Originally Posted by roland stockham
If using an MPPT controller you want to go for high voltage so wire panels in series. One set each side. This is much more efficient and will give more charge at lower sun angles morning and evening. Don't fit blocking diodes as you already have one in the controller but if you don't fit Bypass then any shading on one panel will knock out all the panels n that side. Shading a cell in the chain is like closing a gate, no power can get through to the next section so shading one cell closes the complete panel. Without a bypass know power can get from any other panels upstream so you loose the whole side.
If you wire in parallel you avoid this problem but with small 12v panels you need a stronger sun to get enough voltage to start charging
. You also add all the panel currents together instead of adding voltages so you need much heavier and more expensive controllers and wiring and get lots of problems with voltage drop. 50A at 15v is much harder to handle than 10A at 75v, that's why MPPT controller go right next to the batteries.
There is a lot of debate about which is the most productive way to connect solar panels
on a boat. I still have a slightly open mind, but the majority of evidence favours parallel rather than series connection. Solar
cells produce near normal voltage before they can produce any usable current, so series connection does not significantly help the panels produce power earlier in the morning and later in the day. There are some gains, but they are very small.
Series connection does significantly reduce wire size, as you point out. Marine
cable is both expensive, difficult to feed, and heavy in the larger diameters so this is a helpful advantage. However, it does not generally reduce the cost and size of controllers, the opposite is true.
The output current will be the same in series and parallel connection and the controller needs to have sufficient capacity to handle this current. In terms of voltage, the input voltage under series will be higher and generally the cost is higher for controllers capable of handling high input voltages. In most cases the same MPPT controller is equally capable of managing both series or parallel connection so it is not a big factor, but if you want to string a lot of panels in series a more expensive, rather than cheaper controller may be needed.
If you do want to wire the panels in series, fitting extra bypass diodes is good advice.