The $3,200 Lesson: How I Mis-Sized a 15 kW Propane Generator and Missed a Bad Fuel Pump
December 2019: The call that started it all
I've been handling generator procurement and site power setups for eight years. In that time, I've personally made (and documented) six significant mistakes, totaling roughly $14,000 in wasted budget. Now I maintain our team's pre-deployment checklist to prevent others from repeating my errors.
The one I keep coming back to happened on a cold Tuesday morning in December 2019. A concrete contractor called our office. Their site had lost grid power—the utility was saying seven to ten days for repairs. They needed temporary power for lights, a small office trailer, and, critically, a welder for rebar work.
"We need something reliable, and it has to handle our welding," the foreman told me. "Can you get a Kohler generator out here by Thursday?"
That sentence—"handle our welding"—should have slowed me down. It didn't.
The order I was so confident about
I ran the numbers quickly. Lights: about 2 kW. Trailer: 3 kW. A 220V stick welder, running load roughly 6 kW. Total running load: around 11 kW. I added a 30% margin, landed on 15 kW, and ordered a Kohler 15 kW propane generator from our regional supplier.
Here's the part I'm not proud of. I'd handled dozens of generator orders by then. I knew a Kohler generator—particularly the liquid-cooled standby units—was dependable hardware. The order felt routine. I did not open the product manual's surge section. I didn't ask the foreman for the welder's startup amperage. I thought, "15 kW is more than the running total—what are the odds it won't handle the startup?"
Well, the odds caught up with me on Thursday afternoon.
The Thursday afternoon that unraveled
The unit arrived, got positioned, and was fueled—we'd arranged a propane delivery that morning. It started on the first crank. Good sign, or so I thought. Lights came on, trailer A/C kicked in, everything looked fine.
Then the welder got plugged in. The first strike took the whole system down. The generator tripped, the lights blinked out, and the foreman looked at me like I'd sold him a lemon.
I knew immediately what had gone wrong. Every welder spec sheet I've worked from flags a strike surge well above the running rating—and the arc re-strikes dozens of times a day. My "30% margin" was meaningless against that duty cycle. A 15 kW propane generator—even a good Kohler unit—wasn't going to happily absorb an intermittent strike surge on top of a 5 kW base load without proper load management. I'd sized the running load and ignored the startup behavior.
The fix? We rented a gas generator welder (an engine-driven welding unit rated for the job) and ran the building loads separately off the Kohler. That rental cost us $1,100 for the week. The foreman was decent about it, but I could tell I'd burned some credibility.
The second mistake I found the next morning
The next morning the site called again: the Kohler wouldn't start. No crank, no click—nothing.
Now, this is where the story gets embarrassing. I'd had a fuel pump failure on a portable unit two years earlier, and I'd written the test procedure into our internal notes. But this unit was new, and I'd already been embarrassed once that week. I told myself the no-start was propane-related—maybe a vapor lock from the cold—and dispatched a tech without the pump test.
The tech did what I should have done. First, he verified fuel in the tank and confirmed the shutoff valve was open. Then he disconnected the fuel line at the carburetor, cranked the engine, and watched for flow. Nothing came out. Next, he checked the pump's electrical connection and tested for voltage at the pump connector while cranking—no signal. The pump itself had failed on delivery.
What's the correct way to test if a fuel pump is bad? Our tech's sequence was textbook: confirm fuel supply at the tank, check the line for kinks or blockage, disconnect the output side and crank to observe flow, then check for voltage at the pump during cranking to separate an electrical fault from a dead pump. If voltage is present and there's no flow, the pump is the culprit. If there's no voltage, you trace upstream to the relay, fuse, or control board.
Had I run that five-minute check on Wednesday when the unit arrived (note to self: arrival inspection is not optional on "new" hardware), I'd have caught the failure before the foreman ever tried to start it. The replacement pump—warranty covered the part, but not our labor or the site delay—cost us $280 in labor and a half-day of downtime. I still kick myself for that one. If I'd just followed my own note, the whole Friday morning would have gone differently.
What the $3,200 total actually broke down into
By the time the week ended, here's what the missteps cost:
- Gas generator welder rental: $1,100
- Emergency fuel pump swap labor: $280
- Site delay hours on Friday: roughly $600 in crew standby
- My manager's time cleaning up the customer conversation: hard to price, but real
- The 15 kW propane generator itself: $1,220—hardware that was fine, just misapplied
Call it $3,200 once you include the ancillary hits. The generator wasn't the problem. My process was.
"The unit wasn't undersized. My question was undersized. I asked 'what's the running load' and never asked 'what does startup actually look like.'"
What I changed after that week
I went back through every order we'd handled that quarter. Two things stood out. First, nobody on our team was consistently pulling startup amperage on motor and welder loads—we were sizing on running watts and calling it a day. Second, we had no arrival inspection step for fuel systems on new units. We trusted the factory sticker.
Now our checklist has a hard gate: no generator goes to a site until someone has answered three questions in writing.
- What is the actual surge load, in amps, at startup? For welders, that means the strike rating, not the running rating. For motor loads, it means locked-rotor amps. On one 2021 job, that question alone pushed us from a Kohler portable generator quote to a larger standby unit—and saved the customer a callback.
- What's the duty cycle? A welder that strikes 40 times an hour behaves nothing like a steady resistive load. A 15 kW propane generator might carry a steady 12 kW all day and still trip on the first strike of a 9 kW welder.
- Has the fuel system been verified on arrival? Every unit gets the fuel pump test before it leaves our yard. If there's a flow problem, we find it before the customer does.
That last item is my private atonement. I wrote the procedure, ignored it, and paid for it. Now it's printed on the inspection sheet in bold. So glad I finally started enforcing it across the team—we've caught 11 fuel-related issues in the past 18 months using that checklist, ranging from loose pump connectors to one unit with a completely dead pump out of the crate.
What I'd tell someone placing their first generator order
If you're the person on the other end of this—maybe you're ordering your first Kohler SDMO generator for a job site, or you're comparing a Kohler portable generator against a bigger standby unit—here's the honest version.
Brand reputation gets you a reliable machine. It doesn't get you the right machine. A Kohler generator is a good starting point, but the sizing math, the surge load, and the fuel system check are on you. The 15 kW propane generator I ordered was a perfectly good unit. It still failed the job because I asked it to do something it wasn't sized for, and I didn't verify the fuel side before handing it over.
I'd rather spend 20 minutes on a load calculation than spend a week explaining to a foreman why his welder won't run. That's not a sales pitch. That's what $3,200 in mistakes taught me.
If you take one thing from this: run the fuel pump test before you trust the unit, and size for the surge, not the running load. Everything else is secondary.
