Somebody asks this in one of the truck groups about every week, and they get a pile of answers. "Get a 7,000." "I've run a 3,500 for four years, it's fine." "Buy bigger than you think." Everybody answering is telling the truth about their own truck.
That's the problem with it. A guy cooking on propane with one fridge can run all day on a little inverter unit. A truck with two electric griddles and a fryer needs three times that. Both of them will tell you what worked for them, and neither one has any idea what's bolted into your truck.
And buying wrong is expensive in a specific way. Too small doesn't show up in the driveway. It shows up at noon on a Saturday when a compressor kicks on, the lights dip, and the POS reboots. Then you're buying the right one, and now you've paid for two generators and lost a rush.
The short answer
Most trucks land somewhere between 3,500 and 7,000 running watts.
If you cook on propane and the generator only has to carry refrigeration, lights, a POS, and the hood fan, 3,500 to 5,000 watts usually does it. Move the cooking to electric griddles and fryers and you're looking at 8,000 watts and up, sometimes 12,000 or more, and a much bigger, thirstier machine to go with it. Work at elevation and add to whatever number you had.
That's a range, which is about all anybody can give you without seeing your equipment list. If you want the actual number for your truck, I built a calculator for it. You check off your equipment, put in your elevation, and it does the math below. It's here: food truck generator sizing calculator.
Propane or electric decides most of it
Before you count anything, look at what you cook with. Griddles, fryers, ovens, ranges. If they burn propane, they're not on the generator's tab at all, and the generator's job gets small fast: cold stuff, lights, a fan, the register. If they're electric, they're the biggest thing on the truck by a mile. One 240V fryer can pull more than everything else combined.
One catch. Propane gear still has electric parts sometimes. Ignition, a control board, and on some ovens a blower. The blower is a motor, and motors surge, so count it.
Running watts and starting watts
This is the part that gets people. A reach-in fridge might say 700 watts on the label. The instant its compressor cycles on, it can yank two or three times that for a second, and if the generator can't cover the spike it browns out or trips. It'll pick the rush to do it, too.
Heating equipment doesn't do this. A griddle or a fryer or a coffee brewer is a heating element, and it pulls about the same starting as running. It's the motors that spike: compressors, fans, pumps, blenders.
People get this wrong in the other direction, though. You don't add up every surge on the truck. On mine, everything technically comes on at once, but each compressor's on its own thermostat and they fire at different moments, and the spike is over in a second or two. So you size for your running total plus the single biggest surge. Sizing for all of them landing in the same instant means buying a way bigger generator to cover something that basically doesn't happen, and nobody does that.
The one time it can happen is a cold start with every box warm. The fix is free. Turn the refrigeration on one unit at a time and let each one settle before the next.
The number on the front of the box
It's usually the peak number. A generator sold as 4,500 watts might be rated for 3,500 running, and running is the one you live on. Find the spec sheet and check the continuous rating. If it's dual fuel and you plan to run propane, check the propane rating too. It's often about 10% lower than the gasoline number.
While you're on the spec sheet, look at the per-leg amps if it's a 120/240V unit. It splits its capacity across two 120-volt sides, so you can be under the total watts and still overload one side because all your 120V stuff landed on the same leg. Spread it out.
Altitude
I'm in Colorado, so I'm parked above 5,000 feet most days, and this is the part the sea-level answers skip. Small engines lose power in thin air. A common published rule is about 3.5% per 1,000 feet, plus about 1% for every 10 degrees over 77°F. At 5,000 feet on a hot afternoon that's close to a fifth of the box's number gone. Go work a mountain town at 8,000 or 9,000 feet and it's more.
So a unit that's plenty in Texas can be marginal here. Check your generator's manual for its own derating, and ask whether it needs a high-altitude jet. Some do, and they run rough without it.
If you've got a meter, use it
I've measured my truck at somewhere around 2,000 watts when everything's just running normally. Adding up every nameplate on the truck gives a way bigger number than that, and both numbers are right. The nameplate total is the moment everything's on at once. The meter is the average while compressors and thermostats cycle on and off, which is what a normal hour of service actually looks like.
The 80% rule you'll hear about, keeping the generator under 80% of its rating, is about that meter number. Your typical draw should sit under 80%. The everything-on moment just has to fit, with a little to spare. If you're comparing a meter reading to the calculator, compare it to the typical draw line, not the everything-on line.
What I'd tell you to do
Walk the truck with your phone and photograph every nameplate. Back of the fridge, inside the door, the sticker on the fan. That list is the answer. The group can't see it.
Decide propane or electric for cooking first, because that's most of the number.
Add up the running watts, add the biggest single surge, and check that against the running rating on the spec sheet, at your elevation.
If you're near the edge, go up one size. Not two.
Turn your refrigeration on one box at a time.
And a plain safety line, because everything above is only about electrical capacity. Generators go outside, away from doors and windows and wherever people are working, and they make carbon monoxide. Anything hard-wired or 240V, get an electrician.
The calculator does the adding for you, with the surge and the elevation figured in, and every number in it can be changed, so your nameplate always wins. It's free and there's nothing to sign up for.