Wiring a 12V panel and 24V panel in series

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rseveran
rseveran Registered Users Posts: 2
in Solar Beginners Corner #1
I have a Grundfos water pump that requires at least 30 volts (30-300 VDC). I have a 110 watt panel at 24V, and a 100 watt panel at 12V. Is there any problem/issue with wiring these different voltage panels in series -- to get about 36 volts?

Basically one person has told me it's fine, and another has said it will limit the total power to that of the smaller panel. So I'm looking for some real experience before I wire it up tomorrow.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Wiring a 12V panel and 24V panel in series

    Welcome to the forum.

    Here's what will happen: the Voltages will add up, but the current will be limited to the lower of the two panels.

    So If your 110 Watt panel is actually Vmp 24 it would be about 4.5 Amps. The 100 Watt panel would be probably Vmp 17.5 and about 5.7 Amps. What you'd get in series would be (24 + 17.5) 41.5 Volts but only 4.5 Amps or a total of 186 Watts (roughly).

    I've never seen a 110 Watt 24 Volt panel. That must be one unusual item. Normally a "24 Volt" panel has a Vmp over 30.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
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    Re: Wiring a 12V panel and 24V panel in series

    I should mention also that it's not recommended to wire panels in series if their Imp rating is more than 10% different. The reason being that under certain conditions the higher panel can force too much current through the lower one causing it to burn up. As a precaution, put a fuse in the circuit that is rated between the two panels' currents.
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  • rseveran
    rseveran Registered Users Posts: 2
    #4
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    Re: Wiring a 12V panel and 24V panel in series
    Cariboocoot wrote: »
    I should mention also that it's not recommended to wire panels in series if their Imp rating is more than 10% different. The reason being that under certain conditions the higher panel can force too much current through the lower one causing it to burn up. As a precaution, put a fuse in the circuit that is rated between the two panels' currents.

    Now that is the kind of concerning info I'm looking for. What would you use, a 10 amp fuse? I have had a hard time finding anyone else doing this...maybe this amperage difference on these panels is too big of a problem? It is probably max 3.4 amps versus 5.9 amps.

    I'm actually trying to finish a job that an electrician started. The electrician is going on vacation and yet, the cows would still like to drink! So right now, we're running the Grundfos off a generator. Great pump, but I wish it would run off 24 volts.

    Previous to this one, I can't say I'd ever seen a 24 volt, 110 watt panel either. It can be wired for 12V or 24V, according to the diagram on the back. It is an Evergreen solar panel. At max power, I think the voltage is about 32.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Wiring a 12V panel and 24V panel in series

    You really need to know the current ratings for both panels. If the 24 is a 'true' 24 it would be 35 Vmp and only 3 Amps, which is quite a bit different from 5 to 6. In which case you could use a 5 Amp fuse (you aren't likely to find anything between 3 and 5) so if the larger panels hits its Isc rating due to a short anywhere the smaller won't be subjected to the 2X current.
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