The difference between series and parallel circuits is actually very easy to understand. A simple diagram (as it relates to solar panels and off-grid electrical systems) demonstrates the difference.
But that’s just the tl;dr (too long; didn’t read) version for those who wanted a quick answer. There’s actually much more to this concept. Let’s start at the beginning.
What is a circuit?
Simply put, it’s a path along which electrical current can flow. This is a circuit.
A circuit is a continuous closed loop of conductive material with no breaks. In the diagram above, note that the lightbulb is on. If we were to sever either the red or the black wire, the lightbulb would turn off.
What is a series circuit?
A series circuit is when two or more components are connected in a single path.
A parallel circuit is when two or more components are connected in multiple paths.
The difference is that in a series circuit, any break along that path will cause all of the lightbulbs to turn off. In a parallel circuit, if the path to one of the lightbulbs is interrupted, the other lightbulb may still function normally—although this depends on where the interruption occurs.
So what does this have to do with solar?
First, don’t wire your appliances in series, for the reason mentioned above. (There are more reasons, but that is the one pertinent to this article.) Wire everything in parallel.
Second, you can choose to wire your battery bank in series, in parallel, or a combination.
Third, you can choose to wire your solar panels in series, in parallel, or a combination.
1. Wiring batteries in series
Every battery has two poles. One pole is positive, and the other is negative.
To connect two or more batteries in series, simply connect the negative pole of one battery to the positive pole of another, and so on.
Wiring batteries in series adds voltages. So, if you have (4) 6 volt batteries, connecting them in series would essentially turn the (4) 6 volt batteries into one big 24 volt battery. The amp hours would stay the same.
2. Wiring batteries in parallel
To connect batteries in parallel, you would connect all of the positive poles together, and then connect all of the negative poles together.
Connecting two or more batteries in parallel adds their amp hours. So, if you have 4 batteries that each hold 50 amp hours, connecting them in parallel would give you a total of 200 amp hours. The voltage would stay the same.
3. Wiring batteries in series AND in parallel
It’s possible to connect batteries in a combined series/parallel method if you wish. First, connect individual batteries in series until you reach the system voltage that you need (remember, connecting batteries in series adds their voltages). In this example, connecting (2) 6 volt batteries produces what is essentially (1) 12 volt battery.
You can make sure that you’ve hooked everything up properly by testing the batteries with a multimeter.
Easy, right? So that’s it, you can now take this information and plan a battery bank suited to your needs. If you want to run a 48 volt system, you now know how to wire (4) 12 volt batteries in series. If you want add amp hours to your system, you know how to do that as well. But I should remind you of the dangers of working with batteries:
- Batteries don’t have switches. They are always “on”. If you connect the positive and negative terminals of a battery with any sort of conductor (e.g. by laying a wrench across the top, or by connecting the wrong terminals), you will short the battery, and it will explode. You could die.
- Batteries that are connected in series really do act like one big battery. If you have a pair of batteries that are already wired in series, and if you connect the positive terminal of one battery with the negative terminal of either of the two batteries, that too will short the batteries, and they will explode. Again, you could die.
So, be careful out there, okay? Always work with a buddy around, and have a fire extinguisher available.
WIRING SOLAR PANELS
The exact same method can be used to connect solar panels. Solar panels come with two wires sticking out of the back: a positive wire (red) and a negative wire (black).
- Optimum Operating Voltage (Vmp)
- Open Circuit Voltage (Voc)
- Optimum Operating Current (Imp)
- Short-Circuit Current (Isc)
When calculating voltage and current going from the solar panels to the charge controller, in the planning phase, it’s best to use the Voc and the Isc because those are maximum values. This will ensure that you’re well within the bounds of what the charge controller can handle.
For this example, I’ll use specs from the Renogy 100 Watt Monocrystalline solar panel.
- Voc: 22.5 V
- Isc: 5.75 A
Let’s say that we want to install (6) 100 watt solar panels. There are several ways we could wire the panels into the solar electric system. I’ll walk you through 4 of them. Keep in mind that your chosen wiring method should accomplish two things:
- provide the charge controller with voltage and amperage that are well within it’s acceptable ranges
- minimize voltage drop
1. Wiring solar panels in series
To wire all six solar panels in series, simply connect the negative wire if one panel to the positive wire of the next panel. In solar lingo, this would be called “1 string of 6”.
Connecting solar panels in series adds voltages, and the amperage stays the same.
To connect the 6 solar panels in parallel, connect the positive wires from each solar panel to a single positive wire that leads to the charge controller, and do the same for the negative wires. This would be called “6 strings of 1”.
Connecting solar panels in parallel adds amperages, and the voltage stays the same.
Since there are six solar panels, we can get creative with the wiring, if necessary. This example demonstrates “3 strings of 2”. Each string would consist of 2 solar panels, so let’s start by wiring 2 solar panels in series. We would do this 3 times.
Then, we would connect the loose positive wire from each set to a main positive wire that leads to the charge controller. We would do the same for the loose negative wires.
Combining each string in parallel adds the amperage. So the voltage going to the charge controller is 45 volts, and the amperage is 17.25 amps. Note how these numbers differ from the first two examples.
4. Wiring solar panels using in series AND parallel (Option B)
This last example is very similar to Example 3, but instead of 3 strings of 2, I wanted to show you 2 strings of 3. Note how the voltage and amperage numbers change.
First, we’ll connect 3 panels in series (twice).
In this example, the voltage going to the charge controller is 67.5 volts, and the amps is 11.5. Note how these numbers differ from Example 3.
There are several reasons why you might choose one wiring method over another. Wiring completely in series might be easier for cable management; wiring in parallel might be easier to swap or disconnect panels; wiring in series/parallel combination might be easier to meet the rating of the charge controller. So, you’ll have to decide which method is right for your installation. Make sure that you read the user manual for your charge controller prior to buying all of your gear.
I hope that was an informative and easy-to-understand explanation of the difference between series and parallel circuits, and how those differences apply to building your DIY solar power system. While most explanations on the internet focus on wiring loads and devices in series and parallel, I felt that you really needed to see how it applies to wiring batteries and solar panels. Hope it helps!
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