How do I work out what size battery bank my RV needs?

Start from daily use rather than from the battery. Add up every load in amp-hours per day, multiply by the number of days you want between charges, then divide by the fraction of the battery you are willing to use. That last step is what people miss: a 60 amp-hour daily load does not need a 60 amp-hour battery, it needs roughly 150 amp-hours of LiFePO4 or 240 of lead-acid for two days off grid.

Is a 50 amp RV service twice as much power as 30 amp?

No, it is more than three times. A 30 amp RV service is one 120 volt leg at 30 amps, which is 3,600 watts. A 50 amp service is two separate 120 volt legs at 50 amps each, which is 12,000 watts. The plug is larger because it carries two legs, not because it carries higher voltage to your appliances.

Does a 50 to 30 amp adapter give me more power?

No. An adapter changes the plug shape and nothing else. A 30 amp rig on a 50 amp pedestal still has 30 amps, because the rig inlet, cord and main breaker are all 30 amp rated. A 50 amp rig on a 30 amp pedestal has 30 amps total for the whole coach. Nothing about an adapter increases available amperage.

What do I have to change when converting an RV to lithium?

The charging, in three places. The converter needs a genuine lithium profile or the bank will never reach full charge. The solar charge controller needs its profile set for LiFePO4 rather than left on a lead-acid default. And if you charge from the engine, a DC-DC charger is required, because lithium will draw more current than an alternator can survive if connected directly.

Why do you double the run length when sizing wire?

Because current has to return. It flows out along the positive conductor and back along the negative, dropping voltage in both. A 10 foot one-way run is 20 feet of conductor as far as voltage drop is concerned. Forgetting this halves the calculated drop and undersizes the cable by a full step or more, which is the most common RV wiring error there is.

Can I run my air conditioner off batteries?

It depends far more on the startup surge than on the running load. A 13,500 BTU rooftop unit runs on about 1,400 watts but needs roughly 3,000 watts to start, which a 2,000 watt inverter cannot supply. A soft starter cuts that inrush substantially and is usually cheaper than upgrading the inverter, the bank and the cabling to suit the original figure.

RV electrical power, sized properly

Size the system before you buy the parts

Free calculators for RV house batteries, solar, inverters, converters, shore power and wiring. Every one shows its formula, separates published standards from rules of thumb, and returns nothing rather than a guess when your numbers fall outside the tables.

Start with your daily load All 16 calculators Complete builds

Calculators: 16
Sizing answers: 262
Verified components: 102
Invented figures: 0

Quick answer: what order to do this in

Measure your daily amp-hour use first, because every other number depends on it. Then size the battery bank from that figure and how many days you want off grid, then the solar array or DC-DC charger that refills it, then the inverter for whatever AC loads you run, and last the wire and fuses that connect it together. Sizing in any other order means redoing work.

Start with a calculator

Planning

Daily amp-hour budget

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

Batteries

Battery bank size

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

Solar

Solar array size

array watts = daily Wh / peak sun hours / derate

Inverters

Inverter size

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

Wiring

Wire gauge and voltage drop

drop volts = 2 x run length (ft) x amps x (ohms per 1000 ft / 1000)

Shore power

30A vs 50A budget

30A: 120V x 30A = 3,600 VA on one leg. 50A: 120V x 50A x 2 legs = 12,000 VA

Or find your exact numbers

262 pages, each answering one specific sizing question with the arithmetic already done.

Battery bank sizing

What size RV battery bank you need for every daily amp-hour load from 20Ah to 300Ah a day, in both LiFePO4 and lead-acid

Solar array sizing

What size RV solar array you need for every daily watt-hour load from 500Wh to 3,000Wh, across three to six peak sun hou

Inverter sizing by appliance

Running watts, startup surge, the inverter size each RV appliance needs and the DC current it pulls from a 12V or 24V ba

Wire gauge by amps and length

The smallest copper conductor that carries each current over each run length on a 12V RV system, from published NEC resi

Fuse and breaker sizing

The correct fuse or breaker size for every RV circuit current, sized at 125 percent of a continuous load per NEC 210.20(

DC-DC charger by alternator

What size DC-DC charger each alternator rating supports, so a lithium house bank cannot pull more current than the alter

Run time by bank and load

How long every RV battery bank size runs every load, for both LiFePO4 and lead-acid, with the Peukert correction applied

Shore power load budgets

What actually fits on a 30 amp and a 50 amp RV pedestal, load by load, with the headroom left over and why an adapter ne

Generator sizing by load

What size generator each RV load actually needs, sized on startup surge rather than running watts, and what a soft start

Component spec sheets

Published specifications for every RV power component in our catalog, with what each one needs alongside it and how it c

Published standards and rules of thumb are not the same thing

A great deal of RV electrical advice mixes the two together, which is how bad guidance spreads. On this site every figure is labeled.

Published standard

Conductor ampacity and resistance from NEC and ABYC E-11, overcurrent sizing from NEC 210.20(A), NEMA TT-30 and 14-50 device ratings, and figures from manufacturer specification sheets. These are citable.

Convention

The 50 percent lead-acid depth of discharge guideline, the 100 watts of solar per 100 amp-hours rule, and the usual solar derate factors. Useful for planning, but no standards body publishes them.

Where a published figure does not exist, these calculators return nothing and say why, rather than extrapolating past the end of a table. That is the whole reason the count of invented figures above reads zero.

Complete builds, costed end to end

Campground ladder

Plugged in most of the time

Shore power protection, a converter that suits your battery, and enough capacity to cover the gaps between hookups.

Boondocking ladder

Off grid for days at a time

Lithium capacity, a roof array sized to your own load, and the monitoring that tells you what is actually left.

These are two genuinely different builds rather than one list at two prices. See all six builds and the shopping list.

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Frequently asked questions

How do I work out what size battery bank my RV needs?: Start from daily use rather than from the battery. Add up every load in amp-hours per day, multiply by the number of days you want between charges, then divide by the fraction of the battery you are willing to use. That last step is what people miss: a 60 amp-hour daily load does not need a 60 amp-hour battery, it needs roughly 150 amp-hours of LiFePO4 or 240 of lead-acid for two days off grid.
Is a 50 amp RV service twice as much power as 30 amp?: No, it is more than three times. A 30 amp RV service is one 120 volt leg at 30 amps, which is 3,600 watts. A 50 amp service is two separate 120 volt legs at 50 amps each, which is 12,000 watts. The plug is larger because it carries two legs, not because it carries higher voltage to your appliances.
Does a 50 to 30 amp adapter give me more power?: No. An adapter changes the plug shape and nothing else. A 30 amp rig on a 50 amp pedestal still has 30 amps, because the rig inlet, cord and main breaker are all 30 amp rated. A 50 amp rig on a 30 amp pedestal has 30 amps total for the whole coach. Nothing about an adapter increases available amperage.
What do I have to change when converting an RV to lithium?: The charging, in three places. The converter needs a genuine lithium profile or the bank will never reach full charge. The solar charge controller needs its profile set for LiFePO4 rather than left on a lead-acid default. And if you charge from the engine, a DC-DC charger is required, because lithium will draw more current than an alternator can survive if connected directly.
Why do you double the run length when sizing wire?: Because current has to return. It flows out along the positive conductor and back along the negative, dropping voltage in both. A 10 foot one-way run is 20 feet of conductor as far as voltage drop is concerned. Forgetting this halves the calculated drop and undersizes the cable by a full step or more, which is the most common RV wiring error there is.
Can I run my air conditioner off batteries?: It depends far more on the startup surge than on the running load. A 13,500 BTU rooftop unit runs on about 1,400 watts but needs roughly 3,000 watts to start, which a 2,000 watt inverter cannot supply. A soft starter cuts that inrush substantially and is usually cheaper than upgrading the inverter, the bank and the cabling to suit the original figure.

Electricity deserves respect. Never exceed a conductor's rated ampacity, and fuse every positive conductor as close to the battery as you can get it. A lithium bank needs a charger with a genuine lithium profile and protection against charging below freezing. Never run a generator in an enclosed or partly enclosed space, because carbon monoxide is odorless and kills quickly. Permanent AC wiring is work for a qualified RV technician or a licensed electrician.