Are these figures published standards or rules of thumb?

Both, and the page labels which is which. Conductor ampacity, resistance, voltage drop limits, overcurrent sizing factors and device ratings come from NEC, ABYC and NEMA and are labeled as published standards. Depth of discharge, solar derate factors and inverter efficiency are planning conventions with no standards body behind them, and are labeled as conventions.

Why does this calculator sometimes say there is no answer?

Because there genuinely is not one within the published tables. Rather than extrapolating past the end of an ampacity table or inventing a figure to fill a gap, the calculator returns nothing and explains why. A run that is too long for the current at 12 volts needs a shorter run or a higher system voltage, not a made up wire size.

Do I still need an RV technician if I use this?

For DC work many owners do their own installation, and these figures are the ones that work is sized from. Permanent AC wiring, anything connected to the shore power inlet and anything behind the distribution panel is work for a qualified RV technician or a licensed electrician. Sizing a circuit correctly and installing it safely are two different jobs.

Generators calculator

RV Generator Size Calculator by Startup Watts

Quick answer

A generator is sized by the largest startup surge, not by running watts. A 13,500 BTU RV air conditioner draws about 1,400W running but roughly 3,000W to start, so a 2,000W generator will not start it without a soft starter.

Result

Generator starting watts needed 4800 W

Size to buy 5000 W starting

Continuous output required 1800 W

Surge allowed for 3000 W

The formula

generator watts = total running watts + largest single startup surge

A soft starter cuts air conditioner inrush substantially, which is often cheaper than buying a bigger generator.

The figures shown above are computed with these exact inputs: Total running load 1800 W, Largest startup surge 3000 W, Soft starter fitted no.

Components that match this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

Intermediate

WEN

WEN 56477i 4800W RV-Ready Inverter Generator

$634.99

RV-ready means it carries a TT-30R outlet, so a 30A rig plugs in without an adapter. Inverter output is also clean enough for electronics.

Best for: A 30A rig that needs to run one air conditioner off grid.

Check price on Amazon

Expert

WEN

WEN 6800W Dual Fuel RV-Ready Inverter Generator

$798.00

Dual fuel plus a CO shutoff sensor. Carbon monoxide detection is the feature to insist on with any generator used near a rig.

Best for: A large rig or two air conditioners.

Check price on Amazon

Intermediate

ERAYAK

ERAYAK 4500W Dual Fuel RV-Ready Inverter Generator

$559.00

A 30A RV outlet and dual fuel operation, so it can run on propane you already carry for the furnace.

Best for: Running on the propane already onboard.

Check price on Amazon

Beginner

Pulsar

Pulsar PGD40ISCO 4000W Inverter Generator

$450.07

Light enough for one person to move, which decides whether a generator gets used or stays in the garage.

Best for: Occasional use where weight matters most.

Check price on Amazon

Frequently asked questions

How does the generator size calculation work?: generator watts = total running watts + largest single startup surge. A soft starter cuts air conditioner inrush substantially, which is often cheaper than buying a bigger generator.
Are these figures published standards or rules of thumb?: Both, and the page labels which is which. Conductor ampacity, resistance, voltage drop limits, overcurrent sizing factors and device ratings come from NEC, ABYC and NEMA and are labeled as published standards. Depth of discharge, solar derate factors and inverter efficiency are planning conventions with no standards body behind them, and are labeled as conventions.
Why does this calculator sometimes say there is no answer?: Because there genuinely is not one within the published tables. Rather than extrapolating past the end of an ampacity table or inventing a figure to fill a gap, the calculator returns nothing and explains why. A run that is too long for the current at 12 volts needs a shorter run or a higher system voltage, not a made up wire size.
Do I still need an RV technician if I use this?: For DC work many owners do their own installation, and these figures are the ones that work is sized from. Permanent AC wiring, anything connected to the shore power inlet and anything behind the distribution panel is work for a qualified RV technician or a licensed electrician. Sizing a circuit correctly and installing it safely are two different jobs.

Researched guidance, not professional advice. These results are planning figures from published standards and manufacturer specifications. Never exceed a conductor's rated ampacity, fuse every positive conductor close to the battery, never run a generator in an enclosed space because carbon monoxide is lethal and odorless, and have permanent AC wiring done by a qualified RV technician or licensed electrician.