Simple Solar Power
Light contains energy. When light hits a conductor (or semiconductor) some of the energy is translated into moving electrons, creating current. We can harness the current using solar cells (aka photovoltaic cells). When the sun shines on a solar cell, the current output is mostly constant, which is known as direct current, DC. DC is easy to use and great for charging batteries and powering microcontrollers, but using DC from solar cells can be tricky.
Solar panels come in many varieties. When shopping for solar panels for your system, there are a few specifications you need to follow. The specs below will refer to the Solar Cell Huge, shown above.
- Output voltage range: The panel above will output a voltage between 0V and 10V depending on the intensity of light and what you have attached to the solar panel (i.e. the load). You will need to make sure whatever you attach to the solar panel will be able to handle at most, 10V. This is called the open circuit voltage.
- Power: 5.2 Watts is the total power this solar panel can deliver at its maximum efficiency. The solar panel is most efficient at a point called the maximum power point, which is surprisingly not the maximum voltage, 10V, but a little less, 8V. When the panel provides its peak power, the current draw is around 650mA. This is called the short circuit current. Multiply 8V by 0.650A and you get 5.2W.
- Maximum power point: With no load (nothing connected to the solar cell), in full CO sunlight, the solar panel will output around 10V (Voc). When you start to draw more current (increasing load), the voltage starts to decrease, and the efficiency of the solar panel hits a sweet spot, then tapers off.
The sweet spot for operation is called the maximum power point (top of blue curve) and is around 8V @ 650mA for this specific panel. Sophisticated solar power systems use MPPT controllers to monitor and balance the system to maintain the max power point. To check if your system is at this point, simply measure the voltage on the solar cell with your load attached to see if you are close to 8V. It isn’t too big of a deal if you are not exactly at the maximum power point, the panel will still work, but you will be losing efficiency. Notice, in our testing, we saw 550mA short circuit (Isc), which is outside of the maximum power point.
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