Generators for commercial use are standby or prime power units, usually diesel or natural gas, ranging from about 20kW to 4MW+. They supply a building or site when utility power drops. In our experience, a weak battery is the most common reason one fails to start, with the automatic transfer switch (ATS) second.
We service these machines out of Brighton, Jacksonville, Pensacola, and Austin. One of our shop techs describes the job as mobile mechanic work on “engines the size of houses.” What follows comes from the service calls our team has handled.
What counts as a commercial generator?
At Generator Source, a commercial generator is a 60Hz unit from 20kW up. We don’t sell home standby generators. Our customers include hospitals, data centers, water treatment plants, farms, grocery stores, and construction sites.
A generator that backs up life safety loads, such as hospital circuits, falls under NFPA 110. The unit has to be running and carrying full load within 10 seconds, because a patient on an operating table can’t wait for power. These units also carry extra equipment, including heaters for the battery and the engine block, and they’re exercised on a different schedule. An office running computers and lighting isn’t held to that standard.
Why won’t a commercial standby generator start?
Battery first, ATS second. After that it’s fuel, since a generator with an empty tank won’t run, and forgotten fuel comes up regularly on construction sites and on standby units.
Sometimes the generator is fine and the problem is somewhere else. We’ve had customers report that a unit never started, when utility power had stayed on and a breaker further down the line had tripped. The generator had no reason to start.
A wrong diagnosis can also keep a unit down. One of our Jacksonville technicians went to a landfill where the previous service company had blamed the engine, because the unit shut down every time the gas pump switched on. He ran his own transfer test and found the phase rotation was wrong. The pump was trying to run in reverse and pushing gas back into the landfill mound. If the unit had kept running that way, the landfill could have blown up.
How do you size generators for commercial buildings?
We size from the building’s one-line diagram, an engineering drawing that lists total kW, amperage, and voltage. We also need the inrush of the largest motors, since motors and compressors draw much more when they start than when they run, plus the number of hours the unit has to run on one tank of fuel.
Location changes the answer. A generator’s output is reduced at a mile high in Denver compared with sea level, so we approach a mountain site, a Denver site, and a site on the eastern plains differently, even when the model is the same.
An undersized unit can shut down on overload and trip breakers. An oversized unit running at light load doesn’t get hot enough to burn all its fuel, so raw fuel ends up in the exhaust. That condition is called wet stacking, and it can lead to engine failure. We’ve also seen customers add equipment after installation and find the generator was too small.
Our generator size calculator gives a starting estimate. For pricing and purchase strategy, see our commercial generator buying guide.
What maintenance do commercial backup generators need?
We recommend a visit twice a year for a 31 point inspection. The batteries get tested, and the oil and coolant get checked. Fuel sampling and analysis can be added. Skipped steps cause real damage: leave an old filter in during an oil change and it can clog and starve the engine of oil, and dirty fuel can clog the injector pump.
The load bank test takes a generator from no load up to 80, 90, or 100% of its rating. It also checks that the unit can accept a block load of 60 to 80% all at once. A test typically runs one to four hours. If a generator is going to fail, finding it during preventive maintenance costs far less than an after-hours service call plus parts.
We recommend replacing batteries every 24 months. Some last eight to ten years, but we don’t recommend relying on that for emergency backup.
What happens when an outage runs long?
During a high wind event along the Front Range, the winds lasted two days, and the outages ran as long as seven days for many of our customers. Fuel calls came in from Sedalia to Fort Collins. We had one fuel truck and one technician on fuel, so we brought in an outside vendor.
One Longmont customer had a generator on a 1500 gallon tank, running around the clock at roughly 65 to 70% load. It burned about 400 gallons in 6.5 hours. We rotated deliveries to keep it fueled, and power came back around 9:30 that evening, so the vendor didn’t need to make a second delivery.
That works out to about 60 gallons an hour, so a full 1500 gallon tank covers roughly 24 hours at that load. Facility managers can run the same calculation with their own unit’s fuel consumption and tank size.
Rentals come up in these situations too. One Sunday morning, a new apartment building lost power after a driver hit its transformer. The building had called the big rental companies and gotten no answer. Our service manager was on the golf course when the phone rang. We pulled a team together and had a generator running at the site, about an hour and a half away, before sundown. It stayed for three months until the transformer was replaced.
What are the options if I need power soon?
Lead times on new generators of 1500kW and up run 52 weeks or more, because the large manufacturers are filling data center orders. Used units are one way around that. It’s common to see a 10 or 15 year old generator come in with only 100 or 200 hours on it, since most sit behind a building and run for exercise. Every unit we buy gets a 31 point inspection, an oil sample, a load test, and a filter change before it’s listed.
Our rental fleet runs from about 20kW to 2000kW, usually on terms of two weeks to a year. Browse our inventory, or read our guide to cost-effective generators for commercial applications for the cost side.
Why choose Generator Source?
We’ve been in business for close to 50 years. Some of our technicians have been with us for 30 years or more, and some of their kids now work here. Years of working on used equipment means our techs have seen nearly every make, model, and controller, so they’re prepared for most of what they find in the field. Our service techs hold EGSA certification, and we own every unit in our inventory.
If you’re sorting out generators for commercial use, contact us or call 877-866-6895.
FAQ
What size generator does a commercial building need? Size depends on total load, the inrush of the largest motors, voltage, and runtime. We work from the building’s one-line diagram, which lists total kW, amperage, and voltage. Our generator size calculator gives a starting estimate, and a technician confirms it.
Why won’t my commercial generator start? A weak battery is the most common cause, followed by the automatic transfer switch, with an empty fuel tank also on the list. Sometimes the generator is working and a tripped breaker downstream is cutting power.
How often should a commercial generator be serviced? We recommend an inspection twice a year, with load bank testing as part of the program. Hospitals and other life safety applications fall under NFPA 110, which adds equipment and exercise requirements.
What is the difference between emergency and standby generators? An emergency generator backs up life safety loads and must meet NFPA 110, including carrying full load within 10 seconds. A standby generator backs up the wider building so normal operations continue.
Can I rent a commercial generator? Yes. Our rental fleet runs from about 20kW to 2000kW, usually on terms of two weeks to a year.
Should a commercial generator run on diesel or natural gas? Diesel is still the most common choice because it runs on fuel stored on site, independent of a gas line. Natural gas removes the need for fuel trucks and avoids aging diesel. Our natural gas vs diesel comparison covers the details.