RV Solar Installation Guide: Panels to Battery, Step by Step

Mount the panels first, then run the cable through the roof, then wire the charge controller, then fuse the run, and land the last connection on the battery. Wiring the battery first is the most common mistake, because it means testing the roof entry for leaks with a live circuit already connected, which most installers understandably avoid doing carefully. Doing the roof work first, then testing it with water before anything is wired to the battery, catches a leak while it is still just water and not water near a live DC circuit.

Peak sun hours vary by region and are published NREL insolation data, not a rule of thumb. The system loss derate below is a planning convention.

A solar install almost never fails because of the panel. It fails at the hole cut in the roof for the cable to pass through, which is the one point in the entire install where water actively wants to get in. A dedicated cable entry gland, sealed with a compatible roof sealant and re-checked after the first season, is a smaller and more controllable opening than a generic hole-and-caulk approach, and it is worth the extra few dollars over improvising with parts on hand. Test the seal with a hose before connecting anything electrically, and check it again after it has weathered one season on the roof.

Panels lose output as they heat up, and a rooftop in direct sun runs hotter than open ground, so mounting hardware that lifts the panel an inch or more above the roof surface, rather than laying it flush, lets air move underneath and keeps the panel cooler than a flush mount would. Z-brackets are the common standard mount for exactly this reason, holding the panel above the roof rather than against it. Flush or adhesive-mounted panels are lower profile but run warmer, which is a real efficiency trade to weigh against the lower wind resistance.

Controller amperage on the battery side is roughly the array wattage divided by system voltage, with headroom added for cold-weather output above nameplate rating, which is real: panel voltage rises in cold temperatures and can push a controller above its rated current if it was sized with no margin. Work this out with the charge controller calculator using your actual planned array wattage rather than rounding down to save on the controller. An undersized controller is the more expensive mistake, since it either throttles harvest or trips repeatedly under normal winter conditions.

Fuse the run between the charge controller and the battery, sized to the controller's rated output current, and fuse the panel-to-controller run as well if the array is wired in parallel strings, since a short in one string could otherwise backfeed from the others. Use the fuse size calculator with the actual current the array produces rather than the panel's open-circuit rating, and place the battery-side fuse as close to the battery as it will physically fit, for the same reason any DC fuse goes close to the source it protects.

Wire gauge for the roof-to-controller run depends on the array's operating current and the length of that run, which on some rigs is a longer path than people expect once the cable is routed down an interior wall to reach the battery bay. Run the actual length and current through the wire gauge calculator rather than reusing whatever gauge came with the kit, since a kit's stock cable is sized for a typical short run and may not cover a longer routing on a bigger rig.

Confirm the charge controller shows the correct charge profile for your battery chemistry before leaving the install running unattended overnight. Check the roof seal one more time after the first rain or wash, since a seal that held during a hose test can still show a slow leak under sustained weather. Log the array's expected output against what the controller actually reports on a clear day, so a future drop in harvest has a baseline to compare against.

Series wiring adds panel voltages together while keeping current the same, which lets an MPPT controller work with thinner wire over a longer roof-to-controller run since the current carried is lower. Parallel wiring adds panel currents together while keeping voltage the same, which is the more common approach with PWM controllers that need panel voltage to stay close to battery voltage. A kit's included panels and controller are matched for one wiring approach out of the box; check the kit's instructions before assuming panels can be rewired the other way without also changing the controller.

A single shaded cell in a panel can drag down the output of an entire series string, not just the shaded portion, which is a disproportionate loss compared to how small the shadow looks. Watch the controller's reported output at different times of day against clear-sky expectations for your peak sun hour region, and check for a roof vent, an air conditioner shroud, or an antenna casting a shadow across part of the array at certain sun angles. Repositioning a vent-mounted accessory or adjusting panel placement during the original install is far easier than diagnosing a shading loss after the array is already permanently mounted.

Three failure modes account for most of the solar installs that end up needing rework. The most common is the roof cable entry leaking, usually from a generic hole-and-caulk approach rather than a dedicated gland, and it often is not noticed until an interior stain shows up well after the install. The second is a controller left on the wrong charge profile for the battery chemistry, which silently undercharges the bank rather than throwing an obvious error. The third is a shaded panel dragging down an entire series string's output, which looks like an underperforming array rather than the specific shading problem it actually is. Catching all three comes down to testing the roof seal with water before wiring, confirming the charge profile at first power-up, and comparing actual harvest against expected output on a clear day.

Buy the kit if this is a first install and you want matched components. Buy the controller and panels separately once you are adding a second array or replacing just one failed piece, since a mismatched replacement is cheaper than a whole new kit.