Monday, June 25, 2012

A Year of Electric Energy Numbers: the PV-EV Value Proposition

Here is one year of our electrical energy numbers in simple kilowatt hour and dollar terms.  Just a few numbers plainly demonstrate the value proposition of an electric vehicle (EV) and home solar photovoltaic energy (PV), in conjunction with Time Of Use Net Metering (provided through our utility).  In sum, we use about 50% more electricity than we generate, and yet pay nothing for our electricity, because of the difference between the (higher) value of the electricity we generate compared to the (lower) value of the electricity we use.

Our home use:  about 4500KWH

Our EV use:  about 3000KWH

Our total use:  about 7500KWH

Our PV generation:  about 5000KWH

Our cost:  about $0 (we actually 'gave' about $30 of electricity to the utility)

The value proposition: the electricity we generate is worth about 150% the value of the electricity we use.  This is because Time Of Use Net Metering measures the amount of electricity we generate (i.e., the electricity we put on the grid which is in excess of what we are using at that time of generation) and compares this against the amount of electricity we use at the time we use it.  In a sense, we "sell" electricity to the utility at the time of generation  --  which is, naturally, during the "peak" time of day when the sun is brightest and also when lots of electricity is being used, and therefore the rule of supply and demand means that this electricity is worth more to the utility and to the electricity users of the utility --  and we "buy" electricity from the utility for our home needs and for charging the EV mostly at the "off-peak" time of day when there is little electricity being used, and therefore the rule of supply and demand means that this electricity is worth less to the utility (in fact, often the utilities have excess electricity because they have to keep their turbines spinning even in the middle of the night and there are so few electricity users at that time, which is when we charge the EV).  This is why we can use 50% more electricity than we generate and still come out even).

And just so you have total info:

Our PV cost: about $12,500

Our gasoline and maintenance savings: about $2500

Our electricity savings: about $1000

Complete return on investment break-even point: about 4 years

Projected savings over time: given the rates of increase of cost of gasoline and electricity over the past ten years and in light of the guaranteed lifespan of PV -- over $400,000 with compounding interest (it's true, feel free to check the math). (And while I did not calculate such details as diminished PV output over time, I also ignored several variables on the other side such as the lower cost of PV.)  Of course, this assumes we would instead have kept a car with average MPG, and it assumes a continued even rate of increase energy costs, and these things might not prove to be accurate assumptions.  But then, the increase in the cost of energy may turn out to be even greater over time  --  who knows exactly what will happen over 25 years.

The certainty is that we are economically way to the good.  The certainty is that we have taken the greatest possible step in reducing our pollutant and carbon footprint by getting rid of our gas-burning car, and thereby putting 10,000 fewer pounds of carbon dioxide into the air.  The certainty is that we are supporting American workers and suppliers.  The certainty that we will not send a single one of our patriotic transportation dollars to support petroterrorism.  And the certainty is that I really enjoy driving an EV, knowing that I refuel it with an endless supply of the sun's photons, and therefore I have totally guilt-free spirited motoring.



  1. Thanks. Great explanation.

    I will suggest via Electric vehicle association Scotland (EVAS) that Scotland studies the Californian example.

    Andrew Bissell