What Size Generator for an RV?
A 13,500 BTU roof air conditioner runs at about 1,400 watts and surges to roughly 3,500. That surge, not the running figure, is what decides your generator.
Size power for your rig
Generator
7,500 W generator
Needs at least 4,200 W continuous and 5,450 W to start.
Battery power station
22,400 Wh
5 × 5,000 Wh and up (expandable) — needs 3,350 W of inverter and 5,450 W of surge.
A rv weekend running for 10 hours draws 3,350 W with everything on, peaks at 5,450 W when the rv air conditioner (13,500 btu) starts, and uses 16.6 kWh over the session.
What this assumes
- Running watts assume everything can be switched on at the same time. That is the safe way to size a generator, and it is why the number looks high.
- Starting watts are the running total plus the single largest start-up surge — not every surge added together, because two motors rarely start in the same instant.
- Energy uses each item's duty cycle: a fridge compressor runs about a third of the time, an espresso boiler idles at a fraction of its element rating between shots.
- Battery sizing allows 15% for inverter losses and keeps 15% in reserve so you do not finish the day at empty. In the cooler bands it also allows for the capacity lithium gives up, which is about 85% of rated around freezing and roughly 60% in severe cold.
- Generator sizing never plans to sit above 80% of the continuous rating, which is the same thing as adding 25% to your running total. In the warmer bands it adds roughly 2% per 10 °F, or per 5.5 °C, above 77 °F (25 °C) for the output an engine loses in hot air.
- Fuel burn is modelled from published inverter-generator run times and is accurate to roughly ±20% in the real world.
The roof air conditioner decides everything
Every RV power question comes back to one appliance. A 13,500 BTU roof unit draws 1,400 W once it is running, which almost any generator can carry. Starting it is the problem: the compressor demands 3,500 W for a fraction of a second, and a generator that cannot supply that will stall, trip, or simply refuse while its own display insists it has capacity to spare.
This is why the answer to "will a 2,000 watt generator run my air conditioner" is almost always no, and why the answer changes the moment you fit a soft start kit. A soft starter ramps the compressor up instead of slamming it on, cutting the surge to about 1,800 W while leaving the running draw untouched. A part that costs a few hundred dollars replaces a generator upgrade that costs considerably more, and it is quieter and lighter to carry.
Everything in an RV, measured
| Equipment | Running | Starting | Average | Energy per hour |
|---|---|---|---|---|
| RV air conditioner (13,500 BTU) | 1,400 W | 3,500 W | 980 W | 980 Wh |
| RV air conditioner (15,000 BTU) | 1,700 W | 4,000 W | 1,200 W | 1.19 kWh |
| RV air conditioner with soft start (13,500 BTU) | 1,400 W | 1,800 W | 980 W | 980 Wh |
| RV converter / battery charger | 450 W | 450 W | 280 W | 280 Wh |
| RV water pump | 60 W | 180 W | 6 W | 10 Wh |
| RV fridge (absorption, on electric) | 330 W | 330 W | 200 W | 200 Wh |
| RV fridge (12 V compressor) | 60 W | 240 W | 25 W | 30 Wh |
| RV water heater (electric element) | 1,400 W | 1,400 W | 280 W | 280 Wh |
| RV microwave | 1,000 W | 1,000 W | 85 W | 80 Wh |
| RV TV and satellite receiver | 90 W | 90 W | 90 W | 90 Wh |
| 12 V electric cooler | 45 W | 180 W | 20 W | 20 Wh |
Notice the two fridges. An absorption fridge running on its electric element draws 330 W steadily, while a 12 volt compressor fridge averages a fraction of that. If you boondock, that difference is the single biggest lever on your battery bank, far more than lights or devices.
Which generator clears the surge
Here is every portable class against the 13,500 BTU roof unit's 3,500 W starting requirement, on its own with nothing else running.
| Generator | Running rating | Starting rating | Starts a 13,500 BTU unit? |
|---|---|---|---|
| 1,000 W inverter | 900 W | 1,000 W | No |
| 2,200 W inverter | 1,800 W | 2,200 W | Only with a soft start |
| 3,500 W inverter | 3,000 W | 3,500 W | Yes |
| 5,000 W | 4,500 W | 5,000 W | Yes |
| 7,500 W | 7,000 W | 9,000 W | Yes |
| 9,500 W | 8,500 W | 11,000 W | Yes |
| 12,000 W | 11,000 W | 15,000 W | Yes |
| 15,000 W | 12,000 W | 15,000 W | Yes |
That last column is the whole argument for a soft starter in one place. Two classes of generator move from "no" to "yes" for the price of a part, and both of them are quieter and lighter than the class you would otherwise have to buy.
Know your service before you shop
A 30 amp RV hookup is 120 volts on a single leg, which is 3,600 watts in total. People see the chunky plug and assume more. Compare that ceiling against the table above: roof air plus a microwave plus a converter charging flat batteries will exceed it, and the pedestal breaker will let you know.
A 50 amp hookup is a genuinely different animal, with two 120 volt legs for 12,000 watts. If you are choosing a generator to substitute for shore power, size it against the loads you actually run at once, not against the service rating, or you will buy far more generator than your rig ever draws.
Altitude, heat and the campground rules
Two things thin the air an engine breathes, and both cost you output. Heat is the one the calculator handles: pick the temperature band you will actually run in, and a hot afternoon takes roughly 2% off the engine for every 10 °F (5.5 °C) above 77 °F (25 °C). Altitude is the one you apply yourself, at about 3.5% per 1,000 feet above 3,000 feet, so a 3,500 watt generator at a 9,000 foot campground behaves like roughly a 2,800 watt one. At elevation, size one class up from the recommendation.
Heat cuts both ways in an RV, which is why the temperature band matters more than people expect. On a 95 °F (35 °C) afternoon the engine is weaker and the roof air is working far harder than its duty cycle assumes, so the load goes up at exactly the moment the supply goes down.
Quiet hours are the other constraint that is not electrical at all. Most parks ban generators overnight, which is the strongest argument for pairing a modest generator with a battery: run the engine during the day to charge, and run the fan, the fridge and the devices off the battery in silence after dark. For the arithmetic behind that trade, see generator or battery power station, and for the fuel side, how long a generator runs on a tank.
Frequently asked questions
Will a 2,000 watt generator run my RV air conditioner?
Usually not, and the reason is the surge rather than the running load. A 13,500 BTU roof unit runs at about 1,400 watts, which fits comfortably, but it demands roughly 3,500 watts for the instant its compressor starts, and a 2,200 watt inverter generator tops out around 2,200. Fit a soft start kit and the surge drops to about 1,800 watts, at which point the same generator handles it.
How much is 30 amp RV service in watts?
A 30 amp RV connection is single phase at 120 volts, so it is 3,600 watts in total, not the 7,200 people sometimes assume from the two prongs. A 50 amp service is genuinely different: it is two 50 amp legs at 120 volts, giving 12,000 watts. That distinction decides whether you can run the air conditioner and the microwave at the same time.
Do two small generators work as well as one big one?
Running two inverter generators in parallel is a common and effective answer. Two 2,200 watt units give roughly 3,600 to 4,400 watts of starting capacity, enough for a roof air conditioner plus the rest of the rig, while staying much quieter than a single conventional generator of that size. You also keep the option of carrying just one for trips where you do not need air conditioning.
Does altitude really affect my generator?
Yes, and mountain campgrounds are exactly where it bites. Engines lose roughly 3.5% of their output for every 1,000 feet above about 3,000 feet, so at 9,000 feet a 3,500 watt generator behaves closer to 2,800. The calculator does not ask for altitude, so at elevation size one class up from what it recommends, or knock about a third off a 9,000 foot figure before you compare it against a generator label.
Can a battery power station run my roof air?
For a short stretch. A 13,500 BTU unit averages around 1,000 watts once its cycling is taken into account, so a 2,000 watt-hour battery gives you well under two hours of cooling and nothing else. Batteries are excellent in an RV for the fridge, the water pump, lights, fans and devices overnight. Air conditioning is the one load that genuinely wants an engine or a shore connection.
What about the converter charging my house batteries?
It is easy to overlook and it runs for hours. An RV converter draws around 450 watts while it is bulk charging depleted house batteries, which is a meaningful share of a small generator's capacity. If your generator is marginal, switch the converter off while the air conditioner starts, then let it charge once everything is settled.