As a solar designer, I like to produce solid designs for my clients. Therefore, an important point is to recognize that manufacturers overstate the output of their panels. The rated output is measured under STC (standard test conditions), which is in an ideal laboratory setting. The State of California instituted its own output measurements under more realistic conditions. These are known as PTC (PVUSA test conditions) ratings. So, for example, a 150W STC panel is actually more likely to have an output of 131W PTC.

Caveat emptor.

Additionally, depending on your location, you may need to further derate your panel's performance due to ambient conditions. A 15% derating of PTC is not unheard of.

Solar Mo,

Is there a database somewhere of the PTC ratings of many panels? Alternately, can you provide some sort of link or URL so I can do a little research on it?

I want to design a PV system for my home, but since shipping things to Guatemala is expensive and with things like import duties when shipping internationally, getting it right the first time is rather important. [img]images/smiles/icon_smile.gif[/img]

Thanks!

Hey thanks, that EcoBusiness site is great.

"If you're going with Si panels..." Is there another choice in PV? I pretty much thought all PV cells were silicon-based.

G'night,

PTC is just 85% of STC

if your going with Si panels .. there all pretty much the same. I use Kyocera and have been pleased with the panels as are two other Large installed systems (48 and 72 panels)

The new sharp panels look like an excellent buy at the price per watt but all the PV panels are comming down in price

check out http://www.ecobusinesslinks.com/links/solar_panel_price_surveys.htm

for the lastest prices ... [img]images/smiles/icon_razz.gif[/img]

At http://www.xantrex.com/support/docserve.asp?id=227 they talk about how in a real-world situation, you probably want to derate your system to 67% of the solar panel STC.

Does this mean a 2500W inverter like the SunTie can have 3700 watts of panel connected to it?

Thanks!

Yes ... I'd recommend 2880 to 3200 as the optimal range.

There are 5 systems , that people have reported 3400-3600 STC watts as the panels feeding the Suntie , they are all now getting into the 19-20 kWh/day production range with the spring daylight and weather

My systems are 2880 and usally run in the 2000-2100 watt peak range , though in partly cloudy weather I hit the 2500 watt limit regularly.

Keep in mind , you panel angle will have a large effect , if you not on a tracker or seasonal adjustable mounts , you'll get peak power only for a month out of the year !

My system beats out gordens in the winter months even though he has 3400 STC watts to my 2880 STC watts , the differance is I have 4 season adjustable mounts , his are fix on his roof.

As for the Xantrex document , its very dated , and very low (they made it to counter the ST problems) all before I got involved. I would say you can be in the 72-75% range with 4 season adjustments , ground mount (keeps panels cooler). The inverter is only 9-10% of the loss , every thing else is mounting , tempature and isonalation varibles (smog/fog for example)

I'm far from an expert on this, but I think there are many variables that come into play depending on the setup. I have a small system that is comprised of 8 Sharp 165w panels (1320W STC). My peak output(no edge of cloud)is about 1050w. So my 'real world' derating is about 79%. Also, since I can adjust the tilt on my array, I probably have less loss than someone with a non-ideal fixed angle.

Wow, so if I only have room on the new house for four arrays (in different orientations) rated for 1792, 1024, and (2x)512 watts, I should get inverters rated for 1500 and 1000 watts and ignore the other two (512W) orientations. Is there any realistic way to make use of the 512W arrays?

Maybe I should make the roof less 'interesting' (convoluted) and put up 10KW of panels in two orientations, with three inverters (2x2KW and 1x2,5KW). Hmmm, ugly and significantly more expensive house, or drastically lower power production, what to do?

I would just pair a small array with a big array and use two inverters. The power loss , especially with the small arrays will be minimal

Thats the Suntie advantage , you can put as many 48 volts strings together as you want

Henry

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I would just pair a small array with a big array and use two inverters.
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Ok, but what if I have four orientations available, and two of them can only have 512 watts of panel on them? Should I bother with those orientations? [Turns out the Big Ugly Roof scenario will cost on the order of $100K, so that's not going to happen...]