Solar panel array size for small 24V off-grid system

[DPCabin]

I thought I was good to go on the solar panels and battery components for my off-grid cabin but after watching one of William Prouse' videos I'm not sure. I have 4-12v 100aH AGM batteries that I'm planning to connect to create a 24v system. I also have 6-100watt polycrystalline panels that I was planning to wire in 2 strings of 3 panels each connected in series which = 10 amps & 36V going into charge controller. (I'm a novice so please excuse any inappropriate terminology etc. or errors of other kinds but please comment on them). Is there a problem with this plan? I think I heard that 40V was needed for a 24V system from WProuse. I can buy 2 more panels if need be but I'd rather not if this setup will work. In case this is relevant, this is a weekend cabin with solar to be used for lights and ceiling fans initially. I'm eager to get the charge controller soon so I can get the system running but not sure whether I will get a 40amp or 60amp MPPT one yet. Thanks for any insight into my plans!

 

[DPCabin]

In other words, what does the quote below from the DIYMobileSolar site mean? Will 2 strings of 3 panels (described in my first post) work or would I need to wire all 6 panels in series (=72V) or buy 2 more panels to have 2 strings of 4 panels (=48V)? Thanks.

• You need a higher voltage solar panel array. Not really a con because you should be series connecting your panels anyways. Open circuit voltage of solar panel array must be at least 40+ volts if you plan to charge a 24 volt battery with it. This means that if you are using 12 volt panels (open circuit of 17-20 volts), you will need to series connect your panels to create a larger voltage.

 

[DThames]

Determine the Max Power voltage of the panels. It should be written on a sticker on the back. Add up that voltage in your series circuit. 16v times 3 for example will give you 48volts. Most 12v panels are max power about 16v or so. Now look at your MPPT voltage range of your charger. If the voltage for max power of your panel string is well inside the MPPT range of your charger, then you are good. Also do the same with the open circuit voltage and make sure your series string voltage does not exceed the max input voltage of the charge controller.

 

[DThames]

Unless you plan to add a lot more panels someday, the 40 amp charger is enough considering you say you expect 10 amps from the panels.

 

[DPCabin]

Determine the Max Power voltage of the panels. It should be written on a sticker on the back. Add up that voltage in your series circuit. 16v times 3 for example will give you 48volts. Most 12v panels are max power about 16v or so. Now look at your MPPT voltage range of your charger. If the voltage for max power of your panel string is well inside the MPPT range of your charger, then you are good. Also do the same with the open circuit voltage and make sure your series string voltage does not exceed the max input voltage of the charge controller.

In reading thru my notes and looking at the solar panel output, I realized the max output is 18v not 12v. Phew! That's exactly what you are mentioning. So 2 strings of 3 panels =54v which is enough to charge batteries. Next step while building the solar rack is to decide on charge controller. Thanks for tips on figuring that part out too! The solar industry has close to collapsed in Louisiana due to removal of state rebates so getting help from experts in person is tough.

 

[DThames]

Back to sizing the controller....What you really want to do is to look at the panel power. Say you have 600watts going to a 24v battery. If you got 100% energy transfer (you will not) you could get 25amps on the 24v side for charging the battery. Size based on future plans. If you plan to or might need to add more panels, get a charger that can handle your growth.

Note I have a 10amp MPPT connected to 500w of panels. It will peg out at 9.8 amps on what it sends to the batteries but it only using about 3 or 4 amps on the 72volt Max Power input side.

 

[DPCabin]

I have many questions. Here are another couple. I wired my entire cabin (maybe 1200 sq. ft) for 110v 20 years ago thinking I'd connect to the grid some day. But, now I'd rather not and expect to put a whole house solar system in in a couple of years when trees are cleared, a shed built, etc. etc. In the meantime, I'm setting up the small solar system described above. My reason for choosing to go with 24v is that I'd like to use some of the circuits already in place connected with 12 AWG romex wire. All I plan to run are LED lights, a few ceiling fans, maybe a few more things as I figure out the system and learn more about limits. There will be fuses and breakers as appropriate.

First question: Is this a reasonable approach? Any problem with using existing romex wiring?

Second question: I'm planning to route all the loose circuits into my AC breaker box/panel. Is it possible to run a wire directly from the inverter (e.g. the 2000 watt sin inverter Will mentions on his website for a 24v off-grid system) to the breaker panel? I am confused by the inverters having 3 pronged plugs which are not very applicable to circuits. Surely there are connections to direct wire the inverter to AC circuits, right?

Thanks.

 

[kernel]

Its a vowarranty void, but the plug outlets in your inverter are likely soldered on. Its just a set of wires. You could crimp or wire nut a set of conductors if you needed to hardwire. Place a cover over the outlet holes and place a knockout in it for wire retention or conduit connection.

There is not much wrong with using a double male plug aside from reaching the ampacity of the assembly. If there are two pairs of outlets, they are likely fed from one set of wires.