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 Bank Size Calculator

Quick answer

Battery bank size is daily amp-hours multiplied by days off grid, divided by the usable depth of discharge. At 60Ah a day for two days, a lithium bank planned to 80 percent depth needs 150Ah nameplate, while a lead-acid bank planned to 50 percent needs 240Ah.

Result

Nameplate capacity needed 150 Ah

Usable energy required 120 Ah

Stored energy 1440 Wh usable

Planned depth of discharge 80 % Convention

The formula

nominal Ah = (daily Ah x days) / depth of discharge

Depth of discharge is a planning convention, not a published specification. The nameplate figure on a battery is not the figure you get to use.

The figures shown above are computed with these exact inputs: Daily use 60 Ah per day, Days off grid 2 days, 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

Best for: Budget-led single battery replacement.

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Frequently asked questions

How does the battery bank size calculation work?: nominal Ah = (daily Ah x days) / depth of discharge. Depth of discharge is a planning convention, not a published specification. The nameplate figure on a battery is not the figure you get to use.
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.