RV Generator Size Calculator

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Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

Running watts: The power an appliance uses after it is already on. In the calculator, this feeds Power used after everything is on.

Starting watts: A short burst of power some motors need when they start. Air conditioners, refrigerators, and pumps often need more starting watts than running watts.

Startup burst: The extra part of starting watts above running watts for one appliance. The calculator avoids counting the same running watts twice.

Safety buffer: Extra percent added after the load check. It gives room for small missed loads, hot-weather strain, and normal power dips.

Generator ratings: Continuous/running watts must cover the continuous requirement; starting/peak watts must cover the separate short startup requirement for the required duration. One rating cannot substitute for the other.

RV service limit: The approximate watts your RV inlet or cord can carry, based on volts times amps. This is separate from how large the generator is.

Altitude output factor: The estimated share of sea-level generator output kept at the entered altitude. A factor of 0.79 means about 79 percent is kept.

Mistake: Comparing only one generator rating with the result. Fix: Match running/rated watts to "Minimum continuous rating" and starting/peak watts to "Minimum starting rating". Check the manufacturer's surge duration and appliance starting requirements.

Mistake: Leaving motor loads out of Appliances you want to run , especially an air conditioner or refrigerator.
Fix: Add each item you may run at the same time, with both running watts and starting watts.

Mistake: Entering a starting watt value below running watts in Appliances you want to run because the label lists only one number.
Fix: If the item has no startup burst or you only know one value, use the same value for running watts and starting watts.

Mistake: Treating RV service size as the same thing as generator size.
Fix: Check Approximate RV service limit and Running room left on RV service ; a larger generator cannot push 50 amp power through a 30 amp RV cord.

Mistake: Leaving Camping altitude at zero when you usually camp in the mountains.
Fix: Enter the highest normal camping elevation so Estimated output kept at altitude can raise the generator nameplate estimate.

• The appliance list is only as accurate as the watts you enter. Nameplate labels, manuals, and watt meters can differ from real use.
• The 50 amp RV service estimate assumes a 120/240 V split service and that loads can be shared across both legs. It does not check leg-by-leg balance.
• Altitude loss is a planning estimate. A specific generator manual may use a different derate rate or have fuel, carburetor, temperature, or maintenance limits.
• The startup setting is simplified. Real appliances may start seconds apart, overlap, or use soft-start devices that reduce the surge.
• The common generator size list is a shopping guide, not a standard. Actual models may have different rated watts, surge watts, outlets, noise, and fuel limits.
• This calculator checks watts, not plug shape, breaker coordination, transfer switch safety, grounding, neutral bonding, or code compliance.

How the calculator totals the load

The calculator includes only the appliance rows marked as used at the same time. Running watts are added first. Then the calculator adds either the single largest startup burst or all startup bursts, depending on How to count startup bursts. This follows the practical RV sizing idea that the generator must cover normal running load plus startup surge, often from the largest motor load [2].

running_watts = sum(running_watts_i)

extra_start_i = max(0, starting_watts_i - running_watts_i)

if start_policy = largest_one: starting_peak_watts = running_watts + max(extra_start_i)

if start_policy = all_at_once: starting_peak_watts = running_watts + sum(extra_start_i)

How buffer and altitude change the generator size

The continuous requirement uses running load; the starting requirement uses startup peak. Each is multiplied by the buffer factor and divided by the altitude output factor. This avoids treating a brief motor-starting load as a continuous demand. Confirm the generator's actual derating and surge capability.

needed_watts_before_altitude = max(running_watts, starting_peak_watts) * (1 + safety_buffer_pct / 100)

altitude_output_factor = max(0.5, 1 - (altitude_ft / 1000) * (altitude_derate_pct_per_1000 / 100))

minimum_starting_rating = needed_watts_before_altitude / altitude_output_factor; minimum_continuous_rating = running_watts * (1 + safety_buffer_pct / 100) / altitude_output_factor

"Common generator size" picks a starting-watt comparison class at or above the starting requirement from 2000, 2500, 3000, 3500, 4000, 4500, 5000, 5500, 6500, 7500, 9500, 12000, or 15000 W; above the list it rounds to the next 100 W. "Exact minimum only" retains the calculated starting requirement, subject only to display precision. Neither choice changes the separate continuous requirement.

recommended_generator_watts = first common size >= minimum_nameplate_watts

exact_mode_starting_watts = minimum_starting_rating

How the RV service check works

The RV service check is separate from generator sizing. It estimates the simple watt limit for the selected cord or inlet, then compares that limit with the running load and startup peak.

rv_service_max_watts = amps * volts

running_headroom_watts = rv_service_max_watts - running_watts

starting_peak_over_service_watts = starting_peak_watts - rv_service_max_watts

For this calculator, 20 amp service uses 20 * 120 = 2400 watts, 30 amp RV service uses 30 * 120 = 3600 watts, and 50 amp RV service uses 50 * 240 = 12000 watts. The 50 amp value is a total split-service estimate and does not prove that each leg is balanced.

Mini-example

For a 30 amp RV with 2750 W running load and a largest extra startup burst of 2000 W, startup peak is 4750 W. With a 20 percent buffer at sea level, the requirements are 3300 W continuous and 5700 W starting. The starting-watt comparison class rounds to 6500 W; it does not require 6500 continuous watts. The approximate 3600 W service limit leaves 850 W running headroom, while startup peak is 1150 W above that simple limit. Verify the actual generator, motor-start duration, outlet, and RV service capabilities.