RV Generator Sizing Calculator
Size an RV generator from air-conditioner BTU, microwave, and charging loads, matched to 30A or 50A shore power service.
Common conversions
| Input | Result |
|---|---|
| 30A shore power service | 3,600 W (30A × 120V) |
| 50A shore power service | 12,000 W (2 × 50A × 120V legs) |
| Travel trailer, single 13,500 BTU AC | 3,000-3,600 W generator |
| Class C, dual 13,500 BTU AC | 5,500-7,000 W generator |
| Class A, 2-3 AC + residential fridge | 7,500-12,500 W generator |
| Onboard Onan generator classes | 2,800 / 4,000 / 5,500 / 8,000 W |
| Soft-start kit effect on AC surge | cuts starting draw by ≈65-70% |
| RV microwave, running | 900-1,200 W |
| Converter/charger for house batteries | 600-1,000 W (45-90A charger) |
The math behind it
- AC running watts ≈ 15,000 × 0.293 = 4,395 W
- Add microwave and misc loads: 4,395 + 1,100 + 400 = 5,895 W
- Add AC compressor surge: 5,895 + 1,500 = 7,395 W
Everything you need to know
An RV generator, whether it's a built-in propane or diesel unit under the coach or a portable inverter carried in a truck bed, has to cover the same loads shore power would: the roof air conditioner, the microwave, the converter charging the house batteries, and whatever else is running. The two things that make RV sizing different from a home or job site are the shore power amperage the RV was wired for and the surge from the roof AC's compressor.
Matching generator output to your RV's shore power service
Travel trailers and smaller RVs typically wire to a 30A/120V shore power service, which caps out at 3,600 watts. Larger fifth wheels and Class A motorhomes use a 50A/120-240V split-phase service, which totals about 12,000 watts across its two legs. A generator doesn't necessarily need to match the full shore power rating, since an RV rarely runs every circuit at once, but a unit close to that number lets you use the coach exactly as if it were plugged into a campground pedestal. A 30A trailer is well served by a 3,000-3,600 W generator; a 50A motorhome with two or three AC units realistically needs 7,500-12,500 W to run more than one AC at a time.
Sizing for your roof air conditioner's starting surge
RV roof air conditioners are rated in BTU, and manufacturers typically publish 1,500-1,700 W of running draw for a common 13,500 BTU unit, well below the 0.293-per-BTU estimate used for whole-house or portable sizing, since RV AC compressors are built smaller and more efficient. What actually trips an undersized generator is the compressor's starting surge, not its running draw: that same 13,500 BTU unit can pull a locked-rotor surge of 2,600-3,500 W for a fraction of a second at startup. A soft-start kit installed on the AC compressor cuts that surge by roughly 65-70%, which is often the only change needed to run a 13,500 BTU AC on a 2,200-2,800 W inverter generator that couldn't start it otherwise.
Onboard vs. towable and portable generators
Built-in Onan-style generators run on the RV's propane or diesel tank, start with the push of a dash switch, and stay out of sight in a sealed compartment with its own exhaust and cooling. Portable and towable generators cost less and can be upgraded independently of the RV, but they add setup time, need their own fuel, and have to sit far enough from windows and slide-outs to keep exhaust away from the cabin. Weekend travel trailers most often carry a portable inverter in the 3,000-3,600 W range, while diesel pushers and larger fifth wheels lean toward a built-in unit sized to the coach's full 50A service.
Common applications
Add roof AC + microwave + chargers. A 2,200-3,000 W inverter generator usually covers the load with a soft-start kit on the AC.
Two roof ACs plus a residential fridge typically need 7,500-12,500 W to start both compressors without nuisance trips, matching the coach's 50A service.
A single-AC trailer sized to a 3,000-3,600 W onboard inverter generator lets you avoid hookup sites while still running AC on hot afternoons.
Fifth wheels without a built-in generator often tow a 5,500-8,000 W unit sized to the coach's full 50A shore power draw for extended off-grid stays.
Common mistakes
A 13,500 BTU RV AC compressor can pull 2,600-3,500 W for a fraction of a second at start. Match generator surge spec, not running spec, to this number.
AC running watts plus microwave watts plus a converter charging batteries can exceed 3,600 W, especially the instant the AC compressor kicks in, tripping the pedestal or generator breaker.
Few RVs run every 50A circuit at once. Sizing a towable generator to the full 12,000 W nameplate instead of actual simultaneous use adds needless weight, cost, and fuel burn.
Onboard and portable generators both need their exhaust kept away from windows, doors, and slide-out seals. Carbon monoxide can enter the cabin even from a generator mounted underneath the coach.
Frequently asked questions
What size generator do I need to run my RV's air conditioner?+
Most single roof AC units (13,500-15,000 BTU) need a 3,000-3,600 W generator, or as little as 2,200-2,800 W with a soft-start kit installed on the compressor.
Can a 2,200 W generator run a 13,500 BTU RV air conditioner?+
Only with a soft-start kit. Without one, the compressor's starting surge of 2,600-3,500 W will overload a 2,200 W unit. With a soft-start kit cutting that surge by roughly two-thirds, a 2,200 W generator can start and run the AC.
What's the difference between 30A and 50A RV shore power?+
A 30A service is a single 120V leg totaling 3,600 W; a 50A service is two 120V legs (240V split-phase) totaling about 12,000 W. 50A RVs typically run more than one AC unit and larger appliances at once.
Can I run my RV's AC and microwave at the same time on a 30A generator?+
Often no. A 13,500 BTU AC draws roughly 1,500-1,700 W running plus a 2,600+ W surge at start, and a microwave adds another 900-1,200 W; running both together during the AC's start can exceed a 30A (3,600 W) generator's capacity, especially if a water heater or converter is also drawing power.
Do I need a bigger generator for a Class A motorhome than a travel trailer?+
Yes, usually. A travel trailer with one AC unit is fine on 3,000-3,600 W, while a Class A with two or three roof AC units and a residential fridge typically needs 7,500-12,500 W to run more than one AC at a time.
What does a soft-start kit do for an RV air conditioner?+
It reduces the AC compressor's starting surge by about 65-70%, which lets smaller inverter generators, or even a battery/inverter system, start the compressor that would otherwise trip a small generator's breaker.
Is a built-in generator better than a portable one for an RV?+
It depends on use. Built-in propane or diesel generators start instantly and store out of sight but cost more upfront; portable inverter generators cost less and can be swapped or upgraded but need setup, their own fuel, and careful placement away from windows.
How many watts does an RV converter or battery charger draw?+
A typical 45-90 amp converter/charger draws roughly 600-1,000 W while actively charging depleted house batteries, dropping close to zero once the batteries reach full charge.
Can I use a 50A to 30A dogbone adapter to run a 50A RV on a smaller generator?+
Yes, but it limits the whole RV to that single 30A leg, meaning only about 3,600 W total is available, not the RV's full 50A capacity, so plan to run one AC and skip other high-draw appliances at the same time.
How much fuel does an RV generator use per hour?+
A 4,000-5,500 W propane generator running an AC typically burns 0.5-1 gallon of propane per hour at that load, so a standard 20-30 lb tank lasts roughly 8-15 hours of AC use.