This article refers to experiments using the KidWind 2V/400mA Solar Panel, order code KW-SP2V, $19.
We have recommended a load resistance for each light source listed below. Note that the recommendations are based on using a single 2V/400mA Solar Panel. If you are wiring multiple panels in series or in parallel, the recommendations may not apply.
First, the best light source for any solar energy experiment is the sun. Cloudless skies are ideal. However, depending on where you are located and when the curriculum calls for learning about solar energy, you may not be able to go outdoors.
Recommended load for sunlight: 5 Ω to 10 Ω
Incandescent light bulbs (the kind with a filament) are the next best choice. The most common incandescent light bulbs sold in the US are halogen light bulbs. These can be placed in a desk lamp. Higher wattage incandescent bulbs will allow the solar panel to produce more power, and they will also get hotter with higher wattages. Many desk lamps warn to use bulbs no more powerful than 60 W or 75 W equivalent bulbs. However, some work lights may take 100 W or even 150 W bulbs. Caution students against touching the light bulbs while on and for some time after turning them off if you are using high-wattage incandescent bulbs.
Recommended load for incandescent bulbs: 10 Ω
The next choice is a toss-up between compact fluorescent light bulbs and LED light bulbs. These bulbs do not emit a broad “blackbody” curve of wavelengths, since they typically excite a phosphorescent material rather than putting electricity through a material until it is hot enough to glow. Therefore, these highly efficient and cost-effective light bulbs may emit only small amounts of light with energy high enough to produce much power from a solar panel. 60 W equivalent or 75 W equivalent bulbs are easy to find, however. Use these light bulbs if you have no other options.
Recommended load for compact fluorescent bulbs: 75 Ω to 90 Ω
Recommended load for LED bulbs: 50 Ω to 60 Ω
A note about the load:
When using an energy sensor with any power source, whether it is a solar panel or a wind turbine, there must be a load (resistance) in the circuit. For more information on why you need a load, see TIL 4373: Why do I need a load when measuring energy with an energy sensor?.
Using different light sources with different characteristics will affect the resistance value at which the solar panel will produce the most power. The values in this article are based on our testing using a 2 V solar panel manufactured in 2017. However, we change manufacturers from time to time, and our manufacturers may have a certain amount of variability in their production processes. We recommend that before having students perform experiments, you check your solar panels and the light source you intend to use for power production under different loads to confirm or adjust the values we have given.
If you are using a different solar panel, you may want to do some experimenting to see what light sources and load will work best for you. See TIL 4374: How do I determine the best amount of load to use with my solar panel?
Using solar panels in series generally increases the load resistance at which maximum power is produced, while wiring them in parallel generally decreases the load resistance at which maximum power is produced.