Series or parallel connection?

Whether you connect your solar panels in series or in parallel, the same principle applies as for batteries. Parallel connection increases the amps and serial connection increases the voltage.

Parallel connection

Parallel connection means that the positive terminal of one panel is connected to the positive terminal of the next panel, and the negative terminal of one panel is likewise connected to the negative of the next panel, and so on down the line.

Solar panels connected in parallel

Parallel connection gives the sum of the amps of each panel output without increasing the voltage. 

• In parallel connection, the VMP of each panel does not have to be the same.
• Panels connected in parallel work best with a PWM (Pulse Width Modulation) controller.

Advantages of parallel connection

• You can use mixed sizes of solar panels.
  
• It's safer because you're not dealing with high voltages.
• You can use either an MPPT or a PWM solar controller.
• You can buy a very good PWM controller for much less than a good MPPT controller.
• A PWM controller needs enough power to light up its display. It only really starts to use power when there is more solar than your battery requires and the controller needs to limit the solar coming in. In this case, the power consumed is excess to requirements anyway. So a PWM controller can make use of even small amounts of solar available on overcast winter days.
  
• A PWM controller doesn't produce a lot of heat, so doesn't require special cooling.
  

Disadvantages of parallel connection

• You need more cable.
  
• You need thicker cable.
• If you have multiple solar panels, as their temperature increases, you will not get the combined voltage of them, because parallel connection combines the amps. So in hotter weather, you may not have enough voltage, even from multiple panels, to charge your battery.

Series connection

Series connection means the positive terminal from one panel is connected to the negative terminal from the next panel and so on until you end up with one negative at one end of the string of panels and one positive at the other end of the string of panels. That negative and positive then go off to the solar controller.

Solar panels connected in series

Series connection gives you the sum of the voltage of each panel output without increasing the amps.

Panels connected in series must use an MPPT (Multiple Power Point Tracking) controller to get the amps expected from the number of panels.

Advantages of series connection

• You need less cable.
• Your cable doesn't need to be as thick as that for panels connected in parallel because you're carrying higher voltage and lower current.
• As the temperature of the PV modules in a solar panel heats above 25°C, there is a drop in the voltage that the panel can produce. The higher that temperature is, the lower the voltage you can obtain from the panel. If you have one single solar panel, it's entirely possible for the voltage output of the panel to be too low to charge your battery. However, if you connect multiple solar panels in series, you're combining the voltage of all of your solar panels. So if you have two or more panels, you're more likely to keep your voltage at a level that will charge your battery in hotter weather. 

Disadvantages of series connection

• The higher voltage of series connected panels can be dangerous.
• All Vmp (Voltage Maximum Power) and Voc (Open-Circuit Voltage) ratings should be the same for each panel. If they aren't, you can only harvest solar power to the lowest Vmp and Voc of all the available panels.
• Each panel should be fitted with a blocking diode and a bypass diode.
• A good MPPT controller is quite a bit more expensive than a PWM controller. A cheap MPPT controller will not be a good one.
• MPPT controllers will use some of the energy being harvested from the solar to run the MPPT controller itself. In the early hours of the morning, later in the day or on a poor solar day, this may mean that the small amount of power being harvested is mostly taken by the controller itself, or that harvest may not even be enough to start the MPPT controller. So when you most need to harvest all the solar available because it's a lousy day, the MPPT controller will not give you the best return. MPPT controllers are best suited for large solar installations for this reason.
• MPPT controllers can generate a large amount of heat, depending on how much it has to alter the voltage coming in to suit the voltage of your RV. So, if you're harvesting, for example, 150 volts, to feed into a 12 volt RV, the MPPT controller has a lot of work to do and will generate a lot of heat. Thus, MPPT controllers are manufactured with a large fan and/or heatsink to disperse this heat.

Mixed panel sizes

Technically, you can mix solar panel sizes regardless of whether you connect them in parallel or in series. 

Series connected panels

The bigger panels will just waste the output that is greater than the smallest panel. Sometimes the bigger panel will push through the bypass diode and the smaller panel doesn't contribute at all.

Parallel connected panels

If connected in parallel, and using a PWM solar controller, you can have mixed wattages of panels and still harvest the full capacity of each panel. Your panels don't need to have the same Vmp.

What to look for when buying panels

What type and size of solar panel should you buy? How do you know if a solar panel will do what it claims? Do you need diodes?

Deciphering the back of a solar panel

Maximum rated power, short circuit voltage? What does it all mean, what's important and what is just a number to lead you up the garden path? How to interpret the information on the rear of a solar panel and apply it to your setup.