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Old 15-04-2013, 19:08   #1
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Solar Panel Setup Question.

I thought I knew electrical and Ohms Law but I'm getting some contradictions in some of the solar threads on the forum here. I've just started getting into solar systems.
If I were to take 24V/240W panels and reduce them down to 12V output thru a MPPT controller, does that half the wattage at 12V to 120W? Or am I getting mixed up here?

Are they like batteries, or not?

Quote:
Originally Posted by solarpaneltalk
Here is what I did over the weekend:

1- Connect 175W/24v and 12v battery to charge controller. The display show panel input 12.8V @ 4.6A = 58 watts
2- Connect 100W/12v and 12v battery to charge controller. The display show panel input 12.8V @ 5.4A = 69 watts
Due to space restrictions I'm thinking of going with 2 X 12V/100W panels. It seems if one goes over 100watts the price shoots up with every 5 watts from about $1.25 per watt to $2.90 per watt. But these maybe too long, so I'm considering 3 X 12V/75W panels. All this would be in parallel.

As well I'm considering the Moringstar ST-MPPT-45 controller. But I've read that it has to be hooked to a computer to program it. Or is there something off with that too? Does the TRistar meter TS-RM-2 allow for programing.

There is so much info out there that seems to contradict itself that it's hard to know what's what. My brain is getting numb with the confusion.
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Old 15-04-2013, 19:13   #2
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Re: Solar Panel Setup Question.

watts is power. volts X amps = watts. SO cut the volts in half and you have 2X the amps. Watts is constant. (assumes electronic efficiancy = 100%)
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Old 15-04-2013, 19:35   #3
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Re: Solar Panel Setup Question.

If I were to take 24V/240W panels and reduce them down to 12V output thru a MPPT controller, does that half the wattage at 12V to 120W? Or am I getting mixed up here?

what nicholson58 said. the constant is 240watts. so at 24 volts you get 10 amps and at 12 volts you get 20 amps. volts X amps = watts.

that's about the limit of my electrical knowledge, so i went the simple/cheap route; pwm controller. there's something to be said for simplicity....
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Old 15-04-2013, 19:49   #4
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Re: Solar Panel Setup Question.

Quote:
Originally Posted by Nicholson58 View Post
watts is power. volts X amps = watts. SO cut the volts in half and you have 2X the amps. Watts is constant. (assumes electronic efficiancy = 100%)
So does that mean if I buy a 24V/240W (10A @ 24V) panel and run it thru a MPPT for 12V charging, I'll still get 240W in theory.

"Watts is constant"? Does that mean they doesn't change with voltage?



So with 10A doubled = 20A X 12V = 240W
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Old 15-04-2013, 20:09   #5
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Re: Solar Panel Setup Question.

In theory you are correct. Using a MPPT controller you would get the conversion from 24v down to 14v, so a bit less then twice. (minus conversion losses in the Mppt controller of about 3-4%) But then again a 240 watt panel may only produce about 170 watts under actually outdoor conditions. So you might only see 12.5 amps out of the panel or a bit more. Could be a bit more, but less in summer heat.
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Old 15-04-2013, 20:44   #6
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Re: Solar Panel Setup Question.

Quote:
Originally Posted by sailorchic34 View Post
In theory you are correct. Using a MPPT controller you would get the conversion from 24v down to 14v, so a bit less then twice. (minus conversion losses in the Mppt controller of about 3-4%) But then again a 240 watt panel may only produce about 170 watts under actually outdoor conditions. So you might only see 12.5 amps out of the panel or a bit more. Could be a bit more, but less in summer heat.
If this is the case then why bother buying 12V panels when one can get twice the power out of 24V panels that are slightly larger then a 12V of half the wattage, and twice the price?
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Old 15-04-2013, 20:53   #7
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Re: Solar Panel Setup Question.

My limited understanding as explained by Nolex and others is that the higher output (VOC) you can buy for a given area, the better that will translate into useable charging power thru a good MPPT, when solar energy lessens because of clouds or time of day.
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Old 15-04-2013, 21:17   #8
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Re: Solar Panel Setup Question.

Quote:
Originally Posted by Pelagic View Post
My limited understanding as explained by Nolex and others is that the higher output (VOC) you can buy for a given area, the better that will translate into useable charging power thru a good MPPT, when solar energy lessens because of clouds or time of day.
So, that sounds like a 24V panel will have more "useable charging power" = more power, in unfavorable conditions, yes?

All theses terms>>> Glossary

Quote:
Voc = Voltage open circuit - nothing attached to the panel.
Vmp = Voltage maximum power - what the Voltage would be under ideal conditions with the maximum output current (good luck measuring this in the real world).
Imp = Amps maximum power. The 'I' comes from an old term "Intensity" - before Monsieur André-Marie Ampère got his name on it. In theory, Imp * Vmp = panel Watts.
I feel like I'm about to take on another career.
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Old 15-04-2013, 21:32   #9
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Re: Solar Panel Setup Question.

Disclaimer: I don't know nuthin.

If you buy 7-billion-volt panels and run them through an appropriate MPPT controller, you'll end up with a nominal 12v at whatever amps gets you the rated wattage, minus some reality-factor. The MPPT (more or less) lets you not care about voltage, and uses that higher input voltage to find the "sweet spot" to charge your batteries more efficiently. Watts in = watts out (minus that reality factor).

"Appropriate controller" is the limitation there. The Tristar-45 (which I have) is rated for some voltage (60?) that I'm approaching with my 3 "12 volt" panels (which actually make ~19v) wired in series. Series vs parallel seems to still be a bit of a subjective choice, and I've recently found out that my single 125W panel may be limited by being in series with 100W panels. Whatever - my batteries generally stay full so I'm thrilled with what I've got. (And too lazy to experiment with rewiring!)

The panels I'm running are still around $1.50/W shipped from ebay (user ulsolar). No problems with them so far, and shipping/packaging was great. I think they may have clunkier frames than eg, Kyocera, but the efficiency etc. is pretty much the same (says the shady product literature....).

I can't find any 24v panels for near that price, but I didn't look very hard either.

Bigger panels are generally cheaper per watt, but I couldn't find a place for them.

I built "panels" out of cheap 1x2s and cardboard, and spent a couple weeks duct taping them to random sticks and rails and whatnot in various places on my boat to find the size and placement that would actually work for me. It was a very useful exercise, even though I think my neighbors might think I'm a little weird....
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Old 15-04-2013, 21:33   #10
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Re: Solar Panel Setup Question.

Quote:
Originally Posted by delmarrey View Post
So, that sounds like a 24V panel will have more "useable charging power" = more power, in unfavorable conditions, yes?

All theses terms>>> Glossary



I feel like I'm about to take on another career.
It might help to think of it as water. Voltage (tension) is the water level. Current is how much water runs through a river or pipe. Resistance is like rapids or a restriction in a pipe.

A battery or a solar panel is a power source: think of it as a water well.

Now, when we don't tap water from the well, it's level comes up to a nice level. This is Voc. As soon as you start tapping water, that level will drop but so many gallons per hour run out which is the current. When it needs to pass a restriction, less water can go through plus some energy is spent there bashing through. That splashing water is like heat in a resistor or bad contact.

The total number of gallons you tap per hour (gph) is like the energy used (kWh)

You can basically use this to explain everything about Ohms law.
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Old 15-04-2013, 21:34   #11
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Re: Solar Panel Setup Question.

Also, the morningstar is fully programmable via push/pull pins. You can hook it up to a computer for other magic, but you don't need that to program the thing.
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Old 15-04-2013, 21:53   #12
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Re: Solar Panel Setup Question.

Quote:
Originally Posted by delmarrey View Post
So, that sounds like a 24V panel will have more "useable charging power" = more power, in unfavorable conditions, yes?
:
Not exactly. A single 24v 240 watt panel will put out about the same amps as two 120 watt 12v panels.

You buy a singe 240 watt panel for say $240 and add a blue sky MPPT unit for $225 for a total outlay of ~$500

Two 120 watt panels will cost, oh $350 (best cost I saw for two) plus a $65 morningstar PWM controller, So say $420 ish.

Yes the 240 watt panel with a MPPT controller will put out more power. But that single panel will be effected by shading at least part of the day, which is always an issue with a sailboat. Then the output will be greatly reduced.

I think, in many cases using two smaller panels, at anchor, will give more overall amps in the batteries in a day. This as normally one panel is in full sun.

But it all depends. Clear as mud I know...
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Old 15-04-2013, 22:18   #13
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Re: Solar Panel Setup Question.

Quote:
Originally Posted by sailorchic34 View Post
Not exactly. A single 24v 240 watt panel will put out about the same amps as two 120 watt 12v panels.

You buy a singe 240 watt panel for say $240 and add a blue sky MPPT unit for $225 for a total outlay of ~$500

Two 120 watt panels will cost, oh $350 (best cost I saw for two) plus a $65 morningstar PWM controller, So say $420 ish.

Yes the 240 watt panel with a MPPT controller will put out more power. But that single panel will be effected by shading at least part of the day, which is always an issue with a sailboat. Then the output will be greatly reduced.

I think, in many cases using two smaller panels, at anchor, will give more overall amps in the batteries in a day. This as normally one panel is in full sun.

But it all depends. Clear as mud I know...
So with that, would ya'll think 3 x 12V/75W panels would be better. I sure would be able to fit them in a lot easier then two 100+W panels? Slightly less wattage but less chance of shadowing effects.

s/v Jedi,
I went through electrical courses back in the 70's but I don't remember dealing with watts back then. It was all amps and volts, it seems. So, I do understand the concept, it's just this watts vs amps thing that's got me spinning. I even use to repair CNC machine tooling back in the 80's/90's. I must be losing it in my golden years.

I'm doing research on the controller right now just to get a better understanding of its format.

And thanks Dustymc. That's a relief that I don't need another computer. I had enough of that programing CNG compressor control panels.
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Old 16-04-2013, 00:46   #14
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Re: Solar Panel Setup Question.

The best policy is to try and fit as many watts as you can in the space available and at the lowest cost. The number and voltage of panels only has a small impact on the final output.

Keep in mind that sometimes the delivery costs for large panels can be high. Couriers don't like large glass fronted objects.
With the cheap costs of solar these days the installation costs can become dominant.
So look at the total installed cost per watt rather than the raw panel price.
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Old 16-04-2013, 00:58   #15
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Re: Solar Panel Setup Question.

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Originally Posted by delmarrey View Post
So with that, would ya'll think 3 x 12V/75W panels would be better. I sure would be able to fit them in a lot easier then two 100+W panels? Slightly less wattage but less chance of shadowing effects.
Splitting the panels up does give a slightly higher output with shade, but the difference is small. Sometimes more, smaller panels is easier to fit than fewer large panels, sometimes the opposite is true, this is usually the dominant factor together with the total watts.
Smaller panels can often make better use of the space, but tend to be a bit less efficient in terms of watts per surface area because there is proportionally more framing for smaller panels.

Measure the space and see how you can best squeeze in the most watts.

However 3x75w is 225w so it is always going to better than 2x100w
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