It is a weather-tight box that combines the outputs of a number of solar panels before sending the power to the next level of equipment. It can be simple or very elaborate. I was recently looking at them for a project I have.
Solar panels are sometimes used as singles in parallel or in multiple series strings. Running a wire from the roof or backyard from every one of those panels or strings would use an awful lot of cable and it just isn’t necessary.
For example, I am going to review some of these super cheap $7 thin film panels John has for sale. I had room for a dozen of them come available on my test rack, so there they are. These panels can put out nearly 100volts EACH, so you can’t really string them together for a typical 150v MPPT charge controller. Obviously they were meant for string inverters, but if you are going to be a tightwad like me, you have to be creative.
So what I am going to do is run them all in parallel. Because they are high voltage output and only 70 watts, they are going to make less than an amp each. That means the power from all of them can come back to the control room on a single 10 gauge solar cable. THAT is where the combiner can come in handy. We have to get all 12 pair of wires connected to one pair of wires.
Some combiners are super simple, consisting of just a weathertight box (because this is out in the weather with the modules) and some barrier strips to connect the wires. Some take it a little further and add a fuse for each panel or string. You can even put a surge protector in there. Rooting around on the Sunelec.com website, I noticed one he had has a little bit of DIN rail in it, so you can clip on all kinds of different options.
Some have built in MC4 connectors, the most popular type. These cheapo modules use the older MC3 connectors like my shingles. You don’t see them much, anymore, but there was one combiner box that even had long pigtails spaced for running directly to the modules. Handy, but not cheap. Well, maybe it is cheap enough. Sure, this is not rocket science and you could probably build your own combiner boxes, but by the time you source and purchase all the bits and pieces and then wait forever to get the bits and pieces, you can see that maybe it is well worth the price. Using a combiner box will give you safe and durable connections.–Neal
You will want to occasionally check for loose connections as you would with all of your solar power components.