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.

Batteries calculator

RV Battery Run Time Calculator

Quick answer

Run time is usable amp-hours divided by the load in amps. A 100Ah LiFePO4 battery at 80 percent usable running a 5A load lasts about 16 hours. The same nameplate in lead-acid lasts closer to 8 hours because only half is usable.

Result

Run time 32 hours

Usable capacity 160 Ah

Peukert correction applied No

The formula

hours = (nameplate Ah x depth of discharge) / load amps

For lead-acid a Peukert correction is applied, because lead-acid delivers less total capacity as the discharge rate rises. LiFePO4 is close enough to linear that applying it would imply precision that is not there.

The figures shown above are computed with these exact inputs: Bank nameplate capacity 200 Ah, Load 5 A, Chemistry lithium.

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
Lithium Usable Dod: 0.8	Convention	Common planning figure. Individual LiFePO4 manufacturers publish their own usable depth and cycle life.
Lead Acid Usable Dod: 0.5	Convention	Widely repeated planning guideline, not a published specification
Peukert Lead Acid: 1.25	Published standard	Peukert exponent, typically 1.1 to 1.3 for lead-acid. Published per battery by some manufacturers.

Best for: Budget-led single battery replacement.

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Frequently asked questions

How does the run time at load calculation work?: hours = (nameplate Ah x depth of discharge) / load amps. For lead-acid a Peukert correction is applied, because lead-acid delivers less total capacity as the discharge rate rises. LiFePO4 is close enough to linear that applying it would imply precision that is not there.
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.