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 Living on the Road :: Articles :: Solar 101

Judging by the questions I get asked a lot of motorhomers really don't understand even the fundamentals of a solar power system. Maybe this will help.

In a nutshell a solar power system is simply a voltage source (the panel on the roof) connected to a battery. In theory the panel generates power to charge the batteries during the day and the battery generates power to run appliances during the night. Such a simple system, would look something like the following.

The first problem with this simple model is the fact that solar panels actually produce a far higher voltage than their nominal 12v rating, up to 20v or more in some circumstances.

This is far too high a voltage to connect to a battery so we need a gadget to control or regulate this voltage. Funnily enough the gadget is called a regulator. Note that there are "self-regulating" panels on the market, I have no idea exactly what they do but have heard several people recommend against them.

Blocking diode
The next problem occurs when the panel is not producing a voltage higher than the battery, say at night or in the shade. When this happens the current will flow back into the panel and your battery will discharge.

This can be fixed by the addition of a one-way device called a diode. Diodes will only allow current to flow in one direction, so placing one in line with the wire coming from the panel will allow the panel to charge the battery when light is available but not discharge it when it's dark. In this application we call this diode a blocking diode. Note that some regulators include a blocking diode and some don't so ask before you buy.


More panels
If you can't get by with one panel (and few people could) you can add more, in fact you can add as many as you like right up to the time you run out of money or space on the roof.

Extra panels are added simply by wiring them in parallel, which is to say that all the plus terminals from each panel are wired together and so are all the minus terminals.

Wire size
The size of the wire you use depends largely on the size of your system, ie. how much current you expect to draw. In general a figure-eight wire similar in size to the flex you see on a 240v appliance should do for smaller systems of one or two panels.

Larger systems can also use this smallish wire from each panel but much larger wire from the point where all the panels are connected down through the regulator and on to the batteries.

I would recommend a wire with a cross sectional area of about 16mm, that's about as thick as a pencil.

Remember that even if you currently have a small system you may want to add panels in the future. If there is any chance of this you should use large wire now, otherwise you'll have to rewire when you add panels.

Blocking diodes (again)
Once you start adding a few panels it becomes harder to find a diode that will handle the current produced. It's often easier to use a smaller diode on each panel as shown below.

24 volt systems
With large systems it's more appropriate to use a 24v battery bank because the currents involved get very high with 12v. Doubling the voltage will halve the current and therefore the wire size (for this reason a system on a house will often use 48 or even 96 volts).

As solar panels only come in 12v versions you have to connect two panels in series to get 24v. Below we see a diagram of a simple 24v system.

Now of course there would be little point in such a small 24v system unless you wanted to match the truck voltage so you can charge from the engine's alternator. Or maybe you inherited a 24v inverter or are planning to enlarge the system at a later date.

Once two panels are connected in series as above you can consider them to be a single 24v panel and from there on everything is the same as described above for 12v systems.

Well that's it, as I said this is a very simple tutorial for those with no knowledge at all on the subject. There is of course more to a system than this and you should do your own research but it's not rocket science, any handy man (oops, person) can do it.

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