine for an outage; keep average load moderate |
| Prime | Main supply, variable load | Unlimited hours within its load limits |
| Continuous | Main supply, constant load | Unlimited hours at rated load |
Check the maker’s data sheet for the exact hour and load limits that apply to your set.
Limit 1: fuel
Runtime on fuel is simple arithmetic: usable fuel divided by consumption at the actual load. A set using 100 litres per hour at its typical outage load, with 2,400 usable litres on site, has about one day of fuel. Many emergency systems are designed for a set number of hours or days of fuel on site, with a resupply plan for anything longer. Our pages on fuel storage for emergency fleets and the fuel resupply plan cover both.
Limit 2: lube oil and filters
Engines consume a small amount of oil as they run, so the oil level needs checking during a long run. Oil and filter change intervals are set in running hours, typically a few hundred hours for many diesel generators, which a long outage can reach. Plan to check oil at least daily and to change oil and filters at the maker’s interval, which means stopping the set or switching load to another unit for a short time.
Limit 3: cooling and air
A generator room that was fine for a monthly 30-minute test may get hotter and hotter over a long run. If room temperature rises, the radiator loses capacity and the set may derate or shut down on high coolant temperature. Long outages also expose blocked air filters and radiator fins. A set sized and ventilated for continuous operation in the hottest season will run for days; a marginal installation may not.
Limit 4: light load
Running a diesel set for long periods at very light load causes wet stacking, where unburnt fuel and soot collect in the exhaust. Keeping load above roughly a third of rating, or adding a load bank, avoids the problem. See wet stacking in emergency fleets for the symptoms and the cure.
Planning for a long outage
A good long-outage plan includes fuel contracts with priority delivery, a daily check sheet for oil, coolant, fuel and alarms, spare filters and oil on site, and, where possible, two or more sets so one can be serviced while the other carries the load. For critical sites, an N+1 arrangement makes servicing during an outage routine rather than risky. Our page on N+1 emergency fleets explains how.
When the outage never really ends
On sites where the grid is unreliable for months at a time, an emergency set ends up working as a prime power plant. If that is your situation, the right answer is a prime-rated plant designed and maintained for daily duty, not a standby set worked to exhaustion.
From standby sets to full power stations
USP&E supplies, installs and maintains emergency diesel generators and builds prime power stations when the grid cannot be relied on. We have 150+ power projects in 45+ countries and 25,000 MW contracted since 2002. Our EPC team can move a site from standby to prime power when outages become the norm.
Offices: Colorado Springs (1755 Telstar Dr, Suite 300), Knoxville (800 S Gay St, Suite 700), London (162-168 Regent Street), Johannesburg (22 Kildoon Rd, Bryanston), Cape Town (31 Brickfield Road, Woodstock) and China (No. 1 Ningzhou Road, Wuxi). Regions: USA | AFRICA | UK | UAE | CHINA. ISO 9001 and ISO 45001 certified.
Diesel generator runtime FAQ
How long can a diesel generator run continuously?
With fuel, oil, cooling and load managed, a correctly rated diesel generator can run for days or weeks. Prime and continuous rated sets run for thousands of hours a year.
Can a standby generator run for a week?
Yes, during a genuine outage, as long as it has fuel, its oil is checked and changed at the right hours, and the load stays within its standby rating.
How often should I check a generator during a long run?
At least daily: oil level, coolant level, fuel level, leaks, alarms and room temperature.
Does running a generator at low load damage it?
Long periods at very light load cause wet stacking. Keep load above about a third of rating or use a load bank.
Preparing for long outages?
We can review your set’s rating, fuel autonomy and service plan, or design a prime power solution where outages are routine.
Phone +27 10 822 2324, email info@uspeglobal.com with your site data, or go through the emergency power team. Standby and prime-rated sets are listed in the diesel generator inventory.