01Wiring solar panels in series

Voltage and current in series

Series wiring is simple: connect the positive (+) pole of one panel to the negative (–) pole of the next panel. This increases the voltage but keeps the current the same. The total output voltage is the sum of the voltage of all the individual panels.

For example, if you connect four solar panels of 18 V, 5 A in series, you will get a total output of 72 V, 5 A.

What it's best for

Because series wiring keeps the current the same, it is suitable for low-current systems.

It has no need for fuses, and the wiring is easier and more cost-effective. Higher voltage means lower current through the wire. Smaller wire costs less than larger wire.

Lower current also means less heat loss during wire transport, so the solar system works more efficiently. And the lower heat loss allows you to use longer wires to connect devices. As you know, high temperature is not good for charge controllers, inverters, and batteries. A longer wire makes it possible for them to work in the shade or in a cool room.

Connection precautions

• All panels should have the same rated current. The lowest rated current of any one panel becomes the output current of the whole system.
• Each panel should be unshaded. If any panel is covered by shade, it affects the power output of the entire system. For example, if the current of any panel drops from 5 A to 4 A, the total output power would drop from 360 W to 288 W (72 × 4).
• If one wire is damaged, the whole system stops working.
• You need an MPPT charge controller. It is a DC-to-DC transformer that converts power from a higher voltage to a lower voltage. The amount of power is unchanged (except for a small loss during conversion). For example, if your panel draws 18 V, 6 A, the charge controller will adjust the voltage to 13.8 V and increase the current to about 7 A. That way the battery can get more power because the overall charging power doesn't drop that much.

02Wiring solar panels in parallel

Voltage and current in parallel

Parallel connection means connecting all the positive (+) poles together and all the negative (–) poles together. This keeps the voltage constant but increases the current flowing into the controller.

Connecting the same solar panels (four panels of 18 V, 5 A) in parallel, you will get a total output of 18 V, 20 A.

What it's best for

As each solar panel works independently, parallel wiring is suitable for mixed-light conditions.

In some conditions, like one panel being shaded while the others aren't, you will not lose a lot of power compared to series connection, because you still get enough voltage. Every panel works separately, and one does not affect the others. And because the total voltage is low, it is much safer.

It is also suitable for low-voltage systems, such as some RVs and boats. Because parallel wiring doesn't change the total output voltage, you can choose solar panels that have a similar rated voltage to your battery bank.

Connection precautions

• You can use a PWM controller to reduce the output voltage of solar panels so the batteries work well. PWM controllers are much cheaper than MPPT controllers.
• Parallel connection requires thicker wire, fuses, and connectors. If one panel is shorted, each panel must be fused at the junction of the connector.
• As current increases, the wiring must be larger, which means a higher investment and more heat loss during charging. If you plan to use a longer wire, you need to increase the size of the wire, which further increases cost.
• Use solar panels with the same rated voltage, because the output voltage in parallel wiring follows the lowest rated voltage.

03Which is best for you?

By now you already know the pros and cons of both series and parallel connections. But which wiring method is best for you? The answer is not black and white. It depends on your requirements. There are three main factors you need to consider.

Shade condition

If you use solar power in RV camping life, parallel wiring is usually better. You may park your campervan under trees, beside buildings, and so on, so shade covers some parts of your panels. In that case, if you still use series wiring, the output power of your system drops significantly.

If you are sure there will be no shade covering, you can use series wiring. The higher voltage makes more output at the peak of the sun.

Batteries

When charging your batteries, the total output voltage from the solar panels must be higher than the nominal voltage of your batteries. For example, some 12 V batteries require 12.6 V at the start of charging, while the output voltage of most solar panels is 18–20 V.

In order to meet the voltage requirement of batteries, parallel wiring must use about 70% of its capacity, while series wiring only needs less than 30%. That 30% capacity is easier to get even in the morning, late afternoon, or on a cloudy day.

Charge controllers

If you're using a PWM controller, you have no choice but to wire in parallel. That means more combiners, fuses, and bigger wires — all of which cost more money and installation time.

If you are using an MPPT controller, you can directly connect multiple panels in series to obtain a higher input voltage, and let the MPPT controller handle the charging. No need for combiners, fuses, or oversized wires.

04Other factors to consider

There are also other factors when you consider series or parallel. One of them is the different ratings of the panels. A lower rating will result in lower power output.

The best and simplest way is to use solar panels of the same brand and ratings (voltage, wattage). Also, do not mix old and new batteries.

05A real-world example

Let's say you have a 200 Ah/12 V battery, two 200 W/24 V panels, and an 850 VA/12 V inverter. Should you connect the panels in series or parallel?

In this case, you can connect them in parallel. The panels have a 24 V nominal voltage, which is above the battery's 12 V requirement, and parallel wiring keeps the voltage within a safe range for the 12 V system while adding current.

06The bottom line

Series and parallel wiring both have their place. Series is efficient and cost-effective when your panels are unshaded and you have an MPPT controller. Parallel is more forgiving in mixed-light conditions and works with cheaper PWM controllers, but it requires thicker wire and more fusing.

Before you buy another panel, think about where you'll use it, what controller you have, and what battery bank you're charging. The right wiring method can make the difference between a system that barely works and one that keeps your batteries topped up all day.