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.

Inverters calculator

RV Inverter Sizing Calculator with Surge

Quick answer

An inverter must cover the steady running load continuously and the largest single motor startup as a surge. A 600W running load with a 1,800W compressor start needs about 720W continuous and 2,400W of surge.

Result

Inverter size to buy 1000 W continuous

Surge rating needed 2400 W

DC current at running load 58.8 A from the bank

Minimum BMS rating 75 A continuous

The formula

continuous W = running W x headroom; surge W = running W + largest startup W

The DC current result is the number that decides cable size and the battery BMS rating, and it is where most 12V inverter installs go wrong.

The figures shown above are computed with these exact inputs: Running AC load 600 W, Largest startup surge 1800 W, Battery bank voltage 12 V.

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
Inverter Efficiency: 0.85	Convention	Typical pure sine inverter efficiency. Individual units publish their own figure.

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 P2 2000W Pure Sine Inverter

$285.99

At 2000W a 12V inverter can draw close to 200A, so it needs a battery with a BMS rated to match. Renogy publishes that draw.

Best for: A 200Ah lithium bank with a 200A BMS.

Check price on Amazon

Intermediate

BELTTT

BELTTT 2000W Pure Sine Inverter

$161.99

A 4000W surge rating gives headroom for the startup spike of a compressor without nuisance shutdowns.

Best for: A residential fridge plus occasional high draw.

Check price on Amazon

Beginner

LANDERPOW

LANDERPOW 2000W Pure Sine Inverter

$127.49

The lowest cost 2000W pure sine unit here. Note that it is an inverter only, with no charger and no transfer switch.

Best for: Adding inverter capacity where a converter already handles charging.

Check price on Amazon

Frequently asked questions

How does the inverter size calculation work?: continuous W = running W x headroom; surge W = running W + largest startup W. The DC current result is the number that decides cable size and the battery BMS rating, and it is where most 12V inverter installs go wrong.
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.