Once you have solar panels and a decent MPPT controller you’ll become the recipient of free electrical power from the sun! You can properly harness this power if you understand the electrical connections on solar panels.

Each of the solar panels shown below will produce 22 volts in bright sunlight at 6 amps each. In the right conditions they are rated at 100 watts maximum each

To get the maximum advantage out of your panels you should definitely be using an MPPT solar controller to take care of the incoming voltage. No other type of commonly used controller is as effective. I’ll talk more about controllers towards the end of this article and explain the reason for this advice.

    Neg         Pos    Neg         Pos

The picture above is two identical solar panels that you may buy to start your solar adventure. They may be rigid or flexible panels but in either event they need to be fastened in such a way that high winds won’t damage them.

In the picture above, the positive and negative cables coming away from the panels are represented by pos and neg.

If your boat or RV is a 12 volt system, you can wire up your panels in 2 ways. If you connect your panels in parallel, you will connect the Neg on one panel to the Neg on the other panel and then connect the 2 positive cable together in the same way. You will probably have to extend these cables to reach the solar controller. You now have a power supply that in bright sunlight will deliver 22 volts at 12 amps to the controller. This is easy to do, but not the best solution.

The 22 volts mentioned above is a maximum figure. In dull weather or around dusk the voltage could, and ultimately will, reduce to a point where it can no longer provide a charge to the battery. There is a much better way.

Connect the batteries in Series. To do this (using the picture above) Take the Pos of the panel on the left and connect it to the Neg on the right hand panel. You will then connect the negative on the left hand panel and the positive on the right hand panel to the controller. This will result in a delivery maximum 44 volts at 6 amps to the controller.

The real benefit of this arrangement is that the higher voltage will enable battery charging to begin earlier in the day and later in the evening. It also allows more efficiency in low light conditions. As a bonus, the ‘Maximum Power Point Tracking’ controller has a special attribute. In bright weather there will be an excess of voltage being produced. Alternative controllers that are not MTTP would simply dump this extra voltage and it would be lost without doing any good at all. Your MPPT controller wastes very little. It takes excess voltage and turns it into added amperage!

My own system has three panels like the ones illustrated and they are wired in series. This means that my MPPT controller, that is actually rated to accept up to 75 volts, is more than happy to accommodate the 66 volts my panels can send it. We have an instrument on the dashboard that details in real time the voltage and amperage being sent to the batteries. Although the system is only rated at 6 amps, we regularly see more than 9 amps being delivered. This is free usable electrical power. See the photo above.

This is the MPPT controller I’m using. The 75 is the maximum voltage accepted and the 15 is the maximum amps the controller can produce.

The voltage reading you can see on our dashboard readout is not a mistake. This boat is actually based on a 24 volt system. The 9.23 amps shown is 50% above the rating of the panels because the controller has converted the excess voltage to amps. The 942 shown in the bottom right corner of the picture is the number of free kilowatts the system has given us. 942KWh that we would have had to pay for had we not had the solar on board.

You may be interested to know that an independant small inexpensive solar panel can be used to keep your engine starter battery fully charged if it’s not going to be used for a while. Not a good beginning if after the cold season your engine won’t start! Take a look at another article of mine for full details. Cheap solar for Starter Battery.

If you’re interested in the history of solar panels you’ll really like this article from Wikipedia.