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
925 kilowatt hours
Electric rate reduction
lower natural gas prices
Lower usage
162 kilowatt hours less
Predicted solar production
415 kilowatt hours
Extra solar production
76 kilowatt hours
Feb-Mar 2012 bill
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.