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.