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Recently I bought a 35 watt 12v solar panel, I have a small pond 5 watt panel that supposedly is 12v as well. I wanted to combine and use both of these panels for an off grid application. I want to install these in parallel to keep the voltages the same, I know that I will lose some output wattage due to connecting/mixing panels of different amps. My confusion comes in on how to connect them together, what is the best possible way.

Should I connect them together in parallel first then run one positive and negate wire to the controller? Or should I run both panels, 2x positive and 2x negative wires to the controllers + and - terminals? Right now I’m looking at a PWM controller but I’m not sure how well this will handle what I’m trying to accomplish. Should I use a MPPT controller instead? I rad that its better to keep batteries that are not of the same specs separated, for example using two controllers since most controller only have one + and – terminal. I could plug the 5 watt into one controller and the 35 watt into the other then run the wires from both controllers to the 12v battery. This seems a little excessive for what am doing but im looking for a safe way to do this and get the most watt output possible.

Also have have these panels mounted on a fence post, the post is obviously in the ground, should I be grounding any of the other components ? Im assuming since the post is in the ground that its grounded and so I are the panels attached to it via a metal bracket?

One last thing, I know that panels have many voltage ratings and one is VOC, when I tested the 35 watt panel the VOC was 21 volts as stated in the panel, however the 5 watt was only about 10v, there is no specs sheet on this panel. I know they produce difference amps but is this VOC reading on the 5 watt normal?

My last question is where should the load plug into, I know the charge controllers have a load terminal, but if I want to plug in more then one load I read that I should be connecting directly to the battery, is this correct? I know I might need a bus terminal if this is the case.

I would appreciate any responses and thoughts to this, this is my 1st time doing something like this and I want to get it jut right. Thanks, Paul

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    Under modern conditions (price and availability of PV panels, .vs. prices for controllers/batteries) it's likely not worth crippling your system to include the 5w panel - just find some job it can do by itself (such as run an attic fan or small fountain pump only when the sun shines) and if you want more, either buy a second 35W panel exactly like the first, or look around and buy a significantly larger panel while finding some job for the 35W panel to do by itself, or with a small battery. Having spent quite a while looking into off-grid, I would not go less than 24V for a serious O-G system. – Ecnerwal Jun 18 '19 at 15:11
  • Agreed with @Ecnerwal; I think it's pretty likely you could buy a nice 200W panel for less than the cost of the electronics to properly combine your 5W and 35W panels. Also keep in mind that 5W isn't even enough to fast-charge a cell phone these days. – Nate S. Jun 18 '19 at 16:32
  • OK so I was thinking of not using the 5w as well however let's say I wanted to use it, what's involved in that even though it seems like a waste of money. Just trying to figure out what would be involved in mixing those panel in a efficient way. Thank you both for the replies. – Da P.A Jun 18 '19 at 19:04
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10 VOC for the 5 Watt panel means there is a problem with it. Clean the panel, test again. If still under 12 V it will not help your system anyway.

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Assuming the panels have the same nominal voltage output, you can connect them side-by-side (in parallel), using a blocking diode for each as shown on this page. This won't give the optimal loading to either panel, but you should see roughly the same open circuit voltage irrespective of the wattage, and the on-load voltage is really what matters. For example, if your 35W panel has an optimal load voltage of 17V, so long as the smaller panel open-circuit voltage is above 17V, it will contribute some current to the load.

As mentioned, this is not really worth the effort (much less using one charger per panel). If the panels are closer in relative size, it becomes a more sensible thing to try.

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