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

LiFePO4 vs Lead-Acid Usable Capacity Calculator

Quick answer

A 100Ah lead-acid battery gives about 50Ah of usable capacity, while a 100Ah LiFePO4 battery gives about 80Ah. Matching 80Ah of usable lithium capacity takes 160Ah of lead-acid nameplate, which is roughly 110 pounds instead of 25.

Result

LiFePO4 usable 80 Ah

Lead-acid usable 50 Ah

Lead-acid nameplate needed to match 160 Ah

LiFePO4 stored energy 960 Wh usable

The formula

usable Ah = nameplate Ah x depth of discharge

Depth of discharge is a convention on both chemistries. What is NOT a convention is that the two numbers differ, which is why nameplate comparisons between chemistries are meaningless.

The figures shown above are computed with these exact inputs: Nameplate capacity 100 Ah.

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.

Check price on Amazon

Frequently asked questions

How does the usable capacity compared calculation work?: usable Ah = nameplate Ah x depth of discharge. Depth of discharge is a convention on both chemistries. What is NOT a convention is that the two numbers differ, which is why nameplate comparisons between chemistries are meaningless.
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.