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Wednesday, April 25, 2012

Gotta love sunny days

When you have a solar power system!
by Tom Ferrio

I won't deny it if you say I've become obsessed with analyzing how our new solar electric system is working out. Last month I showed you our electric bill of $39, down from $150 the previous year.

After prompting from some readers I presented an analysis of why the bill was so low.

We just received our March-April bill so here is an update.

I'll start with a comparison of the bottom line on the 2011 and 2012 bills:

Wow, we saved about $80 on this electric bill! But how much of that is because of the solar panels on the garage roof? I used the spreadsheet I developed last month and updated it with the new numbers;

Mar-Apr 2011 bill
719 kilowatt hours
Electric rate increase

Lower usage
28 kilowatt hours less
Predicted solar production
480 kilowatt hours
Extra solar production
70 kilowatt hours
Mar-Apr 2012 bill
141 kilowatt hours

Unlike last month, the electric rate this month is almost identical to last year. Also unlike last month, we used almost the same amount of electricity in our house this year. The really good news is that the solar panels produced about 15% more than predicted by the company that sold them to us. I chalk that up to the sunny and near rain-free month.

Below is our graph of day-by-day usage for this billing period. The dark blue lines represent the amount of electricity we bought from National Grid each day. You can see there are quite a few negative days, meaning we sold sold more power than we used those days.

An interesting tidbit is that almost 20% of our purchase from National Grid for the month occurred on March 25, a laundry day that was cloudy.

Click to enlarge

For those wanting to review the history of my system purchase and installation, all of the articles can be seen here.