As the solar odometer turns

We have a new residential solar electric (PV, photovoltaic) system installed about a month ago, and its "odometer" will soon turn over the 5 in 500 kilowatt-hours (kWh) of electricity generated.

It's pretty impressive, just in a mechanical (or rather, non-mechanical) sense.  It just sits there.  Nothing seems to happen, but the kWh generated daily continues to rise.  No muss, no fuss.  Pretty neat.

But I digress.  The reason for this post was to say something about what 500 kWh will represent, when we get there later today or tomorrow.

In terms of energy, it's the equivalent of 15 gallons of gasoline, or enough to drive one of our hybrid autos about 675 miles.

In terms of generating electricity, the numbers are bigger, because the combustion process for fossil fuels wastes energy.  500 kWh is equivalent to burning about 500 pounds of coal, or 35 gallons of diesel oil (New England, where we are located, still burns plenty of diesel oil to generate electricity).

In terms of carbon dioxide (CO2) emissions, the New England utility system gives off about one pound of CO2 per kilowatt-hour of electricity, so our 500 kWh means about 500 pounds of carbon dioxide has been kept out of the atmosphere.

So what?  Well, that's where the numbers get truly impressive, because of the very long life and heat-trapping effect of carbon dioxide. In a post on his blog, Quark Soup, science journalist David Appell quotes David Archer's book, The Long Thaw: How Humans Are Changing the Next 100,000 Years of Earth's Climate, as follows:

"If we add up the total amount of energy trapped by CO2 from the gallon of gas over its atmospheric lifetime, we find that our gallon of gasoline ultimately traps one hundred billion (100,000,000,000) kilocalories of useless and unwanted greenhouse heat."

Burning one gallon of gasoline results in the emission of 19.6 pounds of CO2. That means that one pound of CO2, over its lifetime, traps 100 billion kilocalories divided by 19.6, or 5 billion kilocalories.

Now, I don't know about you, but I don't use kilocalories much on a day-to-day basis, so, how much energy is that?  Using the handy Google converter for units of measure, I find that it is 2 x 10 to the 13th (10e13) joules--still not a very meaningful quantity.  However, one Hiroshima-size atomic bomb releases 63 x 10e12 joules, and one pound of CO2 traps 2 x 10e13 (20 x 10e12) joules.  In other words, just three pounds of CO2 added to the atmosphere, over its lifetime, traps as much heat as released by the Hiroshima bomb (a unit of measure Mr. Appell calls the "hiro").

This in turn means that, in a little over a month, our residential solar system has kept enough CO2 out of the atmosphere to trap the equivalent, over time, of the heat that would be released by 167 Hiroshima bombs.

(It also says a lot about how much heat you're adding, over time, to the climate system with every gallon of gas--about 7 Hiroshima bombs' worth--and why it's silly to claim that the billions of tons of carbon dioxide we're adding to the atmosphere each year have no impact on the climate.  But that's another blog post.)

I expect to use these numbers more in the future, in other posts and in comments on the Internet, so if you find any errors, please let me know (Mr. Appell has made the same offer, and nothing has turned up yet).

UPDATE (Sept. 8, 2012): The odometer now stands at 626 kWh, which means we have passed another minor milestone--the first barrel of oil equivalent (BOE).  Burning a barrel of oil generates about 600 kWh--and produces 940 pounds (almost half a ton) of carbon dioxide.