Monday, February 27, 2006

How much juice could a PV panel produce ?

Hola solarDwellers:

". . . if a panel could produce juice?" Kind of like the riddle: "How much wood could a woodchuck chuck, if a woodchuck could chuck wood?" The solar question of how much your solar panels will produce can be like a riddle to those looking into installing solar. A good installation company will include this calculation in their proposal to install your panels, but it's good for YOU to have a ballpark figure before that estimate, so you can ask them if it seems like they're "overestimating" or "underestimating" how much your system will produce.

PART I: How much AC or "usable energy" should your DC panels produce after it's converted to that AC

Let me start with the "conclusion" I've come to, about what percentage of electricity loss there is from the solar panels as it travels from roof to your appliances, from AC to DC(in case you don't want to read all the details below in Part II of how I came to my conclusion).

The "loss" figure I'd recommend using in PV electricity production calculations is about 28%, better than the conservative 35% amount I discussed in a previous post. In other words, if your panels' wattage rating adds up to 3000 watts, or 3kW DC, your panels should produce an estimated max AC electricity or "usable energy" of 72% of those 3kW(DC): 3KW x(1-.28) or 3kW x .72 or 2.16 kW or at LEAST in that "ballpark."

The worst conversion rate I found from HomeEnergy Magazine(see below) was 65%(35% loss)of DC, which for the 3kW would be 3kW x .65 or 1.95 kW. The best conversion rate I found from NREL(see below) is better at 77%(23% loss) of DC, which for the 3kW would be 3kW x .77 or 2.3kW.
So, for a 3 kW system, you know to expect a reasonable estimate of an AC power output of between 1.95kW and 2.3KW.
IF the estimate were below 1.95kW in the above example, you would look for a different installer whose PV system has more decent efficiency. If the estimate were much above 2.3kW, you might ask if it's realistic that the installer gets so little electricity loss. If you know the ballpark, you'll know when and what to ask your installer.

(Read on if you're the one who always asks "Why? WHy?? WHY??!!)

Part II for the curious: how I came to that 72% conversion factor (DC to AC)

The low range I mentioned in the abovementioned post was about a loss of 35%(from a 2003 analysis from HomeEnergy Magazine of real installed pv systems in California), meaning if your panels add up to 3kW(DC), they'll produce (3kw x .65) = 1.95 kW(AC). This figure is probably too conservative considering that today's PV is better than the PV systems analyzed in that article, which were installed in 2000. So, what's a more realistic figure? Let's go to "the source" for the info . . . The question: what percentage of loss is there?

First, we travel to the CEC A GUIDE TO PHOTOVOLTAIC (PV) SYSTEM DESIGN AND INSTALLATION(pg 9) produced by the California Energy commission (CEC), which kindly offers this nugget of info from their research: "So the "100-watt" module output, reduced by production tolerance, heat, dust, wiring, ac conversion, and other losses will translate into about 68 Watts of AC power delivered to the house panel during the middle of a clear day (100 Watts x 0.95 x 0.89 x 0.93 x 0.95 x 0.90 = 67 Watts)."

So, no big difference, down to 33% loss(67% of DC) from the other article's figure of 35% loss. So, my last source serves to bring us to more current data, since the CEC's data is from 2001, and a lot happens in 5 years in solar time.

Our last data trip is to the Mac Daddy of research on renewable energy data, from the government folks at National Renewable Energy Laboratory, who have a calculator called PVWATTS . They describe how they figure out the "derate factor" or "percentage of" AC you get from DC that we've been so doggedly trying to nail down in this post:
"Overall DC to AC derate factor= 0.95 x 0.92 x 0.98 x 0.995 x 0.98 x 0.99 x 0.95 x 0.98 = 0.77
The value of 0.77 means that the AC power rating at STC is 77% of the nameplate DC power rating."

(Don't freak about all those multipliers; they each just represent each step along the path from panels to your house that cause the system have electricity loss.) In other words, the PVWATTS calculator says that your panels will produce 77% of their DC rating("nameplate"), or a 23% loss.

So, there you have it, future solarDwellers. When I do my PV calculations, I'll just multiply the panels' DC rating by an average of 72%(a loss of 28%) to get the "real AC juice" they'll give me inside my abode. I hope it works for you too!

--your solarDweller
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Steve Yang, P.E. said...

Dear Jason,

I just came across your Feb. 27 '06 posting 'How much juice....'. It was a nice piece. I now like to invite you to visit our free public service site that offers an instant performance benchmark for any PV installation; given some essential parameter values are provided. This allows a PV owner to perform a quick check anytime, under local sun-lit condition. We are doing this because we see too many PV systems not performing well, thus robbing its owner's Return-on-investment;
not to mention the loss to our environment.
Please check it out and give us a critique. We intend to continually
refining it so it becomes a truly
useful tool for all solar enthusiasts.

Thanks and Take Care,

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