Testing a Solar Panel

102 Hacking Electronics

are lucky enough to live somewhere sunny, then solar power is obviously a lot easier.

Figure 5-13 shows a typical solar panel. This one was scavenged from a security light installation. It is about six inches by four inches and has a swivel mount that allows it to be angled toward the sun. It is the panel I used for the chicken house door.

Projects that use a solar panel to provide power nearly always also use a rechargeable battery. So the panel charges the battery and the project draws its power from the battery.

Small solar cells generally only produce around half a volt, so they are usually combined into panels of many cells that increase the voltage to a level that is high enough to charge a battery.

The voltage you find on a solar panel normally refers to the voltage of battery that the solar panel is capable of charging. So, it is quite common to find 6V or 12V solar panels. When you measure the voltage from these in bright sunlight, the reading will be much higher, possibly 20V for a 12V panel. But, under the load of charging a battery, this drops rapidly.

Figure 5-13 A solar panel

Testing a Solar Panel

A solar panel will have a certain number of watts and a nominal voltage specified for it. These tend to be for ideal conditions, so when I get a solar panel that I want to use in a project, I like to test it to find what it is really capable of. Without knowing how much power it can provide in a real situation where it’s installed, it is hard to make safe assumptions about battery capacities and how low you need to keep the current consumption.

When testing out a solar panel, you should use a resistor as a “dummy load,” and then try out the solar panel in various locations and levels of brightness, measuring the voltage across the resistor. From this, you can calculate the current being provided by the panel.

Figure 5-14 shows such an arrangement for my “chicken house” solar panel. The meter is showing just 0.18V with

Figure 5-14 Testing a solar panel