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.
(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.