That Black Box

What’s that black box on the back of most solar panels? Is it just a place for the wires to come out?

There are all kinds of panels, but the usual 60 or 72 cell module in an aluminum frame is going to have the black box. This is usually called the junction box. Yes, it is a handy way of getting the power out of the panel, but there is more to it.

If you have just a laminate, which John sometimes has in stock, you have the glass panel, but not the aluminum frame or the junction box. The back of the panel has a number of silver metal strips protruding and they are kind of delicate. You don’t have just a positive and a negative. Terminals inside the black box clip onto the silver strips and then the box is glued to the panel with silicone. Silicone helps keep the water out, too.

The typical panel has not one, but THREE series circuits of cells in it. There’s good reason for that. Poly or mono cells can be shut down by a single large leaf or bird splat. Sad but true. You don’t want your rooftop solar to shut down because the chimney is putting a little shadow on one of the panels do you? Of course not. So, they put three circuits in there, hoping you only lose a little power if you get a little shade.

Inside the box, there are some diodes. These are like one way valves for electricity.. They won’t let power flow backwards, but if a string within a panel is blocked or if the entire panel is blocked, power from other panels in series can use the diodes to work around the panel that can’t work and let the other panels in a string get some work done.

Usually these are not a maintenance item, but things happen. Lightning can burn out a diode. Heat can get them, too. Once in a while, like on the old solar shingles on my shed, somebody makes some poor choices and undersized diodes are used. A bad diode can cause hot spots and eventual panel failure. If you ever notice that one cell in a panel is brown or unusually hot, shut it down before it breaks. It probably has a bad diode. Fortunately, this rarely happens.

Crazy things can happen, though. I was once given a panel that had gotten too close to a tractor with a front loader bucket. (Actually, I think it was the other way around.) The story goes that the panel, fearing for its safety, leapt from the rack to the ground. In doing so, it left the cables and junction box behind. That left four shiny strips on the back of the panel. I could have just bought a new junction box. I don’t know if John has them, but numerous people sell them online on ebay. It seemed rather pointless as the glass was broken and the panel was bent, so I just soldered on some diodes that I had in the parts bin. This was not going to be installed in a system. Not even I would do something that appalling. What I did was probably worse. John and I had been having this discussion on how hard it is to destroy a solar panel. The cells, after all, are just finely sliced rocks with wires attached. He advanced the notion that you could probably punch holes in them and they’d still work.

So why not try it? There are neighborhoods in Miami where gunfire is not uncommon, but it is generally frowned upon. Not so in my neighborhood, so I got to have the fun. I proceeded to make small holes with a .45 pistol and large holes with a 12 gauge shotgun. While the power is somewhat reduced, it will still charge a battery!

I wish I could say that no solar panels were harmed in the course of these experiments.

It may seem that I have digressed, but back to the point of diodes in the junction box, the diodes I had tacked back onto the panel allowed the working and non working sections to work it out so that panel still had enough output to charge a battery or operate a car running light.

I am pretty sure that a shotgun blast directly into the junction box would be the end of the panel. Don’t try this at home.

And, yes, the junction box IS a handy place for the wires come out. Take note when ordering what kind of connectors are back there. MC4 is the “usual” but there are others that look similar, but won’t fit, or will fit, but not well enough for a safe connection. Be sure to order the right connectors so you won’t have the installation delayed. You don’t want the neighbors to hear you cussin’.


Will Has a Question

Actually, Will has several questions, but they all boil down to “how do I get started?”

” I am looking into installing solar panels on my roof. I have a straight gable roof that the back slope faces South. I have a 200 amp breaker box and would like to keep that amount of power when I go solar. Looking at solar panels, they come in different amounts of watts per panel. How many watts do I need to support my existing amount of power?”

I have been looking for just the right topic on getting started and Will handed it to me by email. I’m not going to get terribly detailed in this installment. We’ll hit the highlights, this time, and fine tune the details, later.

He wants to go solar. Yay! But how? He does not specify, but it sounds like he wants to go off grid. Again, how? Grid-tie? Hybrid? Off-grid? It can make a big difference in costs and your circumstances may change your options. Pure grid-tie used to be the best bang for the buck, but that has changed in some areas. It can ADD to your power bill! (I am testing a workaround for that.) Do your research and find out what the deal is with YOUR power provider. The major downside is that if the power goes off, it goes off. All those panels on the roof don’t help.

A hybrid system can do grid -tie and give you backup power when the lines are down. Sound good California? OR a hybrid system can interact with grid power, reducing the bill, and NO extra charges for grid-tie. It is almost like being off grid, but the grid is your backup power. In an emergency, you can be off-grid by cutting back on your loads.

Of course, there’s off-grid, too. This is going to cost you the most and may inconvenience you a bit. You don’t have any backup from the grid. If anything goes wrong you run the backup generator or you sit in the dark until parts arrive.

Those are the choices that jump right out. As I said, we’ll dig deeper, later.

I am looking into installing solar panels on my roof,” he says. In many cases that is pretty much the only choice. Rejoice, Will, that your roof faces south. It may not be at the perfect angle for your location, but PLEASE don’t let the installers go with some whacko angle! Keep it low and subtle.

There are questions that must be asked, though. Are you wanting to use conventional panels or some of these solar shingles, like Mr. Musk is promoting? Do you have a homeowner’s association that will pitch a fit over panels? Is your roof strong enough? (I have over a ton of panels on the roof of the Solar Shed.) Are you going to have to hire an engineer to prove it? Does your roof have a bunch of pokies sticking out all over? Are there trees shadowing it? What kind of condition is the roof in?

Most important, maybe, is the condition of your roof. An old roof could not stand the trauma of a bunch of guys stomping around stringing wires and bolting down panels. I’d need to spend 12 grand on a new roof before considering putting solar panels up there. Is your roof in good condition, Will?

“I have a 200 amp breaker box and would like to keep that amount of power when I go solar.” Hold on, there, Will. You have a 200 amp breaker box because the code required it, not because you need it. 200a x 240v= 48,000 watts. I can’t imagine you using anywhere near that much power, even if you were trying. My first house wasn’t big, but everything was electric except the heat. It was an older, post-WW2 house with 60 amp service and I never blew a fuse. After one particularly nasty hurricane at my present house, I fed my generator in through a 30 amp breaker and got by ok if using the loads on a staggered basis. For a real world example, Tom lives in a 25 year old all-electric house, full of computers and home entertainment stuff, central air, served by two 8kw inverters. He could get by on 8kw if staggering loads. 12kw worked fine. The pair of 8kw inverters allows him to get by if one breaks. 16kw/240v is 67 amps, about the same as my old house! What I am saying is, you don’t need 200 amps of solar-generated AC.

“How many watts do I need to support my existing amount of power? “ Good question. First of all, are you currently cutting back on comfort to save a little on the power bill? First thing I did when I got the a/c ported to solar was lower the thermostat last summer! Consider adding a little for a decadence factor! If going off grid, I’m pretty confident that 10 or 16kw of inverter will be plenty. There’s more to it than that, though.

OK, dig around in the drawer and find all your power bills for the past year. Find the biggest one. In KWH. Divide by 30 to make it a month. Where you live and how your house is built will make a tremendous difference, but let’s just call it 1000kwh for the month. That’s over 33kwh per day. Round it up to 34, because you can’t have too much and you don’t want too little. Do a little internet search for solar insolation (how much you get) for your area. Typically, that’s about 5 hours a day at more or less full power at my Gulf Coast latitude. So 34 (make it 35 for easy math and an extra margin) / 5 hours =7kw of solar power. That’s around 21 panels, give or take the model. Oh, it doesn’t matter if you use 250 watt panels or 300 or 400watt panels, if they all add up to what you need, but they should all be the same in most cases. With limited roof space, you might find the newer, more efficient 400+ watt panels may be necessary to get enough power in the available roof space. Yeah, you are going to have to do some measuring and you’ll have to leave some walkway space unless you want someone walking around on that glass! Sizes of various modules can be found on data sheets at and other web sources. Call John’s crew if the spec sheets aren’t posted.

Off grid, you can’t mess around. Traditionally, when solar was really expensive, off-gridders just figured on running the generator some. Today you can probably get by without it, but you need enough solar and enough battery. Tom’s system has 10kw of solar and maybe 400amp-hours of battery. His operates in a hybrid, interactive mode, so solar provides all the power on a sunny day, into the early evening. It cuts out as the a/c runs all night or on cloudy days. He has plenty of solar and not enough battery. Batteries are expensive! My current situation has part of the house on grid and part on solar. I have maybe 14kw of panels presently active and 800+ amp hours of battery. In good weather I could probably run the whole house with a bigger inverter. As it is, though, if I get 3 or 4 days of gloom, there’s a good chance I will switch all of the house over to the grid to save the batteries from running low enough to damage them.

I guess what I am telling you is, it’s complicated. There is a delicate balance between the amount of solar power you have coming in and the amount of storage you have to keep it. If your batteries get charged by noon, but sometimes get wimpy in cloudy weather, then you don’t have enough battery!

A more practical answer might be to figure out if your roof will hold the number of panels you calculated. If so, you have lots of options. Heck, fill it up! If not, go with Zero Export (we’ll talk about that later) grid-tie and save some bucks. You can always change things later.

I’ll go into more detail in Part 2.–Neal