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Understanding your solar system

written April 2003

A solar system converts energy from the sun to electricity you can use in your RV. Since it only provides electricity when the sun is out, it is usually set up to charge batteries that can supply electricity when the sun can't.

A solar system has two main components. The solar panels are outside where they can catch the sun's rays and the charge controller is an electronic circuit usually stuck inside somewhere to manage the system.

A solar panel is a lot like a battery in that it provides electricity at a preferred voltage. A battery depends upon chemical reactions to make its electricity while a solar panel depends upon the sun's rays. Batteries tend towards poorer performance when they get cool while solar panels generally improve performance when they cool. Batteries have problems keeping up the voltage when all of their chemicals are converted and solar panels have the same problem when the sun gets weak. Batteries are composed of 2 volt cells and most RV batteries use six cells for about twelve volts. Solar panel cells are about a half a volt each and panels use 36 cells to provide about 18 volts. The reason solar panels are at a higher voltage than batteries is so they can push electricity into a battery to charge it.

The charge controller's main job is to connect the solar panel to the battery when (a) the battery needs charging and (b) the panel is able to generate enough energy to do the charging. There are other management tasks a charge controller can do as well. Solar panels should be disconnected when they are not producing electricity. Batteries need to be charged properly for fastest possible recovery and long life. Users need to be informed about the status of their solar system and its components. Modern charge controllers use built in computers and power supply technology to make the most of the energy from the solar panel.




Bryan’s solar panel is portable and can be moved with the sun

Don’s solar is installed on the roof of his Airstream

Rick’s solar panel is barely visible on the top of his truck.

How much do you need?

If you look in the store for battery chargers, you will find most of them run around ten amps. This is about the minimum needed to charge a typical RV or automotive battery in a reasonable time. A ten amp solar panel is about a hundred watts. Less than this and you aren't doing any effective battery charging. More than this means you can charge more batteries or charge them fast enough so they are brought up to full charge in only a day or so of full sunlight.

The maximum amount of power you can get from the sun is about a kilowatt per square meter. If we allow about 17% loss for haze, angle of the sun and other factors, we'd have a kilowatt per square yard or about 100 watts per square foot. But solar panels are only 15% efficient in the best case so you'd need to increase solar panel size by seven or eight times to compensate for the poor efficiency. What all this means is that you will need almost a square yard of solar panels to get 100 watts from your panels on a sunny day. You might have 6 square yards on the roof of a typical larger RV but working around vents, air conditioners, antennae, or other things makes filling the entire roof with panels impractical.

The charge controller

The charge controller is the brains of your system. At its simplest level it is answering the question: "should the panel be connected to the battery or not?" To figure out the answer, the controller needs to determine if the solar panel is producing enough energy to move it to the battery instead of vice versa and that the battery is in a condition to accept the energy. The simplest charge controllers take a look at the panel output and battery voltages and then either connect or disconnect. These are shunt type controllers.

PWM or pulse width modulation controllers incorporate modern technology to improve battery charging. They control the current going to the battery in order to charge the battery more fully and faster.

MPPT or maximum power point tracking controllers work like PWM on the battery side and also use a similar technology on the solar panel input side. This means they are not only smart about charging batteries but also smart in how to pull the most power from the solar panel. MPPT controllers work best when the voltage difference between the solar cell and the battery is largest. This happens when the battery is discharged and the solar panel is cool.

Wiring

The wiring can make or break your solar system. With most RV solar systems, the current is only ten or twenty amps and the run is only ten or twenty feet so super heavy cable isn't usually necessary – just somewhat heavy cable such as between 6 and 10 gage. Wires that are outside, such as the wires connecting the panels, need to have an insulation that can withstand the elements. All wires need to be tied down so they don't flap and chafe. Connections need to be properly secured so they don't come loose.

Monitoring

The basic information you need to determine how your system is doing is battery voltage under a light load. Some of the low end systems just use a set of indicator lights to tell you if you are low, OK, or high. Better systems actually tell you the voltage. More advanced systems allow you to see the voltage and current being produced by the solar panels. High end systems keep track of current use and can provide a battery capacity remaining indication.

**Also see a step by step installation of solar panels on an Airstream

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