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.

Planning calculator

RV Propane vs Electric Heating Cost Calculator

Quick answer

One gallon of propane contains 91,502 BTU and one kilowatt-hour contains 3,412 BTU. At 80 percent furnace efficiency, a gallon of propane delivers about the same usable heat as 21.5 kWh of electric resistance heat.

Result

Propane cost 5.46 $

Electric cost 4.69 $

Propane used 1.37 gal

Electricity used 29.31 kWh

Cheaper here Electric

The formula

propane gallons = BTU / (91,502 x efficiency); kWh = BTU / 3,412

Electric resistance heat is essentially 100 percent efficient at the appliance, while a propane furnace vents part of its heat outside. That gap is what the efficiency figure captures.

The figures shown above are computed with these exact inputs: Heat needed 100000 BTU, Propane price 4 $ per gallon, Electricity price 0.16 $ per kWh, Furnace efficiency 0.8.

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.

Expert

Progressive Industries

Progressive Industries EMS-PT30X, 30A

$135.89

An EMS disconnects on low voltage, high voltage, open ground and reverse polarity. A plain surge protector only handles a spike.

Best for: Anyone plugging into parks they have not vetted.

Check price on Amazon

Intermediate

Power Watchdog

Power Watchdog PWD30 Bluetooth, 30A

$139.99

Reports actual pedestal voltage and current to a phone, which is how you catch a sagging pedestal before it damages a compressor.

Best for: Watching pedestal quality over a long stay.

Check price on Amazon

Expert

Power Watchdog

Power Watchdog PWD30EPO, 30A, IP65

$305.31

An IP65 body, which matters because a portable unit sits on the ground at the pedestal in the weather.

Best for: A portable unit left outdoors.

Check price on Amazon

Frequently asked questions

How does the propane vs electric heat calculation work?: propane gallons = BTU / (91,502 x efficiency); kWh = BTU / 3,412. Electric resistance heat is essentially 100 percent efficient at the appliance, while a propane furnace vents part of its heat outside. That gap is what the efficiency figure captures.
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.