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.

Solar calculator

RV Solar Array Size Calculator

Quick answer

Array watts equals daily watt-hours divided by peak sun hours divided by a system derate. Replacing 1,200Wh a day at 4 peak sun hours with a 0.75 derate needs a 400W array.

Result

Array size needed 400 W

Roughly this many 100W panels 4 panels

Controller current 33.3 A at 12V

Controller rating to buy 50 A

The formula

array watts = daily Wh / peak sun hours / derate

The derate covers panel temperature, soiling, wiring loss and controller efficiency. A flat RV roof cannot be tilted toward the sun, which is why the conservative end is the honest planning figure.

The figures shown above are computed with these exact inputs: Daily use 1200 Wh per day, Peak sun hours 4 h, System derate 0.75.

What this calculator assumes

Some of these are published standards and some are conventions. They are not the same kind of number, so each one is labeled.

Assumption	Kind	Where it comes from
Solar System Losses: 0.75	Convention	Common derate for real-world array output against nameplate rating
Solar Per Ah: 1	Convention	The "100 watts of solar per 100 amp-hours of battery" rule of thumb

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

Renogy

Renogy 400W Premium Solar Kit with 40A MPPT

$637.99

Ships with an MPPT controller sized for the array, which removes the most common sizing error in a self-specified kit.

Best for: A full roof array bought as one part number.

Check price on Amazon

Intermediate

ECO-WORTHY

ECO-WORTHY 400W Kit with 40A MPPT

$449.99

Four 100W panels rather than two 200W ones, which gives more freedom to route around roof vents and air conditioners.

Best for: A cluttered roof that cannot take large panels.

Check price on Amazon

Expert

RICH SOLAR

Rich Solar 400W Premium Kit with 40A MPPT

$649.99

Two 200W panels plus mounting brackets, so the roof work is two panels instead of four.

Best for: An open roof where fewer, larger panels install faster.

Check price on Amazon

Beginner

Mndstek

Mndstek 400W Kit with 30A MPPT

$329.99

The lowest cost route to 400W, though the 30A controller is closer to its limit than the 40A units in this group.

Best for: A 400W array on a 12V bank and nothing more.

Check price on Amazon

Beginner

Renogy

Renogy 100W Starter Kit with 30A PWM

$153.99

A PWM starter kit is the cheapest way onto the roof, and at a single 100W panel the loss against MPPT is small.

Best for: One panel, one battery, lowest possible spend.

Check price on Amazon

Frequently asked questions

How does the solar array size calculation work?: array watts = daily Wh / peak sun hours / derate. The derate covers panel temperature, soiling, wiring loss and controller efficiency. A flat RV roof cannot be tilted toward the sun, which is why the conservative end is the honest planning figure.
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.