The Solar Power Expert Blog

How To Build a Simple Solar System

by Chris - April 3rd, 2011.
Filed under: Projects. Tagged as: , , , .

There are three primary components to even the most basic of solar systems: the solar panel, the charge controller, and the battery. This article will take you step-by-step and show you how to construct a simple solar system for use on a boat, an RV, or at home.

Why a boat? Well… because boats are an excellent application of a simple, 12-volt solar system for keeping the boats batteries charged. It’s easy to make the system more complicated, but my goal is for you to have a thorough understanding of the important role each of the three subsystems play in a well constructed solar system.

The trick to building a well designed solar system is sizing. The solar panel, charge controller, and batteries all need to sized to one another. For example, if the solar panel outputs too much power it will fry the battery.

The Battery

The battery is the best place to start as people typically have the least control over how big their battery needs to be. For use in an automotive or marine application, you’ll probably want to look for batteries in a Group 24, 27, or 31 form factor as these are the sizes that typical car batteries come in.

I want to be sure and point out that the physical shape of the battery does not effect the solar system. However, for solar applications, you want to be sure to get deep-cycle batteries – not the typical starting batteries used in cars. Deep-cycle batteries are best suited for solar applications. Many boats will have two separate batteries – one starting battery for the engine and a ‘house’ deep-cycle battery to power the lights, fish finder, depth finder, etc.

Where do you find deep-cycle batteries? You can get deep-cycle car batteries at walmart or if you’re a serious solar system builder, you may want to get batteries that are specially designed for solar applications such as a Sun-Xtender Solar Battery. It’s not critical to get an automotive form factor, but you may want to refer to my article on understanding battery ratings.

The Charge Controller

In a nut-shell, the job of a charge controller is to ensure the battery and solar panel ‘play nice’ together. An oversized solar panel can over-charge a battery and damage it quickly. Likewise, when the sun goes down, a charge controller prevents a battery from discharging into the solar panel which will damage the panel.

The simplest (and cheapest) charge controller is a diode. A common analogy is to compare electrical current to water running through a garden hose. A diode is like a one-way valve that lets the electric current flow from the solar panel to the battery, but not the other way.

In order to get away with using just a diode, you have to very carefully size the battery and solar panel together. The rule of thumb is that the maximum output current of the solar panel should not exceed 1.5% of the batteries amp-hour (AH) rating. As an example, if the battery is rated for 100AH, then the solar panel should not output any more than 1.5 amps. Any higher and you risk frying your battery.

The repercussion of this is that you won’t be able to charge your batteries very fast. This sort of system works well for a sail-boat that is left unattended for days at a time, needs to keep its batteries topped off, and does not have any loads on the battery when the boat is not in use. In order to utilize a higher power solar panel and achieve faster charging, you should look at purchasing a charge controller, such as this one:

Sunforce Seven Amp Charge Controller

Sunforce Seven Amp Charge Controller

The Solar Panel

So finally we come to the solar panel! At this point you know how to properly size the panel, weather or not you’ll need a charge controller or can get away with a diode, and you know the right type of battery to use. Armed with that knowledge, you can make an educated decision on the type of panel you need. Be sure to check out our solar panel reviews to get an idea of some of the available solar panels that you can use for your system.

Also, be sure read my guide on selecting 12v solar panels. Pay attention to the section that talks about choosing a panel with an open-circuit voltage that is 1.5x higher than the batteries nominal voltage. In this case, we have a 12 volt battery, so we need a panel with an open circuit voltage of about 18 volts.

Extending the example above using the Sun-Xtender Solar Battery and just a diode as a charge controller, you would want to use a solar panel similar to this Goal Zero 15 Watt Solar Panel:

Solar System Specifications

Putting it all together, here is how it breaks down:

  • The Sun-Xtender Solar Battery is rated at 89 amp hours. 1.5% of that would be 1.335 amps.
  • This Goal Zero 15W panel outputs an open circuit voltage of 18 volts, which is 1.5x higher than the 12 volt battery – that means our charging voltage is acceptable.
  • 15 watts divided by 12 volts (nominal) equals a charging current of 1.25 amps, which is less than our 1.5% threshold. So this panel should keep the battery topped off without any worries about overcharging.

Hopefully, by following the example solar system on this page, you will be able to piece together your own solar system. Of course, if you purchase the parts here that I recommend, your system will work well too.

2 Responses to How To Build a Simple Solar System

  1. Chris, I would like to light my barn for feeding, or working after dark. I would like 2-4′ fluorescent lights. I was thinking of solar panels, batteries, & inverter. What’s your recommendation for a set up like this?
    thanks Jeff

  2. Hey Jeff,

    Good question! Thanks for asking. I went ahead an wrote up an article covering how I’d go about designing a solar barn lighting system:


    Chris Troutner

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