Wednesday, June 8, 2011
Explaining the $0.02 cents per mile chart
A few folks have asked how Stellar Solar arrived at the numbers used in the charts. Since we did this based on real world experience and cost for both the Solar PV and for the Mini-E, I thought it might be helpful to elaborate.
For a gasoline car they used 20mpg and a 100 mile trip as a baseline.
In San Diego our SDG&E rates vary from a low of $0.14kwh to a high of $0.31kwh. The average is $0.22kwh so they rounded down to $0.20kwh. Some will go TOU metering, some will not depending on how much energy they use.
Most electric cars get around 3.5 mpkwh real world consumption. Some get as high as 4mpkwh but 3.5mpkwh seems to be a good number for most.
Solar PV in San Diego is around $4000+- a kw nameplate. A 1.5kw system ($6000) will provide 2400kwh of electricity, approximately half during peak hours and peak season ($0.27cents kwh) the rest at off peak rate ($0.17 kwh.) The production of 2400khw, half during peak hours-peak season, equals the cost of using 3600kwh a year when charging at night between midnight and 6am, which is when I charge, during super of peak ($0.14cents a kwh)
25 years at 3600kwh of usage is 90,000kwh. 90,000kwh multiplied by $0.07 cents is $6,300 which equals the cost of the system.
$0.07kwh divided by 3.5 mpkwh equals $0.02 per mile.
There are some variables on the solar such as degradation of 0.5% per year and you will most likely need a new inverter around year 12 (enphase inverters are now warranted for 25 years) however the solar PV system will go on producing electricity far beyond the 25 years, so to keep it simple we just calculated that as a wash.
All that boiled down to as simple of a chart as possible.
Oh, and the best part, not included in the chart, those 3600kwhs will drive an electric car 12,600 miles. In a gas car again at 20mpg, you would use 630 gallons of gas or $2,500 of gas per year. 2.5 years of driving on gasoline is slightly more than the cost of the Solar PV system to drive an electric car for 25 years. This gives a 40% ROI for a Solar PV system used to offset gasoline.
Sounds impossible but that is my real world experience and for most the numbers will be similar if installed as part of a larger house system. A small 1.6kw system installed on its own would be higher in price.