Posted by Bob on May 9, 2008, 1:03 am
On May 8, 2:33 pm, d...@02.usenet.us.com wrote:
You are right but the figures from PVWATTS v.1 are odd. Solar
Radiation (kWh/m2/day) Sacramento 5.5, San Francisco 5.45, 0.9%
lower. But AC
Energy (kWh) output for a 4.5 kW system, using the same default
figures for input, Sacramento 6297, San Francisco 6506, 3.3% higher.
Any ideas about the reason for this apparent discrepancy? I have come
across other problems with NREL and PVWATTS data, so I am starting a
Posted by Bob on May 9, 2008, 1:12 am
I didn't start a new thread, I only changed the name of the old
thread :/ Here's the old name back.
Posted by Bill Kaszeta / Photovoltaic Re on May 9, 2008, 5:25 am
A proper calculation of PV system performance will include the temperature
San Francisco is cooler than Sacramento.
The NSRDB database has temperature data and I would assume that PVWATTS
uses the available temperature data. It is possible to also do calculations that
utilize the temperature coefficient of a specific PV module and its NOCT
characteristic (some modules run cooler than others).
Photovoltaic Resources Int'l
Tempe Arizona USA
Posted by Robert Karis on May 9, 2008, 6:22 am
On May 8, 10:25 pm, wkasz...@cox.net (Bill Kaszeta / Photovoltaic
Thanks, Bill, good point. I was aware of the temperature effect, but
forgot about it at my last post. Without doing detailed calculations,
I assume that would easily explain the 4% relative differences.
As I mentioned, I have some other problems with the the data. PVWATTS
v.1 shows insolation in Sacramento only 1% higher than San Francisco,
5.5 vs. 5.45 kWh/m2/day. PVWatts uses the 1961-1990 TMY2 (typical
meteorological year version 2) files to calculate the average solar
radiation per day (personal communication :)
When you look at the NSRDB daily statistics files for the same time
period http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/dsf/ you
find that the averages for Sacramento and S.F. are about 10% lower
than in PVWATTS, and there is a greater difference between Sacramento
and S.F. Sacramento AVGLO (GHI) 4.933, 4.6% higher than S.F. 4.716
kWh/m2/day. Sacramento AVDIR (DNI) 5.505, 9.6% higher than S.F. 5.022
Basically, I don't understand the advantage of using TMY2, an
abstraction, rather than the straightforward 30 year average found in
the daily statistics files.
A nice person at NREL has answered some of my questions, but is very
Posted by dold on May 9, 2008, 8:00 pm
I really didn't ponder how they came to their numbers, nor did I expect
them to be as accurate for me as you seem to expect. I am 50-80 miles
away, and a different altitude and climate from either of them.
I do know what my Fronius front panel said it generated for a year. In 30
years, I might have a 30 year average to post.
Maybe I overreacted to your initial premise that our original poster's
energy produced for the year was too high. I thought you, like Eeyore,
were claiming that it was way off. His and mine are measurements, your
numbers are estimates and projections.
His production compares to mine within a reasonableness test.
Clarence A Dold - Hidden Valley Lake, CA, USA GPS: 38.8,-122.5