Sizing a generator for off-grid RV cooling can be challenging. Lights, outlets, the microwave, even the coffee maker usually seem easy enough to account for when estimating power needs. Then the air conditioner’s compressor kicks on, and the generator that looked perfectly capable on paper is stuttering, stalling, and tripping breakers.
The reason is simple: before an RV air conditioner begins drawing its steady running power, it demands a brief but heavy startup surge—and that short burst can be a rude awakening for an undersized generator.
Thankfully, finding a capable unit doesn’t have to be complicated. A simple power audit focused on startup demand, running load, environmental conditions, and whatever else may be drawing power at the same time usually reveals exactly where the safe margin is. This article lays out the key steps so you can enjoy cooler adventures without paying for more wattage than you need.
Decode Your AC’s Appetite
The first step is understanding how hungry your existing (or prospective) air conditioner really is.
RV air conditioners are primarily sold and categorized by their British Thermal Unit (BTU) rating. This number, which can usually be found on a sticker or metal plate, tells you how much cooling capacity the unit provides. Most RV air conditioners are 13,500 BTUs or 15,000 BTUs, with small trailers and camper vans often having units in the 9,000-11,000 range.
Many RV owners assume that an A/C’s BTU rating directly determines what generator size they need, but this isn’t entirely accurate. Generator sizing depends on electrical demand—or wattage—and there are more factors impacting that measurement than just a BTU rating. Compressor design, control electronics, overall A/C efficiency, and of course, the unit’s startup surge all play a role. This is why two units can share the same BTU rating but have very different power demands.
|
BTU Rating |
Running Watts |
Starting “Surge” Watts |
|
9,000 – 11,000 BTU |
900 – 1,200 W |
1,800 – 2,500 W |
|
13,500 BTU |
1,300 – 1,600 W |
2,800 – 3,500 W |
|
15,000 BTU |
1,500 – 2,000 W |
3,500 – 4,500 W |
To find your A/C’s electrical specs, check the unit’s data plate. This is often found beneath the rooftop shroud or near the interior ceiling assembly. If the label lists only amps and volts, you can calculate running watts by multiplying the listed number of amps by 120 volts (e.g., 5 Amps x 120 Volts = 1,800 Watts). For the most comprehensive electrical breakdown—including both running and starting requirements—look in the unit’s operating manual. (The chart above is just an estimate.)
Since an RV A/C’s startup spike can be more than double a unit’s normal running load, it’s important to have this information before shopping for a generator so you can be sure it can handle the surge.
Calculate Your Power Budget
Even though the air conditioner is usually the largest draw in an RV, it is rarely the only one.
A generator sized only for the A/C—even with startup surge factored in—may still struggle if the refrigerator cycles on, the microwave starts, or someone plugs in a hair dryer at exactly the wrong moment. This is why generator sizing works best as a full power budget rather than a single-appliance estimate.
To correctly match a generator to your power needs, start by listing what you realistically expect to run while the A/C is on. A refrigerator operating on electrical power may draw 300-600 watts. Microwaves often need 900-1,500 watts. A television usually stays under 200 watts, while LED lighting contributes very little. Chargers, laptops, and fans may seem minor individually, but together they can quietly add several hundred watts.
Once those numbers are gathered, total them up. Then add in the A/C’s running watts and its startup surge. The calculation will look something like this: (Sum of Other Appliances’ Running Watts) + (AC Running Watts) + (AC Starting Surge) = Total Power Requirement
For example, if the refrigerator uses 400 watts, lights and electronics add 200 watts, and a 13,500-BTU air conditioner runs at 1,500 watts with a 3,000-watt startup surge, the generator must be able to absorb 5,100 watts. (400 + 200 + 1,500 + 3,000 = 5,100)
This doesn’t mean the generator must continuously produce 5,100 watts. It just means it needs enough surge capacity to survive startup while still supporting normal operation afterward—along with a few “buzzkills.”
Factor in Buzzkills
Even when the numbers in your power audit look right, real-world conditions often change them.
Altitude is one of the biggest overlooked variables. Portable generators typically lose about 3% of their output for every 1,000 feet above sea level because thinner air reduces engine efficiency. That means, at 5,000 feet, a generator rated at 3,500 watts may effectively perform closer to 3,000.
Heat creates a second challenge. Air conditioners work harder when outdoor temperatures rise, increasing compressor demand at the same time that the generators themselves become less efficient. The hottest days are often when both systems are working hardest.
Then there is the quieter load many owners forget about: the RV’s converter or battery charger. If the house batteries are low, the converter may quietly pull several hundred watts while recharging in the background.
These buzzkills are why most experienced RVers build a margin into their power budget rather than stopping at the worksheet total. A 20% safety buffer is a practical rule.
So, returning to the example from earlier—if your power audit reveals that you need a generator with at least 5,100 watts to survive startup and support all your electronics, targeting a unit that provides at least 6,100 watts of available surge capacity usually creates a much more reliable setup. That reserve not only improves startup confidence but also prevents the generator from living at full output all day, where factors like heat, noise, and wear can increase.
Consider a Soft-Start Device
One upgrade that can noticeably improve the math without changing the air conditioner itself is a soft-start device. Installed between the power source and compressor, a soft-start reduces the sharp electrical spike that happens during startup by allowing the compressor to ramp up more gradually.
With a soft-start installed, your standard 13,500-BTU air conditioner—which normally needs 2,800-3,500 startup watts—can see its demand reduced by 50-70%! In many cases, that is enough to let a 2,000- to 2,200-watt portable generator start an A/C unit that otherwise would be out of reach.
However, the key word here is start. Once running, the air conditioner still draws its full normal load. If the microwave, hair dryer, or refrigerator join in, a smaller generator can still become overwhelmed. That means RVers using smaller generators with soft-start systems still need disciplined load management. Refrigerator on propane, microwave off, and charging loads monitored carefully often become part of the routine.
For many campers, though, the trade-off is worthwhile. Smaller generators are generally lighter, quieter, easier to transport, and more fuel-efficient than stepping up to a much larger machine.
Choose Your Generator
Wattage is only the first consideration when shopping for a generator. You will also need to navigate choices like fuel type (gas vs. diesel), installation (portable vs. built-in), and engine technology (inverter vs. conventional).
- Gas vs. Diesel: Gas models remain the most common and accessible for most trailers. Diesel units typically appeal to owners of larger motorhomes already running on diesel systems.
- Portable vs Built-In: Portable generators remain a practical choice for many RVers because they cost less upfront and offer flexibility. In contrast, built-in units offer the ultimate convenience of push-button starting from inside the cabin.
- Inverter vs. Conventional: Inverter generators are the gold standard for RVing because they are quieter, more fuel-efficient, and produce “clean” power that is safe for sensitive electronics. Since inverters are digitally protected against overloads, they also benefit the most from a soft-start device, which prevents the A/C’s startup surge from tripping the generator’s safety sensors. Conventional generators offer more raw power for less money, but they are significantly louder and less refined.
Runtime, fuel efficiency, noise level, electric start, low-oil shutdown, carbon monoxide protection, and parallel capability all deserve consideration, too. For RVers ready to dig deeper into those choices, check out our companion guide on How to Find the Best Portable Generator for Your RV.
Save With Parts Via
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