Sizing Solar System Wiring

Excel Spreadsheet

The process for picking appropriate size wires can be pretty confusing, but we’ve made it simple with an Excel spreadsheet. This allows you to enter your design parameters and see the effects on power loss as well as voltage loss.

Download the Excel Spreadsheet here

If you are interested in a detailed description of the spreadsheet and how to use, check out our YouTube video, or keep reading if you prefer a written description. Disclaimer: I’ve checked these calculations extensively and believe them to be accurate. The final responsibility for selection and installation is yours, however, and we make no claim as to suitability or accuracy in your installation. If you don’t have Microsoft Excel, there are a number of free alternatives that should load and run the file.

Connecting the panels

To make sense of some of the details, it is necessary to know about series connections, parallel connections, and series-parallel combinations. These choices have a major effect on how your system performs and what you have to do about wiring. They all will get the power from the roof to your battery bank, but how it happens varies. There is no “right” or “wrong” or “best”, but what works in your situation is the best for you. The goal with the spreadsheet is to allow you to explore different combinations with an easy to use calculator that shows up just how much, or how little, difference there can be.

Series Connections

Solar panels are simply another version of “battery”, they just need photons to create electricity. So you can put them in series, just like batteries. In fact, you may have 6 volt batteries in your RV with two of them in series to make a 12 volt battery. So you would connect the solar panels just like you would connect batteries in series to get a higher voltage.

We show a combiner, even though you might not even need one with all the panels in series but we will need it later in other configurations.

What happens is that you get the voltage of all the panels added together. If these are conventional “12 Volt” panels (a complete misnomer by the way), then they would produce about 18 volts each at maximum power and 25 volts or so with no load. It’s this latter voltage that we would care about. You would be getting almost 100 volts with no load into your solar controller so you would want to make sure it could take that voltage, preferably at least 15% higher for safety reasons.

But with this higher output voltage, the current is the current of a single panel, in the range of 5 to 10 amps, depending on the panel, so the wire size is pretty small all the way to the charge controller.

Parallel Connections

Just like batteries, you can hook all of the panels in parallel.

The diagram is technically correct, but if you actually wired the panels like this, you would not like the results. By hooking the right most panel to its neighbor, the current from that neighbor is now twice the panel current. That process repeats itself as each panel is added so that the current into the combiner is 4 times the current of any one panel. That could be 40 amps. or more so you would need to be increasing the wire size at each junction to handle that higher current.

A better way is to wire all of the panels to the combiner with individual wires.

You can see that this takes more wire, but it is mechanically easier to do. All of the smaller wires from the panels run into the combiner box and here is where it is really needed and works best. Inside the combiner box will be two heavy bus bars to connect all of the positive leads on one and all of the negative leads on the other. Then heavy wires are run from the combiner to the solar controller. This method consolidates the lighter wires carrying lower currents into two larger wires carrying the heavier current down to the controller without having to splice in heavier wire at several points.

Series-Parallel

But wait, there’s more! In your RV, if it has 6 volt batteries, you might also have four or even six batteries with two in series to make a string, and then two or more strings hooked in parallel to make a large battery bank. We can combine the two methods for the panels the same way to both raise the voltage (keeping it at a safe level) while putting the panels in parallel to get more power.

Since we have four panels, here we show two sets of two panels in series and then the two sets in parallel. Part of this is because we started with four panels, but you could have more sets of two added in parallel, or an arrangement of 3 in series with multiple sets in parallel.

The practical limit in the number of panels in series is the open circuit voltage of the panels and the maximum input voltage of your controller. You can put as many sets in parallel as you want but at some point adding more panels won’t get any more charge current because the solar controller can only deliver so many amps. Or you may simply run out of room on the roof for more panels.

The spreadsheet will let you try different combinations of panel arrays and evaluate what wire sizes give you the desired results and what the specifications have to be on the charge controller to utilize the panels best.

Wiring Effects on Wire size

Starting from the top, we can look at the general effects on the size of wires needed to properly distribute the power. Wire size is important for at least two parameters: 1) voltage loss and 2) power loss.

Voltage loss becomes more important at lower voltages because the inverter (the real load for the solar system) has a low voltage cutoff. Below that point it will not work any more. And it is going to be close to 11 volts depending on the inverter (assuming a 12 volt system here). So you might be starting with a battery voltage not much above 12 volts at full load and a drop of 1/2 volt could mean the difference between working and not working.

But your solar panels might be putting out 50 volts for more and the solar controller can work down to 13 volts, so a loss of 1/2 volt is inconsequential.

The power loss is important because if is due to heat in the wiring. Any loss represents solar power that is not going into the batteries, but dissipated into the air. We want to maximize the transfer of power but keep costs in line as much as possible so there is a trade off between wire size and cost. The other reason is that a drastically too small wire could actually heat enough to cause a fire, but that is unlikely with the size of wire required for high efficiency of the system.

The spreadsheet calculates both voltage and power loss so you can determine which effect might be important at that part of the system. You can also start to see, in some cases, where going to a larger wire size adds very little to the performance but will add to the cost. As a rule of thumb, once you get the power loss below 1%, decreasing the loss means increasing the wire size a lot and you have to consider how good is good enough in your application.

Starting at the top, we see that our distribution system with a series only or a series-parallel arrangement means that the maximum current any wire carries is that for one panel but we have to think about how high the voltage gets. Because this wiring does not come in convenient round trip lengths, the spreadsheet needs to know the total length of all the wires added together. For the other wire measurements we assume the two wires are the same length and only need the distance along the wires between the two devices in the system; the spreadsheet will multiply the distance entries by 2 when calculating the losses.

Since we are combing multiple wires with the same current, we have more current flowing from the combiner to the solar charge controller, so the wires here will need to be heavier. One thing to remember is that the power loss in the wire goes up as the square of the current, so if we double the current we would get four times the power loss in the same wire. The down wires might be shorter than the roof top wiring but will still need to be selected according to the desired power loss.

When we get to the solar controller, it will have most of the voltage generated by the panels in series and its output voltage will be whatever the battery needs for charging. So, if the battery had to be at 12 volts and we had 48 volts coming in, the current going out would be roughly 4 times the current from the panel array. That means even heavier wiring is needed from the controller to the battery, and we probably want the controller as close to the battery as feasible.

The last part of our wiring is from the battery to the inverter. The battery bank must be capable of supplying very high currents to the inverter and the spreadsheet will calculate that for you. Plug in the inverter power and its low voltage cutoff and we calculate the current.

But, perhaps your inverter can supply an even higher short term power level. Think about that when sizing the wire because that current demand can be huge, as much as twice the maximum power operating current. Of course, you probably don’t run your inverter at full load, but it’s good to size the system as if you did.

Expansion

One last point that you should consider when using the spreadsheet: it’s great for playing “what if” games. What if, you decide you need more power later on? It is easy to add in more panels and see if you wiring would be adequate. It is easy to replace the roof wiring. It is usually a lot harder to replace the down wiring or the wiring to the battery so it might pay to over size those areas. You can examine how best to add more panels later, or even replace existing panes with higher power ones.

As a very practical matter, our original system installed some 4 years ago used 160 watt panels, pretty much the best I could find in the physical size I needed at the time. In looking at those panels today, I could replace them with 200 watt panels in exactly the same roof configuration. In a few years, we will probably get even higher power levels as the efficiency of panels improves. So it pays to at least consider options when finalizing what wires to use.

Texas

February 13, 2020