[Note: a gentle introduction to solar is also available]

Solar (photovoltaic or PV) power is an excellent fit for 'dwellers and especially for boondockers. It is silent, nearly maintenance-free, and requires no infrastructure or grid. Downsides include expense, bulky panels, and the requirement for access to direct/unshaded sunlight.

Broadly speaking, solar power systems consist of panels to collect energy, house batteries to store energy, and charge controllers to regulate how those batteries get/stay charged.

Under normal use solar setups charge fully during the daytime and discharge at night; the lowest State of Charge will likely be just before sunrise. On a properly-sized deep cycle system the bank will have used no more than 50% of its capacity at this point. Then the sun comes up and charging begins again. 'Dwellers running small solar setups (or experiencing poor insolation or heavy loads) may have to ration their use of solar power.

The most difficult part of a solar power system (other than paying for it!) is sizing it correctly for your needs.

Maybe not. If any of these conditions are met you probably don't need solar:

• you stay in campgrounds or other places with access to shore power
• you have a lithium or carbon foam batteries that do not require smart (multistage) charging²⁾

• solar panels take a lot of room and must not be shaded. Where will you put them?
• panels are coming down in price but are $$$ to ship. Where will you get them?
• wiring from the panels will have to enter the camper body somewhere, and the entry will have to be waterproof
• lead acid batteries (flooded, sealed, AGM, etc) are extremely heavy and bulky. Where will you put them that is out of the way and that will not negatively impact the balance of your camper? How will you get to them to maintain/inspect them? How will you vent them?
• wiring from power sources will need to be fused or fitted with circuit breakers
• how will you ensure your batteries don't run your batteries too low and damage them?

When you use power from your system can be as important as how much power you use.

There are times (bulk, early absorption) when the system is struggling to charge the lead-chemistry bank; loads should be minimized during these periods.

At other times like late absorption and float the system will have surplus capacity that can be used to run loads.

• Insolation (Intensity of sunlight)
  • strongest when there is least atmosphere between the sun and panels: sun is overhead (noon, summer), nearer the equator, and at higher elevations.
  • weakest when there is the most atmosphere between sun and panels: sun is low (morning/afternoon, winter), higher latitudes, and lower elevations.
• hours of sun each day - more in southern latitudes and in summer; fewer in northern latitudes and in winter
• clouds - panels make more power on sunny days and less on cloudy days, due to increased current (voltage relatively unchanged).
• temperature - panels make more power when cold, due to increased voltage (current relatively unchanged)
• physical obstruction / shade - as when in forested areas
• physical obstruction / covering - as when covered in dust or snow

Solar sizing has been moved to its own page.

Most house banks and accessories are nominal 12v.

Reasons to go 12v:

• it is the most common setup: more accessories, more crowdsourceable knowledge
• you want to charge from alternator
• your rig already has a 12v bank

Reasons to go 24v (or higher):

• to save on copper costs; higher voltages do not require thick copper like 12v. Example: a 10' run of 20A @ 2% loss requires 2awg with 12v but only 5awg with 24v.³⁾
• more efficient inversion to 110vac. If most of the house will be run off an inverter, it will be more efficient at 24v than 12v.
• large arrays and battery banks

Weather and shading have serious effects on solar output.

Cloud cover can have paradoxical effects:

• under full cloud cover current from panels drops significantly while voltage output remains relatively stable.⁴⁾
• under partly cloudy conditions output can be wildly variable.
  • When the sun is behind clouds you get diminished current as above
  • when the sun is not behind clouds you get max current given the sun's position
  • when the sun is near the edge of a cloud you can get spikes in current, called Edge of Cloud Effect (ECE). Here is a youtube video of a charge controller's out during ECE. Here is a simple drawing illustrating ECE. Here is a demonstration of why it happens.

Rainy weather causes a drop in current due to the cloud cover effect. An upside is it can wash dust off panels.

Dusty or snowy weather can cause a loss of output due to physical presence on the panels.

Panels yield the most power in cold temperatures. Since panels are dark and get very hot in the sun they are generally raised rather than directly mounted; this allows air circulation/cooling underneath.

[based on this reddit post]

There are a few issues to consider:

• solar cells get very hot in sunlight, typically 25deg C hotter than ambient temps. Rating lab tests are performed when the cells are 25deg C (0deg C ambient, freezing)
• Panel output is reduced when the cells get hot

hot panels re-radiate heat onto the roof of the van. “On my white van (even with a 5” airgap) it is hotter in the shade under the panels than on the roof in direct sunlight.“ - secessus

• we prefer fewer holes in the van roof when possible
• “flex” panels cost ~2x as much as rigid panels and last for years rather than decades
• airgapped panels will add height for drive-through adventures
• taller racks will likely increase air drag and MPG, particularly if you drive fast

Mounting hard panels to a rack increases airflow (thereby reducing heat-related power losses), reduces heat transfer, and minimizes the number of holes in the van roof.

Mounting hard panels directly to the roof still allows some airgap (the depth of the frame) and the roof corrugations will allow some airflow in the direction of the “valleys”. Taller Z-brackets may give a bit more airgap.

Mounting flex panels to the roof allows some airflow in the corrugations but no airgap. Good news: panel derating won't be so bad because the heat is conducted to your van roof. Which is bad news if it's already hot in your van.

Portable panels allow the vehicle to be in shade and panels in the sun. Also can be tilted for maximal yield.

Opinion: Bonus rant on panel/controller selection.