One question I get from our regional managers more than any other: "What size generator does our facility need?"
My honest answer? It depends. There's no universal spec that fits every building, and anyone who tells you otherwise hasn't managed a real rollout. I started running purchasing for our company in 2020—roughly $1.2M a year across eight vendors, reporting to both operations and finance. In that time I've helped spec backup power for a small corporate office, a parts warehouse, and one 40,000-square-foot assembly plant. They needed entirely different answers.
What I've learned: backup power decisions sort themselves into three scenarios. Figure out which one you're in, and the equipment choice gets much simpler.
- Scenario A: You need lights, computers, and basic HVAC during an outage. A home backup generator-class unit handles it.
- Scenario B: You've got elevators, real HVAC loads, or light production equipment. You're shopping in the 120 kW Perkins generator range.
- Scenario C: Motor-heavy industrial operations, multi-phase equipment, or compliance-critical facilities. Look at a 175 kVA Perkins generator or larger.
Scenario A: Keeping the lights on
If you're managing a small office, retail space, or a clinic where the biggest concern is keeping computers and security systems alive during an outage, you don't need an industrial unit. A home backup generator in the 30–60 kW range does the job, and the Perkins lineup covers that bracket.
Three things I've learned specifying this class:
First, size for what you'll actually run, not the building's total connected load. A small office runs at maybe 40% of its nominal electrical capacity during an outage—lights, network gear, a handful of outlets. Design for that, not for your breaker panel's theoretical maximum.
Second, don't skip the automatic transfer switch. The vendor failure in March 2023 taught me this the hard way. One of our regional sites lost a critical compliance deadline because the facility had a brand-new generator and a transfer switch that had never been tested under load. One missed deadline, and suddenly redundancy didn't seem like overkill. Test the switch under load before you need it—not after.
Third, check local sound ordinances before you pick an installation spot. A generator mounted 20 feet from a medical office's patient wing will create complaints you don't want to manage. That's not a spec issue—it's a placement issue, and it's cheaper to solve on paper than after installation.
Scenario B: The 120 kW workhorse
This is where most mid-size facilities land—warehouses, machine shops, office buildings with elevators and chillers. A 120 kW Perkins generator gives you real capacity without forcing a dedicated powerhouse build-out.
Here's the part that catches people off guard: you rarely run a 120 kW generator at 120 kW. You're buying capacity for starting loads, not steady-state loads. Motors draw several times their running current on startup. That 50 hp compressor can pull 70–80 kW for the first few seconds. An undersized genset doesn't just brown out the equipment—it can trip the generator's own breaker and leave you in the dark anyway.
That's why I recommend load staging for this class, even though it's not the glamorous answer. Sequence your startup: chiller first, then elevator banks, then process loads. A 120 kW unit with programmed load shedding often outperforms a nominally larger unit without one. Worse than expected? No—better than expected, once you set it up right.
On the procurement side: run the fuel system before you sign the delivery acceptance. We learned this in 2024 when a winter startup took 45 minutes because the block heater had been disconnected during transport and the fuel pump wasn't on the acceptance checklist. Which connects to the maintenance questions below.
Scenario C: 175 kVA and the industrial call
When you're running motor-heavy industrial operations—pumps, conveyors, large HVAC plants—the math changes. You're likely looking at a 175 kVA Perkins generator, which is roughly 140 kW at 0.8 power factor. Don't confuse the two ratings; a supplier who flips between kW and kVA without explaining it isn't being careful with your money.
This class comes with real regulatory obligations. NFPA 110, the standard for emergency and standby power systems, assigns a class based on how fast the system must pick up the load. A Class 10 system has to transfer within 10 seconds. It also requires periodic load testing under real load—not a quiet no-load run once a month. I've watched facilities get cited for exactly that gap. If your operation is regulated, generator choice isn't a preference; it's a compliance decision.
Emissions compliance matters here too. EPA Tier 4 standards apply to many stationary diesel engines, and California's CARB has its own stricter rules. A reputable dealer will flag this during the spec; a generalist might not.
One piece of advice I'll defend: don't ask your vendor to "make it fit" with a smaller transfer switch to save money. The penny-wise move—saving $400 on an undersized switch—comes back as a pound-foolish failure. I saw a facility spend $2,400 on a failed inspection and a rushed replacement because someone cut that corner. Not a trade I'd make.
Two maintenance questions that come up more than you'd expect
Every time I've written up a generator spec, someone asks about the small block Chevy oil filter. Yes, really. There's a persistent rumor that a small block Chevy oil filter threads onto certain diesel generator engines. And if you check a crossover chart, the thread pitch might even match.
Here's where I hold a firm line: don't do it. The bypass valve setting, the micron rating, and the anti-drainback design on a diesel oil filter are specified for that engine. A gasoline engine filter is engineered for a different pressure curve and a different contamination profile. Our service vendor put it plainly:
"It might thread on, but it's the wrong filter for this engine."
Save $30 on a filter, risk a $40,000 engine. Not a trade I'd make.
The other question that keeps coming up: how to tell if a fuel pump is bad.
I'm not a diesel mechanic, and I don't pretend to be one. But after processing 60–80 orders a year and managing the resulting maintenance calls, I've learned the symptoms well enough to know when to pick up the phone. It's kinda like that with any specialized equipment: you learn the red flags, but you call in the expert for the actual diagnosis.
- Hard starting, especially after the unit sits. Fuel pressure bleeds down overnight, and the engine cranks longer before firing.
- Loss of power under load. The generator runs fine unloaded, but frequency sags when you add load.
- A low or erratic fuel pressure gauge reading. If your unit doesn't have a gauge, the controller may log the excursions.
- Noise from the pump itself. A whine or chatter that wasn't there before.
- Visible fuel weepage or a fuel smell. The obvious one, but you'd be surprised how long it gets ignored.
When I suspect a fuel pump issue, I don't open the housing. I document the symptoms, photograph the hour meter and any error codes on the controller, and send that to the service vendor. Knowing the boundary between "diagnose" and "dig in" is a skill in itself. That boundary—call it professional modesty—has saved us both money and downtime.
How do you know which scenario you're in?
Here's the process I use:
First, calculate your connected load. Walk the facility and add up the nameplate ratings for the equipment you'd run during an outage. You don't need engineering precision—you need a defensible estimate. Multiply your critical load by 1.25. That's your floor, not your target.
Second, check your starting loads. List every motor that runs in an outage—HVAC, elevators, pumps, air compressors. Add the largest motor's starting surge to your running load. If that number pushes past 70% of a candidate generator's rated output, move up a size.
Third, ask the questions nobody asks early enough:
- Will you grow into this building over the next five years?
- Does your operation have compliance obligations that require a specific class of standby power?
- Who will service this unit, and how far away are they?
Looking back, I should have asked these questions before our 2024 vendor consolidation project. At the time, procurement was reactive—we got quotes, compared prices, placed orders. What I know now is that the best vendors are the ones who admit what they don't do. Our most trusted supplier once told us they didn't service one of our older units and recommended someone who did. That honesty earned them three subsequent orders. I'd rather work with a specialist who knows their limits than a generalist who overpromises.
So: what size generator do you need? The right answer matches your scenario. And if you're still unsure, find a reputable dealer for the Perkins line in your region, give them your load numbers, and ask them to show their work. A supplier who won't put their recommendation in writing isn't worth the purchase order.
