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Read This Before Buying A 12v Solar Panel

by Chris - March 1st, 2011.
Filed under: Solar Education. Tagged as: , , , , .

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This article covers some common mistakes people make when they are looking to buy a 12v solar panel. Installing solar powered products for improving your life is rarely a strait-forward craft. There are always tricky bugs that creep up and a sharp eye needs to always be kept on voltages and power ratings. Read this article to prevent yourself from succumbing to these common pitfalls that are specific to 12v systems using 12 volt solar panels:


Panel Voltage Should Be 1.5x Higher

Many people who decide to purchase a 12v solar panel do so in order to charge a 12 volt automotive battery. They want to use a solar panel 12v to charge a 12v battery – makes sense right? Unfortunately, it’s not that simple.

Automotive, 12v batteries are only nominally 12 volts. During operation they will vary from 13.5 volts down to 10.5 volts. Additionally, a solar panels 12v output will undergo some loss before it actually reaches the battery. These losses come from the internal resistance of the panel, the voltage drop across a protection diode, the resistance of the connecting wires, and finally the internal resistance of the battery.

For this reason, solar power experts recommend that solar panels be rated for an output voltage that is 1.5 times greater than the battery to be charged. In this case, that would be 18 volts (nominal). This gives room for voltage drops between the panel and the battery so that by the time the energy reaches the battery it still has enough voltage to charge it.


Use a Protection Diode

If you are purchasing a 12v solar panel for use in a battery powered system or for charging batteries, be sure the panel either comes with a protection diode or you know how to install one. The best option is to purchase a charge controller to sit between the battery and panel in order to ensure they ‘play nice’ together.


Electrical current is often analogized to water flow. A diode acts just like a one-way valve, allowing electrical current to flow one way. In this case, we want the current to flow from the panel to the battery. At night, the voltage of your 12v solar panel will decrease far enough that the battery will be able to discharge into the cells. This will fry your panel! A simple $0.10 diode will prevent this from happening.


Ensure The Proper Power Rating

In the event you want to use a solar panels 12v output to drive a system directly (no batteries involved), then the situation changes. You do in fact want a 12v solar panel for this system and no diode is needed so long as there is no way for energy from somewhere else in the system to discharge into the panel at night. In this case be very wary of the power rating of the 12v solar panel.

If you are trying to run a 15 watt pump with a 12v solar panel, you would want the solar panel to be rated for 15 watts, right? Wrong.

A pump or other load is typically rated at the power required for operation. The power rating for a solar panel is the maximum power output in full sunlight. If it’s a cloudy day or the panel is partially shaded, you won’t get anywhere near the rated power. For this reason, solar power experts recommend that the power rating of a solar panel be two (and preferably three) times greater than the load you desire to power with it. In this example, you’d want a 45 watt, 12v solar panel in order to power a 15 watt pump reliably.






18 Responses to Read This Before Buying A 12v Solar Panel

  1. I have a panel that im building from scratch. will the system survive if i wire the cells to have only 16 volts?

  2. Hey Brett,

    I’m not sure what you mean by ‘survive’. If you’re not using a charge controller, be sure to have a diode between the panels and batteries. This will prevent the batteries from discharging into the panel and frying it.

    Chris

  3. so i would be charging a 12v / 7.0amp battery, so the solar panel will have an output of 18 volts but what about amps?

  4. You’ll need a charge controller (or at a minimum a diode) to control current flow into your battery. Check out this page on how to build a simple solar system.

  5. Hi,
    After reading your artical it would seem this solar charging is a minefield!
    Can I use a 24 volt panel and reduce the voltage to charge a 12 volt battery bank so that there is no voltage drop?
    Regards, Barry.

  6. Hey Barry,

    Theoretically, yes, you can step the voltage down for charging. Realistically, no, it’s not practical to do this with simple components.

    If you already have a 24v panel, then your best bet is to put two batteries in series and find an inverter rated for 24 volts for using the power. Alternatively, you may be able to find a 12v charge controller that is compatible with a 24 volt input.

    If you don’t have the panel yet, then look for a 12v (18v really!) panel to use. If you stick with 12v technology, then you’ll find that you can pick up a lot of what you need at your local automotive store. It will make everything easier and cheaper to stay with 12 volts.

  7. More questions for you! You cited solar power experts recommending that the power rating of the panel should be 2-3x greater than the load. Is this pertaining to the system with or without batteries? Assuming the rule applies to the system with batteries, and the average sun-hours here in Washington falls somewhere between 3.5 and 4 hours in winter, and using your equation from the DIY Barn post, I figured a range of 72W-82.28W. So I would need a panel between 160-240W? But, so far, I have found that the higher wattage panels are between 24V-36V or higher. Also, which voltage rating do you consider? The peak voltage? A few I’ve viewed have nominal voltage ratings as well. I did find one panel at a reasonable price that was 100W 18V mono from Home Depot for approximately $300, but I’d hate to have to link 2-3 together when I could get a 250W 24-36V mono for around $500-$600. But then, I couldn’t use the free inverter I got that’s 1100W 12V. You also mentioned that a charge controller might be found with a 24V input. If that were utilized, then would I still have to change the inverter? Thanks!

  8. Another question…would it be possible to run a trough defroster with a solar panel? I’d assume that I’d need a battery if I wanted the defroster to run at night, but I was wondering if it would be worth it, or if it would be very inefficient?

  9. Hey Ashley,

    Good questions!

    The 2-3x greater rule-of-thumb applies to both system with and without batteries. A single panel may be hard to find for the power ranges of 160 to 240 watts. However, like batteries, panels can be put in parallel. You could put two of these 120 watt 12 volt solar panels in parallel to get the power you would need. The voltage rating to pay attention to is the Maximum Power Voltage. This is the voltage when the panel is under full sunlight and putting out maximum power. It should be between 16 and 19 volts, typically.

    In regard to the defroster, no, you wouldn’t want to run it off your solar system. Electricity is a very inefficient energy source for heat. I’d recommend you look into a propane or other gas-powered device whenever heating is concerned. People forget how energy dense fossil fuels are. Also, when you burn a fossil fuel for the purpose of heating, you operate near 100% efficiency. I don’t know much about defrosting food troughs though, so I don’t know if this is a realistic option.

  10. Hi Chris-

    I am trying to run a 12V, 20W pump for a trickle irrigation system. The pump says input voltage can be 9-12V. I’m worried if I use a 12V solar panel, it is actually 17.3V and will fry the pump?

    Could you recommend a solar panel that I can use and any other parts? I’d like to keep the system battery free if possible.

    Cheers!
    Keith

  11. Hey Keith,

    Pumps are pretty flexible when it comes to voltages. It’s the power rating that you should pay attention to. If the pump is 20 watts, then look for a 20 or 25 watt solar panel, like this 25 watt solar panel by HQRP. This panel would be perfect for your pump.

    While the operating voltage is stated as 17.4, the short circuit current is only 1.53 amps. Your 20 watt pump will draw 1.67 amps at 12 volts. The voltage and current out of this solar panel will equal out and meet the operating point of the pump. Worst case, your pump will draw all the current that the panel can put out and the voltage will stabilize to whatever it needs to be – probably between 13 and 15 volts.

    That’s a long winded way of saying that as long as your in the ball park with voltage, but your power ratings match, you’re going to be OK.

    Cheers!

  12. Dear Chris,

    I am little confused about solar panel voltage rating. At one place you have mentioned that for 12V battery, we need 18V (nominal) solar panel. At other place, you have mentioned 1.5 times greater voltage for solar panel for Open circuit voltage.

    Sent for clarification

    Thanking you, I remain

  13. If you’re charging a 12 volt battery, then 18 = 1.5 * 12. So it’s both. The voltage of the panel at max power (Vm or Vmp) should be somewhere between 16 and 19 volts. It doesn’t have to be right at 18 volts.

    The open circuit voltage of the panel may be quite a bit higher than 18 volts. The max power voltage is what really matters.

  14. Dear Chris

    First of all – great document, thanks for sharing this.
    I am having my leadacid battery 100Ah, and wanted to use it for 3 hrs a day with 4 cfl bulbs (18W) and 2 36″ ceiling fan’s, i might need to look after an inverter too, but after reading ur document, i realised that i can charge this battery with solar panel, just blinking on how to arrive this wattage requirement. If it is 100watt panel, then 12vdc*8.3Amps, could this wattage be sufficient? i am having 6 hrs clear sun light. pls help

  15. Hi Chris I made the mistake of buying 2 270 watt 24v panels and my system I have used for a long time is 12v. Is there a way to split the voltage coming off of the panels? When I pulled the cover off the panel were the wires come out I saw 4 tabs coming off the panel? There are 6 diodes in there too?What do you think?

  16. Use a voltmeter to test the voltage across the different diodes. With some careful soldering and cutting, you will probably be able to separate the 24v plan into two 12v panels. However, you should really hire an electrician or someone familiar with electrical concepts to do this for you.

    Cheers!

    Chris Troutner

  17. I have aquired a an oddball solar panel. It is a 92v dc high voltage @ 14amps 240 plus watts(!!!). It does not have a voltage regulator.
    two questions,
    I am wondering what the original application was for this panel?
    and if I attempt to charge a bank of 6v or 12v batteries will a diode protect the panel?

  18. I have some “12v” panels that seem to be putting out only around 10 volts. Is there any way I can bump this up so that I can charge a 12 volt battery?
    Thanks much

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