Solar panel sizing

[Bags4266]

Hello, just wondering when looking at smaller solar setup why wouldn’t I want to buy a 200 watt single panel as opposed to (2) 100watt panels? One 200 watt panel is smaller than (2) 100 watt panels.

[grandseahawk]

I was wondering the same thing... then I did the math.

I went with Renogy/Rich Solar) Panels. I was going to install three 100 watt panels, but opted for two 170 watt panels (they are actually 200 watt, not sure why the eBay listing said 170 watt)

Each 200 watt panel is @1573 square inches. (58.7" by 26.8")

Each 100 watt panel is @933 square inches (45.5" by 20.5"). So, @1866 square inches total for two 100 watt panels (200 watts).

1573" vs. 1866"

The real estate used "per watt" is less using the bigger panels. Also, the cost per watt was a little less in purchasing the bigger panels.

My roof can easily fit the bigger panels, so the choice was simple.

[Ynot4me2]

With 2 panels, you get a little redundancy. You can also plug them in series for greater voltage. But don't quote me on that last statement. I'm also fairly new at solar.

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[MoonShadow_1911]

With 2 panels, you get a little redundancy. You can also plug them in series for greater voltage. But don't quote me on that last statement. I'm also fairly new at solar.

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Yes, and yes. You get some kind of redundancy, depending on your setup, and if in series, more voltage. You can also get more current (amperage) out of parallel.

I would caution you on using 24vdc (series) to charge a 12vdc system.

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[grandseahawk]

I am still looking into that as well... trying to wrap my brain around it too.

The slow-blow and/or inline fuses typically only go up to 24 volts (two 100/200w panels in series will be 24v). I'm still undecided on running my panels in parallel or series too. Some solar converters (Renogy/Rich) will automatically step down the 24v from the in-series panels to the @12v level the batteries need (if your batteries are run in parallel). Like Moon Shadow_1911 stated above... I learned that if you run more than 2, 3, 4, etc. panels in series, you will certainly have higher voltage and lower amps. But, and a big but, I think it's virtually impossible to find 36v, 48v, 60v etc. (DC) fuses. Maybe they exist, but I could only find 12v -24v (DC) fuses that would work in these applications... meaning 2 panels in series only (24v). So, that means with 4 panels... an idea might be to run 2 in parallel and those two sets of two in series to get 24v for your 24v fuse. Then from 24v fuse to solar controller and let it step down your power to the 12V (if your are running you batteries in parallel). Still need to look into if that's actually possible... Can any electrical/mechanical engineers reading this care to chirp in???

My main concern ether way... parallel or series is the crappy #10ga wire that was used from the roof to the front basement area. In my 2020 399TH it's not MTW (machine tool wire), or THHN (Thermal High Heat Nylon coated, aka "thermoplastic high-heat-resistant, nylon-coated wire"), THWN (a better version of THHN), etc. In fact, it's not even pure copper; it's CCC (copper coated copper) which is typically from China and may only have 50.1% real cooper in it. It's a cheap wire that can have lead, steel, pot metal, recycled trash, etc. in it... who knows. Anyways, it's a poor quality wire.

*For the record... I was an electrician for many years and still dable here and there. I have been a full time firefighter/paramedic for the past 20. But, I know my wire; I'm comfortable at least with that.
Honestly though, until late last year I was 100% CLUELESS about Solar, 12v fuses, 12v step-up and down, solar controllers, solar cells/panels and especially anything RV Solar. In fact, I would put my ignorance and "dumbness" at the level of "Dangerous Ignorance." It's very different from the Commercial/Industrial Electrical I was familiar with. I might be just slightly above that ignorant level now though ;-) I am still learning a lot, reading, studying, watching online videos, etc. (I learn a lot from people in online videos who give "bad" advice too... at first I did not know the difference.) There is a lot of great advice and input on this particular site, I think it's pretty much the best RV site. There are some pretty sharp folks on here who I think know their stuff. Respectfully, I would recommend reading through all the older posts, watching videos and research online from "legitimate" sources to anyone learning this solar stuff.

Rock On...

[Scramjet]

The slow-blow and/or inline fuses typically only go up to 24 volts (two 100/200w panels in series will be 24v). I'm still undecided on running my panels in parallel or series too. Some solar converters (Renogy/Rich) will automatically step down the 24v from the in-series panels to the @12v level the batteries need (if your batteries are run in parallel). Like Moon Shadow_1911 stated above... I learned that if you run more than 2, 3, 4, etc. panels in series, you will certainly have higher voltage and lower amps. But, and a big but, I think it's virtually impossible to find 36v, 48v, 60v etc. (DC) fuses. Maybe they exist, but I could only find 12v -24v (DC) fuses that would work in these applications... meaning 2 panels in series only (24v). So, that means with 4 panels... an idea might be to run 2 in parallel and those two sets of two in series to get 24v for your 24v fuse. Then from 24v fuse to solar controller and let it step down your power to the 12V (if your are running you batteries in parallel). Still need to look into if that's actually possible... Can any electrical/mechanical engineers reading this care to chirp in???

My main concern ether way... parallel or series is the crappy #10ga wire that was used from the roof to the front basement area. In my 2020 399TH it's not MTW (machine tool wire), or THHN (Thermal High Heat Nylon coated, aka "thermoplastic high-heat-resistant, nylon-coated wire"), THWN (a better version of THHN), etc. In fact, it's not even pure copper; it's CCC (copper coated copper) which is typically from China and may only have 50.1% real cooper in it. It's a cheap wire that can have lead, steel, pot metal, recycled trash, etc. in it... who knows.

Grand sea hawk is right regarding your question on panel size. You pick the panels that fit your real estate the best.

As far as Grand Seahawk’s question above (and trying not to hi jack the OP’s question); when looking at DC fuses you will go by the amp rating not by the voltage rating. You should pick a max fuse amp size that first protects the wire rating and the min fuse amp size should be about 25% above your expected operating current for the equipment.

The 10 gage wire run length is also important for voltage drop (that translates to heat in the wire which is your greatest enemy for wire size). Keep your wire run as short as possible. There are numerous calculators online. For a 200 watt system, 10 gage cheap wire can work with some efficiency losses as long as it is a shorter run. Try to keep the losses no more than 2-3%. Series connections will double the voltage while maintaining the rated current allowing you to have a smaller fuse and a longer wire run. This all assumes that you have a MPPT controller that handles the high voltage (which I highly recommend in any case).

Also remember that your solar panels will rarely be at maximum output.

Hope this helps,

Brian

[CoChris]

As far as Grand Seahawk’s question above (and trying not to hi jack the OP’s question); when looking at DC fuses you will go by the amp rating not by the voltage rating. You should pick a max fuse amp size that first protects the wire rating and the min fuse amp size should be about 25% above your expected operating current for the equipment.

Hope this helps,

Brian

@grandseahawk Sorry, I have to disagree with Brian's comment on voltage. The voltage rating of the fuse must always be higher than the peak voltage see across the fuse. Otherwise there is a chance the fuse can arc/conduct after it has opened. This will not protect the circuit. That being said there are many fuses with voltage rating well about 24v. Check most solar or marine supplier sites and you will easily find them into the hundreds of volts rating.

Chris

[Scramjet]

@grandseahawk Sorry, I have to disagree with Brian's comment on voltage. The voltage rating of the fuse must always be higher than the peak voltage see across the fuse. Otherwise there is a chance the fuse can arc/conduct after it has opened. This will not protect the circuit. That being said there are many fuses with voltage rating well about 24v. Check most solar or marine supplier sites and you will easily find them into the hundreds of volts rating.

Chris

Chris,

Well you are technically correct and I should’ve stated that. You would be hard-pressed to find a fuse that would not work in the 24 to 30 V range of a 200 W system connected in series. The cheap blade fuses are rated up to 32 V and many in-line fuses are 600 V rated. So yes I agree with you and thank you for the comment but I stand by the fact that the main concern is amps.

I do have a BS in electronic engineering.

Brian