Any solar gurus on here?

[andoland Senior Member Find all posts View Profile Close andoland]

I've just bought my first solar panel and am currently (no pun intended ;D) testing it out. It is meant to be a 120W panel, it has 36 cells and the specs are:
MAX POWER 120W
MAX POWER VOLTAGE(Vmp) 18V
MAXIMUM POWER CURRENT (Imp) 6.66A
OPEN CURRENT VOLTAGE (Voc) 21.6V
SHORT CIRCUIT CURRENT (Isc) 7.20A

I've measured the open current voltage at close enough to 21.6V.

On the weekend turned my Waeco on to it's coldest setting and let it run for most of the day to drag the battery down so the solar panel would be able to pump some current in. I turned the fridge off when the battery was down to about 12V then I connected up my new solar panel and MPPT controller to charge it up. It was a hot day (>30C) with clear sky here in Brissy yesterday when I was doing this. I had it set up in full sun but the MPPT controller was showing that it's only putting out about 80W. When I connect it up it ramps up to 80W straight away and then sits around 79-82W. The MPPT controller says it is putting out a constant 5.3A to the battery. The battery is a 80AH sealed lead acid.

While the solar panel (semi-flexible type) was draped over the windscreen of the Prado during this time, the sun was pretty much straight on it. At one point I did pick it up and move it around to see if it made any difference but I couldn't get the power output to get over about 82W. I did move it at one point and the output went up to 85W, but still far short of 120W.

So I'm trying to understand why it won't go over 80-ish watts. Is it the case that the regulator will only draw in the power that it needs so if I had a bigger battery or more drained battery it might have taken in more power? Have I been sold a dud panel, i.e. told it's 120W but it's really only 80W? I wanted to get some advice before I call the seller.

Here's photo of the panel and the read out from the MPPT controller. The controller is an MPP solar PCM3012 which is meant to be reasonable quality, it's not a $50 cheapie.

[Guest Close Guest]

The 120W is rated at a specific cell temperature.

[andoland Senior Member Find all posts View Profile Close andoland]

Yes I believe solar cells are typically rated at 25C and the drop off rate is 2.5% for every 5 degrees above that, so even if my panel was at 60 degrees C that should still be 100W (120W x 0.975^7).

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

Two factors are at play, the maximum power point is 18V, if the panel is below that voltage you
won't get 120W out of it.

As already discussed temperature is the other.

If you getting 5A or better your doing well.

[andoland Senior Member Find all posts View Profile Close andoland]

Two factors are at play, the maximum power point is 18V, if the panel is below that voltage you
won't get 120W out of it.

I think this gets to the heart of my question. If the panel is getting full sun on a Qld summer day and can't reach max voltage (temp issue aside) when is it going to reach max voltage?

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

Hi,

The performance of the panel will taper off as it gets hot, 18C day in Melbourne in bright sunshine you'll probably get your 120W if you have a good MPPT though that is still a question mark. Hot day in Queensland you not going to get it.

I have a 100W and a 120W blanket and that just keeps up with running the electrics in my Vista when up North, April to September.

The panel voltage is dependent on load, if the battery is low ie 12V and your not using a MPPT controller, the controller will pull the panel down to around 12V, as the battery charges and the charge current reduces the panel voltage will rise but by the time the battery reaches a point where the panel can get to 18V the battery will be nearly fully charged.

MPPT can make the most of the panels output though they are question mark, I have found there's not much difference in output with my one panel. If you want to make the most of them I feel you need a unit that can accept high voltages ie 100V and have two or three panels in series.

Simplest way to test if you have a MPPT is run the battery down, connect it to charger then measure the voltage out of the panel, you should see around 18V on the output of the panel.

[andoland Senior Member Find all posts View Profile Close andoland]

Simplest way to test if you have a MPPT is run the battery down, connect it to charger then measure the voltage out
of the panel, you should see around 18V on the output of the panel.

While it was running I put my multimeter across the Anderson plug terminals on the inlet to the controller. The voltage seemed to fluctuate up and down from around 13V up to 15V. It was quite a regular fluctuation over a few seconds, as though the voltage was being affected/controlled by the controller. That's why maybe I thought the controller was limiting the total power draw from the solar panel.

Maybe I'm over analysing - if I had a controller that didn't have a digital readout I would be none the wiser and would've just thought the system did a good job of recharging my battery:-|

[krypto Avid PP Poster! Find all posts View Profile Close krypto]

Until recently I have been using the same controller as you, PCM3012, and I was pretty happy with it for the past 3 years. I did notice a lot of voltage fluctuations and finally decided to try a different controller because I was worried about the effect of this on potential battery life.

Having spent a lot of time mucking around with solar I think the controller is as important as the panel. I have noticed with my new controller (an ep solar) that there is a lot less fluctuation and it seems to be quicker to adjust the voltage. It also has a battery temperature sensor to ensure the correct temperature compensation.

[andoland Senior Member Find all posts View Profile Close andoland]

Thanks Krypto. I don't really want to have to buy another controller after just having bought this one. What I've found frustrating is trying to identify good quality solar gear that doesn't cost the earth and it seems that there's no consistency - stuff that is expensive isn't necessarily any good and there are so many generic items out there, with vendors and brands that come and go, so it's hard to find any long term history of what works well.

I paid a bit more for my solar panel than the generic ebay/internet cheapies , with vendor assuring me it has genuine Sunpower cells, so I want to be confident that I've got what I paid for. At least if it is the controller not the panel the wasted money will be a bit less.

[stanmiko Member Find all posts View Profile Close stanmiko]

I would be questioning something else. If your panel is rated at (nominal) 6.66 Amps, then why is it only showing up as 5.3 Amps. I have 2 x 140W panels on my van, wired in parallel. They are each rated at 7.8 Amps and my controller is constantly flickering between 7.7 and 7.8 Amps... not directlt facing the sun at any time, just flat on the roof of my van.
I still think something is a go with your panel... or rather NOT a go.

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

Two panels rated at 7.8A that's 15.6A even with them laying flat at midday I would have thought you would see over 12A with low batteries? My projects blanket is 110W and I see 110W out of it laying on the ground.

[Symo Advanced Member Find all posts View Profile Close Symo]

With anything solar it all comes down to the controller. A quality MPPT controller will make any panel perform at 100%. Even cheap eBay ones.

I get 8.8A from my 120w eBay cheapie all because I bought a quality MPPT controller and wired the panels in series 48v, prior to this the most they would make is 4.5A.

Total system cost $280, look on people's faces when it out performs a $500-$1000 brand name, priceless.


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[stanmiko Member Find all posts View Profile Close stanmiko]

With anything solar it all comes down to the controller. A quality MPPT controller will make any panel perform at 100%. Even cheap eBay ones.

I get 8.8A from my 120w eBay cheapie all because I bought a quality MPPT controller and wired the panels in series 48v, prior to this the most they would make is 4.5A.

Total system cost $280, look on people's faces when it out performs a $500-$1000 brand name, priceless.


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What controller do you use?

I have 2 x 140W (each rated at 7.8A) wired in parallel, feeding through a 30A controller, powering 2 x 105Ah Deep Cycle batteries. Should I be doing it the other way around, i.e. wired in series?

[Symo Advanced Member Find all posts View Profile Close Symo]

What controller do you use?

I have 2 x 140W (each rated at 7.8A) wired in parallel, feeding through a 30A controller, powering 2 x 105Ah Deep Cycle batteries. Should I be doing it the other way around, i.e. wired in series?

I have 15A Victron Solar controller and with MPPT the more volts, the better. Series is definitely the way to go because every time you double the voltage you halve the current and 1/4 the cable size needed to run the power from the panel to the controller.

Most 12v panels lose a bit on the cable run, going 24v fixes this issue instantly. The MPPT converts the excess volts to more amps.

Win, win.


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[krypto Avid PP Poster! Find all posts View Profile Close krypto]

Perhaps a silly question as I haven't looked into this but if I use my panels in series at 24v (they are currently in parallel) will I need a special controller to run a 12v system behind the controller?