The graph below shows our monthly electricity costs for 2017 (blue line) plus costs averaged over years 2009 through 2011 prior to solar system installation (red line), and costs averaged for years 2012 through 2016 after installation (yellow line). Numbers are taken from CPS billing statements and include not only charges for power used, but also CPS fixed charges, City of San Antonio service fees, and taxes. In 2016 these added fees totaled about $24, and set a minimum for the monthly CPS bill.


Comparison of total costs over time is not straightforward because of changes in rates and other fees. However imprecise, the graph indicates that the solar power system provides substantial savings: the average annual cost for years 2009 - 2011 (prior to installation) was $3,308, while average annual cost for 2012 - 2016 was $1,288. This is an average savings of $2,021, or about 61% of the costs before the system was in place.

Based on current electric rates, the payback period for our initial investment is about 10 years from the date of installation.