When Get System Max Output?

System · August 2007

Hello,

I just had a new system installed. It has 2 arrays of 6 Sanyo 190s, one facing South and one facing West. They feed into a Xantrex. I'm in the CA Bay Area, and it has been clear and around 80deg today. The theoretical max output that the installer quoted is right around 2.0kW, but at 12:45pm today, I only got 1727W according to the Xantrex. The Sanyos supposedly have a tolerance of -5%. There is a bit of dust on the panels, but otherwise there is no shading at 12:45 and no old fruit, bird droppings, etc. I took a single, cheap solar cell that I had and the max short circuit current I could get was 80mA - is there some way to translate that into an actual irradiance value at that time or is it only useful as a relative value?

When should I measure to try to get the theoretical max output number? Or do I need to call the installer back?

Thanks.
tsshfs2005

niel · August 2007

Re: When Get System Max Output?

you can get a rough idea of irradiance with that single cell and you would need to know what the short circuit current should've been at 1000w/m^2 to be able to compare with. do clean off the pvs as that will take away from the pvs output even if only a slight amount of dust. one other factor that will effect the output is that one of the pvs is not aimed optimally south and that i would suppect to be the biggest problem with you not seeing full ratings. turn that pv a bit more towards the south and watch the power go up given the same irradiance conditions, that you can measure with that one cell to be sure it is the same. have the cell in the same place and measure at the same time of day for comparrisons.

BB. · August 2007

Re: When Get System Max Output?

I too am in the SF Bay Area (San Mateo). For me, I very rarely (like 2 or 3 times when I looked over two years) see maximum rated power on my system (3.5 kW of panels, ~3.0kW peak).

Typically, it is around 2.3-2.5 kWatt peak--sometimes over 2.7 kWatts peak on a bright, cool, clear, day. That would equal about 1.6 to 1.7 kWatts peak on your system (using my numbers). Orientation (south best) and roof tilt will affect your readings too. Right now, on clear days, I am generating about 17.5 to 18.5 kWhrs per day (I have some early/late shading, pointing SSE, good nominal latitude roof pitch).

I rinse my panels every once in a while--but I have not washed them with soap and water yet (2 years next month). 2nd story with ladder--getting too old to do that often (in my youth--used to play on the roofs--now I hang on for dear life).

I am guessing, but I probably am losing about 5% because my panels are not spotless (looking at my skylights--There is quite a bit of grunge from the trees and pollutants that don't just rinse off with water).

-Bill

Solar Guppy · August 2007

Re: When Get System Max Output?

86% of name plate into the grid is ABOVE typical performance ... there is nothing to complain about!

The biggest derating is the temperature of the panels, next would be you'll have a smaller loss with the split arrays facing different directions.

Also the panels are not perpendicular to the Sun as they are roof mounted

Enjoy your new system, they did a GREAT job on your total system efficiency!!

Roderick · August 2007

Re: When Get System Max Output?

Hi,

You know who I am, and I don't live far from you, so our climate conditions should be comparable.

I will say that for our polycrystalline panels, we were getting the PTC power, maybe PTC+5%, from the system as a whole (that is, taking into account inverter losses), near the solar noon (1 pm DST) on cool days in winter and spring, as long as there were no clouds.

In the summer heat, the power peaks at PTC-15% at solar noon, no better. However, the days are long, and almost uniformly cloudless, so it looks like June and July will turn out to be our best months as far as total energy.

I'm not sure how well this experience maps to your panels, since you have those Sanyo HIT's. It's very possible that your best days are yet to come, when it gets cooler.

If you really can't wait to know, I suppose you could be a Scientist about it, and chill your panels with a fine spray from a garden hose at noon or 1 pm this weekend, while someone watches the inverter output.

If the installer showed his/her calculations to get the theoretical max number, you might post them here, and there will surely be someone who can comment. In particular, they may have just taken the PTC power of the panels and multiplied it by the PTC efficiency of the inverter, as this is all that's required on the California rebate form. Was the orientation of the panels disregarded?

Lastly, there is free software to monitor the Xantrex inverters. You might look at the *shape* of the graph over the course of a clear day, to see whether the west-facing roof is giving you a strange profile.

mike95490 · August 2007

Re: When Get System Max Output?

My array faces west, which is fortuitous for for me, I have AM fog/overcast, and a clear view to the sunset. My array wakes up in and AM, and lounges around untill about 11:30, then the panels start producing, peaking about 3pm, and doing just OK till sunset. Other folks with south panels, get an earlier peak - at noon, but are off axis all afternoon, and if it was a foggy/overcast morning (common in the So Calif coastal area) they loose the entire pre-noon harvest.

System2 · August 2007

Re: When Get System Max Output?

Thanks for everyone's info. I went and checked the quote again and found the following info in a spreadsheet the installer attached:
peak = 12*rated=12*190=2280W
rated = 12*ratedCEC*inverter_efficiency=12*178.7*0.945=2026W
actual = 12*rated*0.77=12*190*0.77=1756W

It appears that the quote gave the RATED but I am getting the predicted actual? From what you all are telling me, it sounds like if indeed I'm getting the predicted actual right now in August is actually pretty good and hopefully it will do better with cooler weather.

When they submitted for the CA rebate, they did indicate that one array faced South and the other faced West - for the South one, I'm supposed to get back $2.45/W and for the West one, $2.43/W. I guess since the rebate uses the RATED number, I thought I'd actually be able to see that number on the Xantrex.

I didn't bother having them run a serial cable from the Xantrex to my computer room so I can't really run GT-view. It's the old fashioned "taking data by hand" right now.

BTW, is the power number displayed by the Xantrex in DC or AC?

Thanks.
tsshfs2005

Solar Guppy · August 2007

Re: When Get System Max Output?

tsshfs2005 wrote: »

BTW, is the power number displayed by the Xantrex in DC or AC?

Thanks.
tsshfs2005

AC Watts ...

You can do wireless RS232 for your inverter and get full unit AC/DC performance and will probably find a better peak number as you likey to miss it just looking at the GT's LCD display .. Mike can tell you all about options as he runs wireless, My forum is full of ideas on this. Also, running a Cat5 wire, even from one end of the house to the other is only an hour or two's work.

If you ever want or need to have the installer take you seriously, you'll need data logged data to get their attention

System2 · August 2007

Re: When Get System Max Output?

Thanks. I looked into a wireless RS232 adapter (by AirCable) a few months ago but got email from the rep that it needed an external power supply, so I dropped the idea since I don't have a nearby power outlet. Oh well. I guess if I never get close to rated power when the weather cools, I'll try to figure out how to connect something up and log it.

mike95490 · August 2007

Re: When Get System Max Output?

It's easy to use a length of shielded twisted pair wire, & 2 DB9 connectors, and make your own RS232 cable as long as you need (couple hundred feet appears to be fine.)
You don't have an outlet anywhere near your inverter to power a wall wart for a wireless ?

BB. · August 2007

Re: When Get System Max Output?

I debated about hooking my 3kW Xantrex GT inverter to a computer--but decided that additional power required to log the data with a, relatively, low power laptop PC was still going to be 0.5 kWatt*hours per day (if I left it running 24x7--a desktop computer could suck much more). I go to a lot of trouble to save that by switching off lights and other phantom loads (like old stereo, various wall transformers, etc.).

So, instead I just log the total power generated each day, and the cumulative kWH counter. More than enough data for me to watch for daily power changes (clouds, temperature), and checking my monthly power usage to make sure it does not sneak back up again.

In the end, the peak power (at any minute of the day) is not nearly as interesting as the cumulative power comparisons (daily, monthly, yearly) in determining system health... I log the data into Excel and do a year to year comparison to watch for any big differences.

I don't know what I would do with minute by minute data logs...

-Bill

System2 · August 2007

Re: When Get System Max Output?

I probably won't keep a laptop going all the time to log everything, but thought it would be nice to occasionally get representative data. Since my nearest outside outlet is at least 50 ft away from the Xantrex, I'll go look at the solar-guppy forum threads for cable ideas.

Thanks to everyone for the tips and info.

mike95490 · August 2007

Re: When Get System Max Output?

tsshfs2005 wrote: »

my nearest outside outlet is at least 50 ft away from the Xantrex,

duck soup for RS232 cable.

Laptop with HDD spun down, and backlight off, I doubt is more than 30W.

BB. · August 2007

Re: When Get System Max Output?

30w * 24 hours per day = 720 WH per day...

More power than I really wanted to toss--I can get probably a 1kWH/Day increase in my solar panel power if I were to climb a 2 story roof panel installation with my ladder and wash it once per month... I prefer to just manually log the data and save the monthly trek and get the "same" power output with less work.

-Bill

Solar Guppy · August 2007

Re: When Get System Max Output?

I've measured 11 watts for mini-itx based Pc's and older laptops under 10 watts with everything shutdown as Mike Suggests ... plus you can get them on eBay for 25 bucks and work fine for data logging

The single biggest draw on a laptop is the backlight, easily over 1/2 the power draw ... let the HD spin down, run GT-View by itself writing to flash stick and on new laptops, windows will downshift the clock rate to keep power draw very very low ...

When Get System Max Output?

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