Key takeaways
- Starting watts set the size, not running watts. The biggest startup surge in your rig is the number to design around.
- The air conditioner is almost always the surge. A 13.5k unit runs near 1,400 watts but spikes to 2,800 or more at startup.
- Add the AC surge to everything else running. The generator has to carry your steady loads and absorb the spike at the same time.
- Most boondockers land at 3,000 to 4,000 watts. Enough to start a rooftop AC, run some extras, and charge the bank.
- A soft start is the cheap shortcut. It cuts the AC surge and can drop you to a smaller, lighter, quieter generator.
Running watts and starting watts, and why the difference is everything
Every electrical thing in your rig has two numbers. Running watts are what it draws once it's humming along. Starting watts, also called surge watts, are the brief spike it pulls at the instant it switches on. For most things the two are the same. A coffee maker or a hair dryer is a resistive heater: it draws the same watts the whole time it's on, no spike. But anything with a motor or a compressor, an air conditioner above all, demands a big jolt to get the motor spinning. As the generator-sizing guides put it, the starting wattage of a motor load can be roughly three times its running wattage. Size your generator for the running number and it may run the AC fine once it's going, but it'll stall trying to start it. That's the trap.
Why the air conditioner sets the number
For nearly any rig with rooftop air, the AC is the appliance that decides your generator size, and it isn't close. A 13,500 BTU unit, the most common size, draws roughly 1,300 to 1,600 running watts but surges to 2,500 to 3,000 when the compressor starts. A 15,000 BTU unit runs 1,500 to 1,800 and surges 3,300 to 3,800. The RV-AC sizing guides line up on the practical answer: a 13.5k unit wants a generator of at least 3,000 watts, and a 15k unit pushes you to 4,000 or more, precisely because the generator has to survive that startup spike. Nothing else in a typical rig comes close to that surge. So the honest way to size a generator isn't to total everything up. It's to find your biggest surge, which is almost certainly the AC, and build from there.
| Air conditioner | Running watts | Starting watts | Generator, ballpark |
|---|---|---|---|
| No AC | 0 | 0 | A small inverter does it |
| 11k BTU | ~1,000 | ~2,400 | ~3,000 watts |
| 13.5k BTU | ~1,400 | ~2,800 | ~3,000 to 3,500 watts |
| 15k BTU | ~1,700 | ~3,500 | ~4,000 to 4,500 watts |
The actual math, in plain terms
Here's how the sizing really works, and it's simpler than the spec sheets make it look. First, add up the running watts of everything you'll have on at once: your baseline house load (the converter charging your batteries, the fridge controls, lights, and chargers, usually 250 to 600 watts), plus the AC running, plus any extras like the microwave. That total is what the generator carries steadily. Then add the single biggest startup surge on top of that total, because the worst moment is when your AC compressor kicks on while everything else is already running. That sum is your peak demand. The generator sizing guides recommend adding a margin, usually around 20 percent, so the generator isn't pinned at its ceiling. Round up to a real generator you can buy, and that's your size. The calculator below runs exactly this math on your rig.
The other loads, and what they add
The extras matter, but mostly for the running total, not the surge. A microwave pulls around 1,000 to 1,500 watts while it's on, a coffee maker or kettle 900 to 1,200, a hair dryer about 1,500. Those are resistive, so they spike very little at startup. Power tools are the exception: a motor tool surges like a compressor does. The practical rule is that these extras raise the steady load the generator has to carry, which can nudge you up a size if you run them at the same time as the AC, but they rarely become the surge that sets your number. That stays with the air conditioner. The boondocking generator guides land on the same place most weekend dry campers do: a 3,000 to 4,000 watt inverter generator runs the AC, a few extras, and charges the bank.
The soft start, and how it shrinks the generator
There's a cheap way to beat the surge instead of buying a bigger generator. A soft start is a small device wired into the air conditioner that ramps the compressor up gradually instead of slamming it on, which cuts the startup spike dramatically. The soft-start makers show the effect plainly: with the surge tamed, a 13.5k AC that needed a 3,000 watt generator can often run off a 2,000 to 2,200 watt portable inverter, the small kind people actually want to carry. If your generator is sized entirely by the AC surge, and most are, a soft start is usually the better spend. It buys you a smaller, lighter, quieter generator instead of a bigger, heavier, louder one.
One common story in the boondocking forums goes like this. Someone buys a 2,000 watt inverter generator because it's light and quiet, gets it home, and finds it won't start the rooftop AC. The running watts were fine; the startup surge tripped it. They either return it for a 3,000 watt unit they didn't want to carry, or they add a soft start to the AC and the little generator handles it after all.
The takeaway is the one this page is built around: size for the surge, not the running watts, and decide up front whether you'd rather carry a bigger generator or fit a soft start. Figuring that out before you buy saves a return trip and a heavier box in the bay.
Run your numbers
Size your generator in about 60 seconds
Tell us your rig, your air conditioner, and the extras you'll run. We'll size the generator you actually need, show you the startup surge that sets the number, and tell you whether a soft start could drop you a size.
The mistakes that leave you with the wrong generator
Sizing by running watts. The running total tells you what the generator carries, not what it has to start, so it sizes the generator too small to fire the AC. Ignoring the surge stack. The worst case is the AC starting while the converter and fridge are already running, and that's the number to plan for. Buying the smallest generator because it's light. A 2,000 watt inverter is wonderful until it won't start your air conditioner; check the surge first. Skipping the soft start. If the AC surge is your whole problem, a soft start is cheaper and lighter than the next generator size up. Forgetting it's a backup. The better your solar and battery, the less the generator runs at all, which is the next number to size.
RV generators, frequently asked
What size generator do I need to run my RV AC?
What's the difference between running watts and starting watts?
Will a 2,000 watt generator run an RV air conditioner?
How do I calculate what size generator I need?
Does a soft start really let me use a smaller generator?
What size generator do most boondockers use?
Should I get an inverter generator or a conventional one?
Next: size the solar that keeps it off
A generator is the backup. The less you have to run it, the better the trip sounds and the less fuel you burn. The Solar calculator sizes the panels and the bank that keep the generator off most days, the same way this one sized the generator. About 90 seconds.
Start the Solar calculator →Sources: running and starting watt ranges and the surge-sets-the-size framing from Cummins and Jackery; the sizing method and margin from EcoFlow; boondocking generator sizes from Boondocker's Bible; soft-start effect from Micro-Air.