I purchased a "24v" solar panel thinking it would output only at 24v, or less if there wasn't enough light. Immediately tested it, in indirect light it's producing 38V. Checked the label on the back and it says:

Open Circuit Voltage (Voc): 43.5V

Voltage at Pmp 36V

On the box it says Nominal Output Voltage 24V.

I want to use this solar panel to power 24V fans, I'm worried providing that much extra voltage to the fans may shorten their lifespan. Is there something I can use to limit the voltage to no more than 24v? Is my concern about too much voltage to the fans legitimate?


Edit: Would a product like this work for me? http://www.amazon.com/DROK-Switching-Regulator-Adjustable-Stabilizers/dp/B00BYTEHQO/ref=sr_1_1?s=electronics&ie=UTF8&qid=1426729713&sr=1-1&keywords=24v+voltage+regulator
It says an input voltage of up to 40V. Is this too little because of the VOC? Or is it OK because the Voltage At pump is only 36V?

  • \$\begingroup\$ Vmax can be limited with a resistor that takes the absolute Voc edge off - you lose a few Watts only.| A 10 Watt zener would do - with series R maybe. Discuss.| A regulator like your example is OK BUT wattage needs to be higher probably. You can get higher powered ones on ebay for much the same $. | Yes - buck converter gives you a gain in Iout at lower Vout. Expect ABOUT Pout ~= 80% x Pin. More if lucky. | Watch heatsinking even with a switching supply. | Box was wrongly labelled as others note. \$\endgroup\$ – Russell McMahon Mar 19 '15 at 5:08
  • \$\begingroup\$ The rated voltage is measured open-circuit. When it is connected to the MPPT controller it will take excess voltage (as your panels are wired in series) and turn it into Amps at the proper voltage. My 6 panels provide over 120 Volts but under load they might be 70-80, and the MPPT controller provides from 26-29 volts or so for the load. The load includes both the battery bank and the inverter. \$\endgroup\$ – SDsolar Mar 25 '17 at 7:34

Solar panels act a little differently than you might think. For example, all "12 volt" solar panels actually output 18 volts, as high as 20 volts Voc in bright sunlight. You have to remember this is Voc and there really is no situation in which Voc is output when the panel is actually attached to a load and providing current.

I have read a couple of theories on why panels are made this way. One is that under lower light conditions you will still get the minimum voltage required for your load. This is the most common answer that I have read, but it doesn't make sense to me because under low light conditions amperage drops considerably making the panel not very useful.

The other more logical answer has to do with the power curve of the solar panel. If you graphed the current versus voltage curve of the panel you would find that for most cells, you get the maximum current (given the same sunlight) at about 75% of the max voltage of the panel or approx 14/18 volts.(when charging 12 volt lead acid batteries will rise to 14 volts and even a little higher)

So how does this apply to you? Well solar panels are interesting creatures and as soon as you hook it up to your 24 volt fan in bright sunlight, it will drop to 24 volt output and problem solved. No need for extra electronics. I am not sure of the correct terminology, but I believe that the panel's resistance is incredibly low, so that anything you hook it up to has a higher resistance. As a result the voltage of the panel drops to the voltage of the load.

For example my 12 volt panel that reads 20.5 volts in the bright sunlight drops immediately to 10.5 volts, to when I connect it to my depleted lead acid battery (which is 10.5 volts when completly depleted). I would think that the same should happen to your fans.

However, sunlight comes and go, birds, clouds, etc cause the panel's output to fluctuate and thus your fans would likely stop and start repeatedly. Have you considered hooking the panel to a battery first and the fan to a battery? It could be a relatively low capacity battery that could feed the fans a constant voltage and amperage and the panel could feed the battery. You would want a cheap charge controller for the battery. Two 12 volt lead acid batteries in series would work well. I do that with my wife's water fountain. It would shut down for a few seconds when a cloud flew over or the dog ran in front of the solar panel. I got a free lead acid battery that was essentially dead (measured capacity less than an alkaline C battery!). The panel charges the battery while the battery feeds the fountain. Works well even though the battery is almost dead, the battery only really providing full power for a few seconds at a time.

Hope this helps.

  • \$\begingroup\$ I don't mind if the fans are inconsistent. I'm using them to help with attic intake ventilation during hot summer days. I will try out tomorrow and see if the voltage drops once the fans are connected. Thanks! \$\endgroup\$ – user1308743 Mar 19 '15 at 9:29
  • \$\begingroup\$ They're "made that way" because a solar cell is, after all, just a silicon diode, and it has the same characteristics as any other diode, except that it has a current source (driven by the light) in parallel with it. The magnitude of the curent is proportional to the light intensity. A panel is just many such cells connected in series. \$\endgroup\$ – Dave Tweed Mar 19 '15 at 12:51
  • \$\begingroup\$ @DaveTweed - Thank you for providing much needed clarity. \$\endgroup\$ – Filek Mar 19 '15 at 22:46
  • \$\begingroup\$ @user1308743 - let us know what happens! \$\endgroup\$ – Filek Mar 19 '15 at 22:49
  • \$\begingroup\$ Connected up 4 fans and voltage dropped to 22V. Connected just 3 and it was right about 24V. It's interesting that it doesn't say 24V on the panel anywhere, I really thought a 36V panel was packaged by mistake. Thanks @Filek for saving me from buying a voltage regulator. \$\endgroup\$ – user1308743 Mar 19 '15 at 23:28

The box was mislabeled. The "PMP" is a reference to the panel's point of maximum power output, and that's saying that the panel's nominal voltage is 36V, not 24V as it says on the box. Maybe they're implying on the box that you'd use this panel in conjunction with a nominal-24V storage system, but that would require a separate MPPT/charger module.

In any case, assuming you want to use the panel to run some 24V fans anyway, a good approach would be to get a DC-DC converter module that accepts a nominal 36V input (they usually have a fairly broad range of voltages that they'll accept, at least 2:1) and a regulated 24V output. Such modules are available from several vendors — check Digi-Key and Mouser.

  • \$\begingroup\$ Thanks. If I use a DC-DC voltage regulator, do I get the extra amperage to get the full 30 watts that it would normally produce, or does a third of the power get wasted? \$\endgroup\$ – user1308743 Mar 19 '15 at 3:55
  • \$\begingroup\$ With a proper DC-DC (as opposed to LDO or something) you get the extra amps. :-) See: en.wikipedia.org/wiki/Buck_converter \$\endgroup\$ – R.. Mar 19 '15 at 5:59
  • \$\begingroup\$ You actually want a MPPT DC-DC converter. The issue is that solar cells aren't Ohmic (they do not have a linear V-I curve). \$\endgroup\$ – Aron Mar 19 '15 at 8:29
  • \$\begingroup\$ @Aron: MPPT is only useful if you have a load (such as a bank of batteries) that can absorb all of the available power. If you're just driving some fans at a fixed voltage, the power consumption is fixed anyway, and MPPT adds no value. \$\endgroup\$ – Dave Tweed Mar 19 '15 at 12:42
  • \$\begingroup\$ @DaveTweed That is incorrect. Without MPPT you will not have matched impedance. In fact, given that the OP wants to use motors, this is even more a reason to use MPPT, due to the start up current. A non MPPT DC-DC converter might not be able to drive the start up current because it might try to put too much load on the solar cells. \$\endgroup\$ – Aron Mar 19 '15 at 12:50

The panels are not mislabelled, btw. 24V panels usually have a no load in the 41V range, and maximum power, Pmp, is somewhat less. The 38V is the max power you can get out of the panel. A MPPT (max power point) charger keeps the power at the max point, which is the advantage of these chargers. If you are using a 24V battery bank, your panels are pulled down to the battery voltage, and you loose that much power. Let's say you have a 100W panel. That's 2.3A at 41V, or something like that. If you are charging a 24V battery (@26V), you have 2.3A x 26V or only 60W. With an MPPT charger, you can get the full 100W out of the panel. All other things being equal of course.


My 315 watt 24vdc panels normally output 39vdc to the MPPT charge controller, this is the way that amperage is created in the panel through voltage (amps convert to volts in dc without a load to consume them), you need to use a charge controller between the panels and fans to limit the voltage into the fan (PWM controller will work just fine as long as it is 24v) , your voltage will generally fluctuate throughout the day. I would suggest you put a little more into your system like: 2 - 12v batteries (they can be car batteries or barbie doll car batteries or deep cell, don't mater) and a PWM charge controller plus I would add a timer to control the fans and limit them to just daytime use or use a cheap 12v thermostat (same type in your home) and wire it into the fans to go on when the attic hit a preset temp.


All 24v panels can hit over 37v and as high as 50vdc at Voc

MPPT has no bearing here, so the many mentions of that are not particularly helpful.

An inexpensive PWM regulator is fine for loads, MPPT just maximizes panel output which is not necessary or useful here since there are no loads mentioned beyond the fans which could make use of the extra energy which can be extracted by a more expensive MPPT solution.

  • 3
    \$\begingroup\$ I think you have a major misunderstanding of how this site is meant to be used. This is not a forum in the traditional sense, and promoting your business is explicitly forbidden. If you have comments on others' answers, post them as comments on those answers, not as a new "answer" that doesn't even answer the question. \$\endgroup\$ – Hearth Sep 14 at 19:05
  • \$\begingroup\$ New users are not allowed to post comments, but that doesn't mean they get to abuse the answer form to do so. If you want to re-write this as a simple explanation of why PWM is sufficient and MPPT necessary, that can be an acceptable answer. But state the facts in a way that would make sense and be useful to the asker if this were the only answer on the page. Remove the rant, or you will likely see this deleted. \$\endgroup\$ – Chris Stratton Sep 14 at 19:08

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