Sizing residential off-grid solar configurations is relatively easy: calculate power needs in the worst case scenario (December in a particular geographical area)²⁾ and pay for it.

Campervan solar configurations have a more variables:

1. worst case scenario loads (the max loads you will apply in any 24hr period³⁾)
2. worst case scenario sunlight, usually December in your geographical area due to shortest daylight hours, but also affected by tree canopies, mountains, “urban canyons”, or other forms of shading
3. days of reserve (ie with no solar charging) you want
4. whether or not you have access to other forms of charging: shore power, alternator, generator
5. the kind of battery chemistry - some⁴⁾ charge more efficiently than others
6. the kind of camping you do. Full-timers will need more robust systems. Weekenders can "get by" then recharge batteries fully at home.

These factors will dictate the system capactity you will need. If the system you need will not physically fit on/in your vehicle, you will have to take a different approach.

The first step in (and difficult part of) designing a solar setup for your camper is to understand your real power needs. Most people running on solar power only (as when boondocking) will need to reset expectations to be more in line with reality. Necessities like charging phones, LED lighting, fans, etc. are easy to power.
“Niceties” like electric coffee makers, toasters, hot plates, induction cooktops, and microwaves require big/costly setups.
Some luxuries like A/C are impractical on typical solar installs and are therefore relegated to shore power or generator scenarios.

As a sanity check of what's actually being done out there, here is a list of some descriptions of working (rather than theoretical) solar setups.

Before you dig in, consider that it is far easier and cheaper to use less power than it is to make more power.

Power requirements are usually expressed in Amp-hours needed every day (Ah/day), although thinking in Watt-hours (Wh) might be simpler less complicated.

There are two different kinds of loads 'dwellers usually run, AC (110v) and DC (12v). Appliances run off an inverter will incur additional efficiency penalties (see below).

Luckily there are stickers on most products that say how much power they take to run. Sometimes it will be expressed in Amps and Sometimes in Watts. That's ok, because the math is easy: Watts (W) = Volts (V) * Amps (I).⁵⁾

Let's say you are a minimalist and the only 110v appliances you take are a hot dog cooker and lava lamp.

Hot Dogger: 110v * 5A * 02.5hrs = 137.5Wh
lava lamp: 110v * 1A * 4hrs = 440Wh

Combined this would be 577.5Wh. But inverter losses apply to all 110v loads in your van, so multiply by 1.1.
440Wh * 1.1 = 484Wh.

And since we are going to run it off a nominal 12v system we will divide by a typical “12v” voltage, which is usually stated as 13v.⁶⁾
484Wh / 13 = 37.23Ah

DC appliances are usually labeled in Amps, so the math is easy.

Batman projector light: 2A * 1hr = 2Ah
Heated bunny slippers: 5A * 3hrs = 15Ah
Spinning disco ball: .5A * 3hrs = 1.5Ah

This is 18.5Ah/day.

37.23A + 18.5A = 55.73Ah/day⁷⁾

• Unbound Solar offgrid calculator
• AltE offgrid calculator
• BatteryStuff 12v load calculator
• GoPower calculator
• RVSSO calculator

Also see the EU PVGIS modeler.

[see also days of autonomy]

…one that really [effed] with me was running out of power or spending a few days in the dark cause the weather is bad. – hellexual⁸⁾

Solar power is highly dependent on local conditions like weather, season, and latitude. One cannot assume getting a full charge every day in every condition.

To account for this decide how many reserve days of power⁹⁾ you need if there is little solar power available. You may need few in desert regions and many more in the Pacific Northwest. Long stretches of little solar charging may necessitate rationing solar power.

It is safest and simplest for solar newbies to assume a day of poor solar harvest = zero solar harvest. In actuality zero harvest is highly unlikely; more experienced or risk-tolerant RVers can run with less reserve or no reserve at all.

Now that we know the total amount of capacity we need (from 24hr loads X number of days needed) it is time to figure out how that capacity is met by various chemistries.

It may not be obvious, but one cannot consistently use all the capacity in a battery without damaging it. Deep cycle batteries are usually cycled to 50% Depth of Discharge (DoD), so you will need a deep cycle bank 2x the size of your previously calculated needs. Starting batteries may tolerate only 10% discharge. LiFePO4 batteries are happy to go down to about 90% discharged.

Example: You need 100AH x 1 day, so your bank will be 200AH cycled 50% each day.
Example: LiFePO4 can be drained down to 80%-90% DoD so you would need only 110AH of rated capacity with that chemistry. 120AH if you are conservative.

A rule of thumb is flooded lead acid batteries in offgrid scenarios should be charged between C/10 and C/8. For a 200AH bank this would be 20A-25A. Charging at lesser rates may result in not getting fully charged each day. Charging at greater rates may result in increased outgassing and the need for a lengthier Absorption stage.

and that's just to get through a 24hr period with good sun. If you want to get through a day (or more) of terrible solar yield (rain, snow, fog) it requires even more battery capacity (reserve), money, and cargo capacity. These numbers are both very rough but will illustrate the pattern:

Lead batteries take something like ~13% more amps to charge from 50% DoD than were consumed. So if you need to replace 50A it will take ~56.5A the next day to replace it. While you are running your normal loads.

Lithium batteries have very high charging efficiency, so 50A consumed will take ~50A to recharge.

Most camper solar installs run at nominal 12v. Because of this most accessories are 12v.

Some installations utilize 24v electrical systems. This is common in boats, and occasionally some very large commercial RVs. The common theme of an advantage to 24v is that you're using half the amperage to transfer the same amount of power, which can result in cheaper components

You may want to consider running a 24v system if:

• You have a lot of solar, like 600w or more.
  • Ohm's law tells us that 600w / 12v = 50A but 600w / 24v = 25A. Therefor, you can use much smaller (and cheaper) wiring to connect your solar panels to the charge controllor
• You will be using inverters heavily
  • Inversion from 24vdc to 110vac is more efficient than inversion from 12vdc.
• You have large, power-hungry appliances that can run natively off of 24v
  • Some DC refrigerators and induction cooktops can run off of both 12v or 24v, and are slightly more efficient when running off of 24v.
  • Off-grid Air Conditioning can be slightly more efficient with 24v native systems, and will allow smaller wires to be used.
• You have a lot of long, large cable runs
  • 24v doesn't have as much voltage drop as 12v over a similar distance, and can use smaller/cheaper cables.
• A 24v system can use a much smaller/cheaper charge controller

There are downsides to 24v, however. There are fewer 24v accessories, and you cannot use a chassis ground as it is already grounded to the coach's 12v system. It can also make alternator charging difficult, as DC-DC boost converters get quite expensive when they are sized to handle the required loads.

Pick a solar panel array output on daily power requirements and battery bank capacity. A common rule of thumb for matching panels to battery bank is that at a minimum there should be 1 panel watt per amp-hour of nominal bank capacity.

“…the person draining their batteries to the 50% range regularly would do much better with a 2 watts to 1Ah ratio, or even 3 to 1.” – SternWake¹²⁾

Here are some examples using this model:

• 1:1 -
  • shallow cycling; or
  • deep cycling with augmented charging in the morning
• 2:1 - deep cycling and charging from solar only
• 3:1 - deep cycling and charging from solar only in areas with low insolation like the Pacific Northwest (PNW)

A crude rule of thumb for panel output per day is a horizontally mounted panel on a sunny day will generate Ah (amp hours) roughly equal to Rated Wattage / 3.¹³⁾
Example: a 100W panel could output 33Ah/day.

Another rule of thumb for panel output per day is a horizontally mounted panel on a sunny day will generate Wh (Watt hours) roughly equal to Rated Wattage x 4.¹⁴⁾
Example: a 100W panel could output 400Wh/day.

After the bank and panels have been sorted one must size the charge controller to the panels.

There are two general rules:

1. voltage from panels must not exceed controller voltage, ever.
2. controller rating should be the panels' output plus a safety margin, unless you are overpaneling an MPPT controller on purpose.

Solar power often works best when paired with another power source. This is because the first half and second half of smart charging have different time/power ramifications

1. high current / short duration during Bulk ←- generator, shore power converter and alternator charging do this best
2. low current / long duration once Absorption gets underway, and during Float ←- solar does this best

Joe Vandweller needs 50AH of power each day. He wants to get all of it from solar.

If he lives in the desert southwest he might need

• 1 day of reserve, totaling 100AH of power
• 200AH of lead acid (2p1s GC2 golf cart batteries) or 100AH of LiFePO4
• 200W of panel
• a 20A charge controller

if he lives in the Pacific northwest he might need

• 3 days of reserve, totaling 200AH of power
• 400AH of lead acid (2p2s) or 200AH of LiFePO4
• 400W - 800W of panel
• one or more 40A charge controllers

If Joe is willing to augment his solar with another charging source he can significantly reduce panel and charge controller requirements. The battery bank would stay the same; it just gets filled by more than one source.

In 'dweller solar configurations the order of sizing calculations may be different due to space or budget limitations. Few of us will be able to fit all the solar panels we need to run every load on our wishlist. In this case we size our systems for the maximum amount of panels we can fit in the space allowed. This might be what fits in open space on our roof, or how much portable panel we are willing to deploy.

1. max solar wattage you can install (or afford)
2. whether your have access to other forms of charging: shore power, alternator
3. max battery capacity you can charge fully/consistently (or afford, or install)
4. these factors will tell you what kind of loads you can run.

See the self-reported capabilities of real life configurations.