Our electricity bill for the year was about $8, about double that of last year.

To start off the comparison, here are the charts for the 2 years, 2010 and 2011. Of course, December is still empty for this year.

The total yearly production in 2010 for the first 11 months was 4010 kWh while it is 3910 kWh for the same period in 2011. In other words, 2011′s production is 97.5% of 2010′s production.

The first thing that struck me was that the peaks were off. June and July of 2010 produced about 490+ kWh of energy each month. The maximum we had this year was July with 469 kWh. Here are the month charts for July 2010 and July 2011:

The average temperatures, hours of sunlight and cloud cover look about the same between the two years, but there were more days where the day’s cloud cover may have been responsible for the reduced output. July 2010 had no days where the output was less than 15kWh while July 2011 had 8 days where the daily output was about 13kWh. Here is a the output for the lowest production day for July 2010, July 20 compared to the equivalent in 2011, July 25:

This shows that while the average cloud cover was higher on the specific day in 2010, the cover either blew away quickly or was during darker periods. One check that this isn’t a sign of a deteriorating equipment is to pick a good day this year that had as good a production and we have such a day, July 14.

Interestingly, the average cloud cover on this day is 60%! Much higher than on the lowest day which only had 38%. So, I wonder if this may be a local effect i.e. cloud lingering over the panels rather than everywhere or that the cloud cover was higher during non-production hours. Sigh! You know what they say about statistics: Statistics is like a bikini: what it reveals is interesting, what it conceals is important.

But another thing manifests itself. The monthly chart shows July 13, 2011 as having produced 17 kWh of energy while the day’s page only shows 15.59 and July 14 has a higher production, 15.97 kWh. Checking the 2010 charts, I see the same thing for some of the days. Hmmm, I wonder what is going on.

Getting back to data, the first three months of the year had roughly the same production: 787 kWh. The next two months, April and May, 2011 actually outproduced 2010. Then June 2011 showed up and just blew a hole in the production: 413 kWh compared to 490 kWh. The next quarter, July-September, 2011 fell behind 2010 by 5% (1297 kWh compared to 1358 kWh). Oct and November production is identical between the years again, 554 kWh. So, what happened in June ? Comparing the outputs of the two years:

We see that there were some really crummy days this year, days where production didn’t even touch double digits while 2010 had no such days, consistently hitting 15 kWh per day.

Getting back to the production and costs, a 2.5% drop in production resulted in doubling the bill ? No, I’ve been using the treadmill a lot more this year. Plus we had more guests than we’ve ever had in a long time. All of this must’ve contributed to the increased bill.

One other significant piece of news in this connection is that the manufacturer of our solar panels, Evergreen Solar, declared bankruptcy earlier this year. What this means to our warranty and such is something I’ve still to determine. Cheaper panel production from Chinese manufacturers were blamed. A pure US manufacturing entity is unsustainable. Compunding the problem is the plummeting price of the solar panels. Along with the Solyandra debacle, this is a sign that investing early in a market maybe risky. 

With that, we end the last post of the month. Good night, and good luck.