Solar panels convert solar energy to electricity. A panel is a collection silicon wafers called “cells” wired together to get the required power and voltage. Ideally, all your panels should be exactly the same. In our case they are not, but they’re “close enough” since our old and new panels are all 280 watt, 60 cell panels.
A little spot of shade on the corner of a panel, or a thin line of shade from a rope might not seem like much, but it can drop the power output of the shaded panel significantly. This is because the voltage of the shaded cells drops, rendering them nearly useless. In older panels this shaded area can actually suck power from the sunny cells. Luckily newer panels have bypass diodes that disconnect and bypass the shaded area. The bad news is panels have only three or four bypass diodes, so even a small shadow can result in a large drop in output. This is a big problem on sailboats with masts and rigging casting shadows. I’ve even seen panels mounted with straps over them!
We use mono-crystalline panels because they perform slightly better under less than ideal conditions. You can recognize mono panels because you can see the individual cells which are actually thin slices of a silicon ingot. Multi-crystalline panels are poured, so the entire panel will be one continuous sheet of silicon, often with some multi-color light refraction. The performance difference is pretty slight, so don’t be afraid of multi-crystalline panels if the price is right.
Ideally, solar panels should be at a right angle to the sun’s rays. Doing so gathers maximum solar energy per square foot of panel. Static installations can easily approximate this, but on a moving boat it’s quite a challenge. I’ve seen a few articulated panels, but to be effective they take some baby sitting. Most boaters just deal with less output from un-aimed panels.
Solar panels are usually wired in series (+ to -, – to +) to create “strings”. Wiring in series increases voltage, but not amps, so wiring is cheaper and easier because higher amps requires thicker wire. Although there used to be problems with shade on one panel having a negative impact on the entire string, bypass diodes effectively fixed that issue. Strings are then wired in parallel (+ to +, – to -) at the charge controller to keep voltage at manageable levels.