# Performance of solar panel vs. DC power supply

I am building a solar swamp cooler for burning man (hot dry desert environment).

Purchased a 15w solar panel that outputs 0.9a at 20V in full sun (verified as true with my voltmeter). The solar panel apparently provides 0.8a at 17V under load.

I applied the output to power a 12V fan rated at 0.4a, and it seemed to work fine, although a little slow.

THEN, I applied my PC power supply output to the fan, and it hummed along superfast, providing an awesome air current.

What's the story here? If the fan draws 0.4a, what is the solar panel lacking?!

Thanks for any insights!

A solar panels current rating is based on short circuit current (short the wires together) and it's voltage is open circuit.

When you attach a fan (an inductive load, relatively heavy load too), the voltage of the panel will drop and is probably less than 12V. However, because there is some current flowing through the fan coils, it will spin, albeit slowly.

Your solar panel CAN'T provide 800mA @ 17 V. It can provide up to 800 mA current and up to 17 V. Give or take.

The plug in power supply can deliver however many A @ 12 V. It will maintain 12 V. A solar panel won't.

You should use a battery and solar regulator to supply the power to the fan. As long as there is more power going into the battery than the fan is consuming over a long period, you should be ok. Remember the battery won't charge during the night so the panel needs to deliver enough power during the day that will be used in a 24 hour period. That assumes a sunny day every day too...

Power is voltage multiplied by current, per Ohm's law. (P = IE)

Even though the solar panel is showing a few more volts than your PC power supply (17 versus 12), the PC power supply has a great deal more available power than the solar panel (probably a few hundred watts or more).

Supposed available power: $$17V * 0.8A = 13.6W$$

Fan's required power: $$12V * 0.4A = 4.8W$$

The fan is rated for 400mA, and the solar panel is supposedly capable of 800mA, but the question is "What is it really providing?"

Measure the current draw of the fan from both power sources and see what the actual current is. My hunch is that the solar panel isn't actually delivering as much as the fan would like.

The specs given on the few solar panels I have worked with have always been more of an "ideal situation" rather than reality, sadly.

Also keep in mind that a 12V fan may run erratically when given a higher voltage. In the case that the panel is providing enough current, the fan might actually be having problems due to the higher voltage.

• I appreciate the response, will measure current draw when the sun is up again. Meanwhile, I'm curious about the erratic behavior. This case fan came with a detachable speed-control knob. When I turned the speed down (increased resistance per my meter), the fan actually sped up a bit. Is this what you mean by erratic? – nimble Aug 8 '13 at 0:55
• If you saw faster speed with more resistance while connected to the solar panel, it's likely that the voltage drop on the potentiometer was such that the fan received closer to the 12V it is rated for. If you exceed that voltage, it may actually go slower, run hotter, and die prematurely. – JYelton Aug 8 '13 at 1:30