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.

Planning calculator

RV Daily Energy Budget Calculator (Amp-Hours)

Quick answer

Daily amp-hours is each load's current multiplied by the hours it runs, added together. A 12V fridge at 3.75A for 10 hours of compressor run time, LED lights at 2.5A for 5 hours and a water pump at 5A for half an hour comes to about 52Ah a day.

Result

Daily amp-hours 112.2 Ah per day

Daily energy 1346 Wh per day

DC load energy 640 Wh

AC load drawn from the bank 706 Wh Convention

The formula

daily Ah = sum of (amps x hours per day) for every load

AC loads must be converted to DC amps first, including inverter losses. That is what the inverter efficiency field does.

The figures shown above are computed with these exact inputs: DC loads 80 W running, DC hours per day 8 h, AC loads through inverter 150 W running, AC hours per day 4 h, System 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

Victron Energy

Victron SmartShunt 500A Battery Monitor

Price varies, check the listing

A shunt counts amp-hours in and out, which is the only accurate way to read lithium state of charge. Voltage alone cannot do it on a flat discharge curve.

Best for: Any lithium bank you plan to boondock on.

Check price on Amazon

Intermediate

Renogy

Renogy 500A Battery Monitor with Shunt

$63.10

Includes a wired display head, so you get a reading without needing a phone in your hand.

Best for: A panel-mounted readout.

Check price on Amazon

Intermediate

Litime

LiTime 500A Bluetooth Battery Monitor

$109.99

Covers 8V to 120V, so the same shunt works if you later move the bank to 24V or 48V.

Best for: A build that may change system voltage later.

Check price on Amazon

Beginner

HENGSHAN

AILI TR16 Shunt Battery Monitor

$42.00

The cheapest true shunt-based monitor here, and a cheap shunt still beats an expensive voltmeter for reading lithium.

Best for: A first real state of charge readout.

Check price on Amazon

Frequently asked questions

How does the daily amp-hour budget calculation work?: daily Ah = sum of (amps x hours per day) for every load. AC loads must be converted to DC amps first, including inverter losses. That is what the inverter efficiency field does.
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.