The Perkins Generator Buying Checklist I Use as an Office Buyer (Interlock vs Transfer Switch Included)

If you're the person responsible for buying a backup generator for a small or mid-sized business, this checklist is for you. I'm the office administrator for a 70-person facility, and I manage the facility budget—roughly $250k a year across about a dozen vendors. In 2022, I ran the buying process for a Perkins generator after a two-day outage cost us a customer order. I'm not an electrician, but I've learned what to check before signing.

What This Checklist Covers

Use this when you're past the 'are we ready for an outage?' conversation and are about to spend money. It works whether you're looking at a 40 kW Perkins home generator (a genset built around a Perkins engine) or a 125 kW Perkins electric generator feeding a plant. The steps are the same; the numbers change.

Step 1: Write the Load List Before You Look at Anything

Don't start with price or brand. Start with what has to run when the grid goes down. I know that sounds basic, but the first quote we got in 2022 was for a 60kW unit. Our actual critical load, after adding the IT rack, two HVAC units for the server room, and a water pump, was 94kW. The sales rep had based his number on square footage.

Go through the panel with your facilities person. Identify the circuits you actually need. Then add 25–30% headroom for motor-starting and future loads.

Step 2: Understand Generator Interlock vs Transfer Switch—It Changes the Whole Project

This is the step most people skip because it sounds boring. It's not boring when a technician is working on a live panel.

Generator interlock is a mechanical bracket that connects the main breaker and generator breaker so they can't both be on. It's cheap, simple, and works for a lot of residential and small business applications. We used an interlock on our old equipment shed and it was fine.

Transfer switch is a separate device that detects power loss, starts the generator, and switches the load. For a 125 kW Perkins electric generator on a commercial building, I'd argue the transfer switch is usually the safer route. It prevents human error and gives you automatic start if nobody's on site. If the switch isn't UL 1008 listed, it shouldn't go in a commercial building.

From a buying perspective, the price gap is real. In our 2024 vendor consolidation project, an interlock kit was around $180. The automatic transfer switch quote was $1,600 installed. But the interlock requires someone to be there to flip it, and if the outage happens at 2 a.m., you're paying overtime or losing product. Per NFPA 70's Article 702, transfer equipment has to prevent accidental parallel operation—manual interlocks are allowed for manual transfer, but automatic setups need listed transfer equipment. Verify with your local inspector before ordering.

The most frustrating part of the interlock vs transfer switch debate? A lot of quotes treat it as an upsell, not a code question. You'd think the electrician's recommendation would be the deciding factor, but sometimes it's just 'this is what we stock.'

Bottom line: if it's a 40 kW Perkins home generator and someone is home most of the time, an interlock is a legit way to save money. For a 125 kW Perkins electric generator keeping a business running, budget for a proper transfer switch.

Step 3: Size for Real Loads, Not 'Biggest Generator in Stock'

You'll see a lot of ballpark rules. 'A 100kW generator for a 10,000 sq ft building.' Ignore that. A machine shop and an office have completely different starting loads.

Our final spec was a 125 kW Perkins electric generator. The 125kW rating covers the 94kW critical load plus a 30% headroom buffer. It also kept us out of the weird 100kVA derating zone—125kW vs 125kVA is not the same thing. If you see a 'kVA' number, multiply by 0.8 to get 'kW' on a typical 0.8 power factor set. This is where I almost made a $4,000 mistake: I quoted a 125 kVA unit thinking it was 125kW.

If you're looking at a 40 kW Perkins home generator for a big house, do the same load math. Electric range, well pump, septic pump, and a shop welder together can hit 35kW. But if you don't have motor loads, 40kW is probably more than you need—my house runs on a 12kW unit.

Step 4: Be Honest About the Fuel Source

Diesel isn't the only answer. If you already run an Onan propane generator at a remote site, you know the pros: propane stores indefinitely, it doesn't go bad like diesel, and it's cleaner. The downside is tank size, delivery coordination, and winter BTU derating. Onan's propane units are solid, and for a smaller site they make sense.

For a 125 kW Perkins electric generator, diesel is the more common choice because the engine is designed for it and fuel availability in an outage is easier if you have a bulk tank. Diesel does need maintenance—fuel polishing, additive, and regular testing. I learned this after our unit sat for nine months and the fuel turned into something the service tech called 'murky.' That was a costly training moment.

Step 5: Plan for Parts Before You Need Them

This is where an admin's perspective helps. Everyone compares the generator price. Almost no one compares the cost of a fuel filter, oil filter, air filter, and labor over five years.

For our Perkins engine, the service intervals are pretty standard, but the filters vary by engine model. When I order parts, I use a cross-reference check—the same idea as the Walmart Fram oil filter finder. Seriously, if you're comfortable looking up a filter for your truck, you can do this. Just don't stop at 'this one fits.' Check thread size, bypass valve setting, and the OEM part number. I once bought a cheap fuel filter with the right thread but the wrong micron rating. It worked—briefly—until the injection pump complained.

If you're buying a Perkins generator, keep one full set of filters on the shelf. The cost is small compared to the downtime.

Step 6: Get the Installation Details in the Contract

The generator is a machine, but the installation is a construction project. Before you sign, you need all of this in writing:

• Concrete pad and vibration isolation
• Fuel tank size and routing
• Exhaust system
• Automatic transfer switch model
• Permits and code inspections
• Load bank test with results attached

The load bank test is the step I regret skipping. Looking back, I should have insisted on a 2-hour load bank test before final payment. At the time, the salesman said the factory test was enough. It wasn't—when we threw the building on the generator for the first real outage, the voltage dipped under load. If I could redo that decision, I'd add a third-party acceptance test to every quote.

Common Mistakes I Keep Seeing

After five years of managing these vendors, I still kick myself for not documenting things better. Three specific mistakes stand out:

1. Underestimating Fuel Consumption

The spec sheet for our 125kW unit said roughly 7-8 gallons per hour at 75% load. That's 175+ gallons for a 24-hour outage. The '48-hour' amount we originally stored was nowhere near enough. Run the math on your own critical load and then double it.

2. Forgetting That Noise Matters

Our first generator was a little too close to the office window. The exhaust was technically within clearance. It was still super annoying. Put the sound level in the spec, not just the emission limits.

3. Choosing a Vendor on Price Alone

My rule after 2020: the lowest quote has to be 10% lower before I'll even look at it. That rule came from a project where the upside was $2,000 in savings. The risk was a vendor who didn't know transfer switch ratings. I kept asking myself: is $2,000 worth potentially failing a building inspection? It wasn't. Of course, I learned that after the fact.

So that's the checklist. It's not glamorous. But if you're the person who has to explain to your finance team why the generator didn't run, the boring steps are the ones that save you. And if you're on the fence between an interlock and a transfer switch, let the load list and the night-shift reality make that call for you.