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Old 07-12-2018, 09:54 AM   #1
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High voltage low amps vs. low voltage high amps solar panels??

Hey yíall, Iím getting ready to finally get some solar panels but Iím stuck at this point. I would like to use higher volt panels but I donít know which way is better for the batteries.

Is it better to have something like 600w 24v panels giving 25a or 600w 12v giving 50a.

Do I need more than a 30a charge controller (mppt) for the 24v panels since they might be coming in at more like 30v and maybe converting extra volts to amps?

Are there any drawbacks to using less larger panels as opposed to more smaller panels?

600w of panels is a little too much if Iím up north but might be needed down south in the winter, Iím still figuring out the sizing with concerns like daylight hours of locations. We plan on chasing warm weather. Thanks in advance and if there is a previous thread on this I couldnít find it but would be happy to check it out as any info helps.
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Old 07-12-2018, 10:06 AM   #2
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30A is about as high as anyone will recommend on a 12v system.

Also, while you may have 600W in panels, that's your maximum and you'll only come anywhere near that in absolutely peak conditions, keep that in mind when planning your power system.
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Old 07-12-2018, 12:32 PM   #3
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With higher voltage panels, you will need a charge controller than can properly convert the power (MPPT) so the actual voltage out the panels has nothing to do with the voltage that the batteries see.

I tend to think of 50 volt panels being "high voltage panels". You might be able to use a less expensive PWM controller with a 24V array - not positive, you'll have to check on that.

A positive to higher voltage panels is using small size cables from the array to the charge controller.

Typically, charge controllers are rated by their maximum output - some allow you to pick the output voltage. Therefore; a 30 amp charge controller will put out a maximum of 30 amps regardless of the input power (there are limits). Note that there is an advantage here when working with higher output voltages (a 24VDC battery bank vs. a 12VDC bank) - that is another subject...
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Old 07-13-2018, 06:29 PM   #4
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I found out where my confusion lies, if anyone is wondering.

What I really need is to do some more reading on how mppt charge controllers work. I did a little research watching YouTube and understand a little what mppt controllers do. With that knowledge I began to answer my question but need to figure out a few more things before I move on.

If anyone has a good resource on sizing solar panels with mppt controllers I could use any info I can get. Thanks again!

Also, I might be making this harder than it needs to be? Iím not sure.
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Old 07-13-2018, 11:15 PM   #5
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The theoretical advantage of having panels in series to produce a high voltage at a low current is more relevant when cable runs are very long, such as when a PV array is some distance from the house. No bus has cable runs anywhere near that length, so don't worry about saving money by using smaller-gauge wiring for higher voltages! Because buses and boats often have less-than-ideal insolation compared to an ideally-located home installation, it makes more sense to wire panels in parallel, then if one panel is shaded it won't greatly affect the array's overall output. A parallel-wired array's voltage is no higher than any individual panel's voltage, and the few feet of cable run from the panels' combiner box down to the charge controller(s) can easily be a moderately-heavy cable and still have minimal voltage loss.

One more thing to consider: MPPT charge controllers (and if you're thinking of having about 600W of panels, it would be wasteful to instead use a PWM charge controller for that much power) are most efficient when the voltage drop between panels and battery is as little as possible. For example, the Morningstar TS-MPPT-60 has an efficiency in the high-90s percent when the voltage down-conversion is no more than about 2:1, but its efficiency drops a lot if it's asked to down-convert a series-wired array at a much higher voltage. This is one of several reasons that each string of my panels is in parallel, sending about 30V at 34A to each CC that then produces about 14.7V at up to 60A for the batteries. The CC is running very cool at that low stepdown ratio: heat is the biggest enemy of electronics, so (I hope) my CCs will last a very long time. I use 10AWG cables from each panel to the combiner box, a 12A fuse there for each panel, a 12-foot length of 4AWG cable to bring the power down to the CC with a 50A circuit breaker, then a short 2AWG cable to take power from the CC to the batteries with an 80A breaker. The really heavy 4/0 cables are elsewhere!

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Old 07-14-2018, 06:05 AM   #6
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Quote:
Originally Posted by Joee View Post
If anyone has a good resource on sizing solar panels with mppt controllers I could use any info I can get.
John's point about conversion efficiency is a good one and also reason that a higher battery bank voltage (like 24V vs 12V) might be a better choice (stepping 50 volts down to 28 volts is more efficient than stepping it down to 14 volts).

"SCC" = solar charge controller

Other panel array to SCC "sizing" considerations include:
1. SCC minimum/maximum voltage. As John said, you can connect the panels in such a way to meet these requirements.

2. SCC total output capacity. This is related to battery voltage. For example, a Morningstar TS-MPPT-45 can output a maximum of 45 amps. If your battery bank is 12V, this is about 630 watts (charging voltage of 14V). If your battery bank is 24V, this is about 1260 watts (charging voltage of 28V).

3. The real world output of the array does not match the theoretical maximum. Solar insolation is the biggest factor but how the panels are mounted can also be a significant factor. Observing high voltage panels on my last two coaches over the last four years (all in the mountain west - south to north), I get ABOUT 60% of rated output during the winter months and 80% during the summer months. Factor this into your SCC considerations.

4. An MPPT SCC can be 'overdriven' to some extent. This means that a 700 watt array can be connected to a 45 amp (12V battery) without damaging it. Using the theoretical maximum, the array could produce more power than the SCC can output (45 amps) but the SCC will simply clip off the excess. However; this is a poor example as, in the real world, a 700 watt array is unlikely to produce a full 700 watts (#3 above).

Depending on how and where you plan to use your system, #3 can be a biggie. We often think that installing 600 watts of panels will get us 600 watts of power. That is not usually the case.

I don't claim that this is the best approach but I tend to build things with some future growth/expansion in mind - at least when it is possible/affordable. For example, one might conclude that a 30 amp SCC would be sufficient for a 600 watt array to a 12V battery bank. This would be borderline (undersized) and would have zero expansion capability. I would instead go with a 45 or even 60 amp SCC so I could later replace that array with a larger array. Same with the cables, oversize them a bit.
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