RV Generator Size Calculator
Work out the running and starting watts your RV generator needs, with headroom and elevation derating.
Work out the running and starting watts your RV generator needs, with headroom and elevation derating. Free, private, and it shows its working.
How to use this tool
Add up the running watts of everything you want on at the same time, then enter the extra starting watts of your largest motor load, usually the air conditioner. The calculator adds headroom and adjusts for elevation.
Look for a generator whose rated (running) output meets the first number and whose peak output meets the second. A soft-start kit on the air conditioner can lower the starting figure.
Worked example
Here is what the RV Generator Size Calculator returns with its starting values. Replace them with your own numbers for a real plan.
| Input | Example value |
|---|---|
| Running watts of everything on at once | 2200 W |
| Extra starting watts of the largest motor load | 1500 W |
| Headroom | 20 % |
| Camping elevation | 0 ft |
| Result | Value |
|---|---|
| Running (rated) watts to look for | 2,700 W |
| Starting (peak) watts to look for | 3,700 W |
| Elevation derate used | 0% |
How the calculation works
- Running need = 2,200 W x (1 + 20% headroom) = 2,640 W.
- Starting need = running 2,200 W + largest surge 1,500 W = 3,700 W.
- Elevation: about 3.5% less output per 1,000 ft (a common rule of thumb) = 0%, so both figures are divided by 1.
When this tool helps most
Use it before you buy or rent a generator, and whenever you add a big appliance.
Common mistakes to avoid
- Sizing to peak watts and forgetting the running rating.
- Ignoring the air conditioner’s starting surge.
- Forgetting that generators lose output at high elevation.
Related guides
This tool is for planning and education only. It does not store your entries. Read our disclaimer.
Understanding your results
- Running watts: what the generator must supply continuously.
- Starting watts: the surge needed to start an air conditioner or pump.
- Altitude derate: generators lose output at elevation.
Two quick scenarios
One air conditioner
Air conditioners need a starting surge well above running watts; a soft-start device can lower it.
High elevation
At mountain campgrounds a generator produces less power, so a unit sized at sea level may struggle.
Before you rely on the numbers
- Never run a generator inside or near open windows.
- Follow campground generator hours.
- Use carbon monoxide alarms inside the RV.
- Check fuel type and storage rules.
Keep planning with RV generator guide and RV Propane Usage Calculator.
Official references
Frequently asked questions
Add the AC’s running watts to everything else on at the same time, then add the AC’s extra starting watts for the peak. Check the AC label or manual for exact figures. Yes. Engines lose power in thinner air; the calculator uses a common rule of thumb of about 3.5% per 1,000 feet. Your generator’s manual gives the exact derating.What size generator do I need for an RV air conditioner?
Does elevation change generator output?