List what runs together
Do not enter every appliance you own. Enter the loads that might overlap on the same hot afternoon.
Generator wattage planning
RV generator size calculator
Estimate the generator class needed for RV air conditioning, battery charging, kitchen loads, startup surge, altitude derating, and real planning margin.
Quick generator read
This generator is undersized for the entered overlap
After the 7.5% altitude derate, the entered generator has about 3,238W running and 4,163W surge available. The entered overlap needs about 3,450W running and 4,700W at startup before buffer.
Recommended class
4,500W running / 6,000W surge target
3,500W running / 4,500W surge entered
Running target
3,967W
3,450W before buffer; 3,238W available after derate
Startup target
5,405W
4,700W before buffer; 7.5% altitude derate
This is load-overlap sizing only. Confirm generator manual ratings, exhaust clearance, grounding, transfer equipment, fuel handling, and CPSC carbon-monoxide guidance before running a generator.
Add charger draw
If the batteries will charge while the AC or microwave runs, the converter or inverter-charger is part of the load.
Derate before the trip
Use altitude and a real planning buffer before you decide a generator class is safe enough.
Start over
This calculator stores inputs locally in this browser. Clear saved inputs when stale values are getting in the way.
Generator size calculator
Check whether a generator can carry the overlap you actually use.
Add the AC loads that may run together, include battery-charger draw, then derate the generator for altitude. This keeps the question practical: can it start and keep running the stack you plan to use?
Simultaneous AC loads
Include loads that can overlap. Leave out anything you promise to turn off.
Generator size check
This generator is undersized for the entered overlap
After the 7.5% altitude derate, the entered generator has about 3,238W running and 4,163W surge available. The entered overlap needs about 3,450W running and 4,700W at startup before buffer.
Review CPSC generator carbon-monoxide safety guidanceRunning load
3,450W
3,967W with buffer
Startup load
4,700W
5,405W with buffer
Altitude derate
8%
3,238W effective running watts
Recommended class
4,500W
6,000W surge target
Margin after buffer
Running margin
-729W
107% raw load on derated running watts
Surge margin
-1,242W
113% raw load on derated surge watts
Watch-outs
This is a planning calculator. Final generator selection should follow the generator manual, appliance labels, transfer switch limits, grounding instructions, ventilation requirements, and campground or public-land rules.
Startup math assumes the listed running loads are already on and the single largest startup surge happens next. If two compressors or motors start at the same time, real surge can be higher.
Altitude derating varies by generator model and fuel system. Use the manual's derate table when you have it, then adjust the derate input here.
Startup surge is driving the recommendation. A soft-start device or changing load timing may do more than buying a much larger generator.
Recommended next move
Move one load out of the overlap, reduce charger watts, add a soft-start device for compressor loads, or shop in the recommended generator class before relying on this setup.
Why this exists
Generator size is about load overlap, not just the biggest appliance.
RVers often ask whether a 2,200W or 3,500W generator is enough for air conditioning, but the better question is what else runs at the same time. A converter, microwave, electric water heater, or high-elevation campsite can erase the headroom that looked fine on a spec sheet.
This is sizing math, not installation approval.
Still check the generator manual, RV manual, transfer equipment, exhaust clearance, grounding, and fuel handling before wiring or running anything.
Use this with
Generator runtime calculator
Use this after wattage looks workable to estimate fuel, charging hours, and quiet-hour fit.
Air conditioner runtime calculator
Use this when the generator result makes you reconsider running AC from batteries instead.
Inverter size calculator
Use this to compare the same appliance stack against inverter surge and DC current.
Generator vs solar guide
Use this when you are deciding whether generator capacity, solar harvest, or battery storage is the better fix.
Tool notes
What the generator size estimate is actually saying
This output helps you decide whether a generator class has enough running and startup margin for the loads you plan to overlap. It does not approve a wiring method, transfer switch, fuel setup, or installation.
Running watts
The calculator adds battery-charger draw to every listed AC load that may run at the same time.
Startup watts
Startup demand is estimated as the full running stack plus the largest single startup surge delta.
Altitude and buffer
Generator running and surge ratings are derated by the entered altitude factor, then compared against a buffered planning target.
Avoid these traps
Common mistakes before buying
Shopping by surge watts only
Surge watts help start motors, but air conditioning, battery charging, and microwave use are sustained loads. Running watts decide whether the generator keeps up.
Forgetting converter or inverter-charger draw
If the batteries are charging while the AC or microwave is running, the charger is part of the generator load.
Testing at home and camping at elevation
A generator that starts the AC near sea level can feel much smaller at 7,000 feet. Derate before the trip, not after the first hot afternoon.
Treat the calculator result as a planning range, then verify the relevant manufacturer guidance, safety limits, installation requirements, and local rules before changing the rig.See assumptions
Gear to compare after the math
Spec-checked products to compare after the math.
These handoffs match the calculator family, not a one-click prescription. Verify fit, specs, clearances, and install limits before buying.
SOK SK12V100P 12V 100Ah LiFePO4 Battery
Best for
Budget lithium builds after a smaller bank result
A common 100Ah building block when the calculator result still fits a straightforward 12V lithium bank.
Current listing
SOK SK12V100P 12V 100Ah LiFePO4 Battery at SOK.
- Checked model
- SK12V100P
- Spec fit
- A 100Ah LiFePO4 building block for smaller 12V banks when the calculator result is framed around modular battery count and charger compatibility.
Victron SmartShunt 500A/50mV
Best for
Making battery runtime visible instead of guessed
A shunt-based monitor is the usual next buy when the calculator result depends on real state-of-charge tracking.
Current listing
Victron SmartShunt 500A/50mV at Victron.
- Checked model
- SmartShunt 500A/50mV
- Spec fit
- A shunt-based monitor handoff when calculator results depend on real state-of-charge and consumed amp-hours instead of voltage guessing.
Victron Orion XS 12/12-50A DC-DC Battery Charger
Best for
Improving recharge support for larger daily loads
Use this class of DC-DC charger when the battery answer only works if drive-day recovery is part of the plan.
Current listing
Victron Orion XS 12/12-50A DC-DC Battery Charger at Victron.
- Checked model
- Orion XS 12/12-50A
- Spec fit
- A 50A DC-to-DC charger handoff when route-day charging is part of the battery recovery plan and alternator reserve is verified.
Frequently asked
Questions RVers usually ask next.
Should I use running watts or surge watts from the generator box?
Use running watts for the generator rating and surge watts for the temporary startup rating. If the box only advertises one big number, check the manual or spec label for continuous output before trusting it.
What should I enter for battery charger draw?
Use the AC input draw from your converter, inverter-charger, or charger manual if you have it. If not, estimate DC charge watts divided by about 0.82 to 0.90 efficiency, then round up.
Does a soft-start device change the result?
Yes. A soft-start device can reduce the startup surge of a rooftop air conditioner, which may move a setup from surge-limited to running-watt-limited. It does not reduce the AC's normal running watts very much.
Is this a safe generator installation plan?
No. This is sizing math for load overlap. Follow the generator, transfer switch, RV, and appliance manuals for grounding, neutral bonding, ventilation, exhaust clearance, fuel handling, and connection details.