When people think of a power supply, they typically think of a device that will put out a constant voltage and supply current to a device as necessary.
A solar panel, on the other hand, will vary its voltage and current based on a characteristic curve called an I-V curve. The performance of the panel will always follow the curve.
If the current is limited to a value, the voltage will change to the point on the IV curve where that current value is.
The figure below shows the IV curve for a 10W solar panel and an ideal 10W USB power supply.
In the figure, the green diamond shows the panel's maximum power point, which is the only point that the panel will produce 10W. The power output will be less at any other point on the IV curve.
This particular panel could be used to charge a 12V battery. If the panel is connected directly to a depleted 12V battery with a voltage of 10.5V, the panel will produce power at 10.5V and 0.72A.
As the battery charges, the voltage of the battery and panel will increase together, and the current will decrease based on the IV curve shown above.
If an inexpensive USB lighter adapter was connected to the panel and to a phone to charge, the panel voltage would be the same as the output of the USB adapter, which is approximately 5V.
At 5V, this panel will output 0.74A or 3.7W. So even though the panel can produce a maximum of 10W, it will not produce 10W in every situation. In contrast, the 5V USB power supply will output up to 2A at 5V, making the full 10W available at 5V, as shown in the IV curve above.
With the PowerFilm-tested 12V USB adapter, you can achieve higher power out of the 10W panel than a USB adapter that would otherwise pull the panel down to 5V. Our tested and recommended 12V USB adapter can supply up to 7-8 watts to a phone or phones, assuming they don't try and draw exactly 10 watts.
Phone(s) that draws 10W will collapse the voltage of a 10W panel. The reason for this is the adapter is not 100% efficient, it's more like 85% efficient, so at 10W, the adapter is drawing 11.76W.
This, of course, is more than the solar panel can produce. When this occurs, the panel goes to maximum current with minimal voltage on the IV curve, and the adapter shuts down. It will restart, and the phone or phones will either lower their power draw or not charge at all. So for a USB-to-phone ratio, PowerFilm recommended a min of 10 watts of solar for each cell phone.
This is why power point tracking systems are employed.
These systems hold the panel at maximum power despite the output voltage requirements.
If a power point tracking system was used, it would enable the panel to produce 10W and provide 10W to the USB adapter by decoupling the panel from the device it is charging.
Solar panels are fantastic solutions for remote power and allow us to stay charged in nearly every location.
That being said, it’s important to understand how they are measured, what we can do to optimize performance and how they differ from power supplies.
Do you have other questions you’d like answered or need more information about PowerFilm’s custom design, engineering and manufacturing abilities?
Contact us or leave a comment below.