Menu Bar

Home           Calendar           Topics          Just Charlestown          About Us
Related Posts Plugin for WordPress, Blogger...

Thursday, April 5, 2012

Better than expected! - the math geek version

by Tom Ferrio

Yesterday I published a report on our latest electric bill with our solar electric system in full operation. That led to some questions so I couldn't resist digging into the details some more.

Read past the break for the gory details.


I found out that my energy monitoring system (image at right) can give me a spreadsheet with our daily solar production and purchases from National Grid.

(An aside: We have had this system for years to help understand our electricity usage.)

Those daily usage numbers and a study of the bills helped me understand the details better. There are several factors involved:

  • The rate we pay for electricity is considerably lower this year than last year, probably reflecting the very low cost of natural gas now.
  • We did use about 15% less total electricity this year, probably mostly due to the warm weather.
  • The solar electric system produced about 20% more power than the computer model for our system predicted for this time of year. The weather was almost certainly more sunny than normal last winter.
I put all of that in a spreadsheet to compute the savings for each component and this is how it worked out:

Feb-Mar 2011 bill
$150.39
925 kilowatt hours
Electric rate reduction
-19.44
lower natural gas prices
Lower usage
-22.74
162 kilowatt hours less
Predicted solar production
-58.27
415 kilowatt hours
Extra solar production
-10.68
76 kilowatt hours
Feb-Mar 2012 bill
$39.25
249 kilowatt hours

So I conclude that the solar system saved us almost $70 in the February-March billing month. The rest of the savings were due to the lower price for electricity and our lower usage.

Disclaimer: the kilowatt hours don't exactly add up because my energy monitoring system is not revenue grade and I don't know exactly when, what time, National Grid read the meter each day.