Forget the fact that you’re an investor with a penchant for green opportunities, and instead consider yourself a hard-nosed businessman looking for the best return on your money. You run a spread sheet to determine the ROI of electric cars, based on current gas prices, and suddenly you’d be looking elsewhere for investments that would ensure your retirement.

I’ve been reading about a new crop of garages that convert hybrid cars such as the popular Toyota Prius into all-electric vehicles. Once you consider the economics of this folly, you see why electric cars are a fallacy for both the owner and investors looking for a toehold in this burgeoning market.

I read my first story a few months ago. It was about the Luscious Garage in San Francisco. Run by women, they carved out a juicy niche for themselves by yanking out the gas engine in hybrid cars and replacing it with another set of batteries. The job cost about $7,500.

More recently, I read another story about a similar operation — this one across San Francisco Bay in Berkeley. Run by two guys, they’ll do the same thing for $7,000.

Regardless of who you used for the conversion, the results were pretty much the same. Instead of filling up at the pump, you plugged your car into a socket. Let’s forget, for a moment, that you’re still burning up fossil fuels from your local power plant and instead focus on the feel-good economics of actually operating the cars.

In both cases, the writers of these articles interviewed the cars’ owners. What you get are comments like this when asked about shelling out $7,000 and more for the conversion…

“My carbon footprint concerned me more…”

“We don’t like what oil is doing to the world…”

One piece in CNN included a gentleman named Dave Moore. A resident of Washington state, CNN said Moore had “started down the electric-car path when he became worried about climate change. On the waiting list for an $85,000 Audi sports car, he decided to buy a Prius instead and got it converted to a plug-in car for about $10,000. He figures he has a green car for less than half what the Audi would have cost.”

That may in fact be true, but that doesn’t make it a valid business case for investors.

When it comes to making investments in green technology for the road, only one thing matters: ROI.

Carbon footprint, sticking it to the sheiks or that butt-tingling smugness as you sit in rush-hour traffic won’t cultivate a new industry. When you’re pitching fuel economy, it’s all about the economics.

So I ran a few numbers to see what the ROI would be on a hybrid-to-electric conversion just to get an idea of whether or not electric cars are something for investors should consider for future growth. Here’s what I came up with…

The average price of a gallon of regular gas today is $1.65 according to Consumer Reports. And the average driver logs 12,000 miles per year, using 550 gallons of gas, says the AAA.

For the $7,500 conversation, the ROI is 12 years for your average driver at today’s gas prices. For the $7,000 conversion, the ROI is 7.7 years.

Anyone can see that a 12-year ROI is ludicrous in terms of trying to sell an electric car. We’re not even sure the batteries would last 12 years. And as far as 7.7 years goes, well that’s marginal – again assuming that all the batteries can hold a charge of that long.

Now, what I didn’t take into account is the approximate $5,000 premium that people are paying for the privilege of buying that stock, hybrid Prius in the first place over the price of a gas-powered economy car such as the Honda Civic. If you want to tack on that extra $5,000, you’ll see that the ROI is simply absurd (or maybe not as absurd as Moore’s $10,000 Prius conversion).

What do these numbers mean in the real world of major automakers?

It means that electric cars could become nothing more than window dressing for empty showrooms. Sure people will flock to the Chevrolet dealership to see the impending Volt for about $35,000, but they’ll drive out with an econo-box Aveo for $12,600.

And why not?

You would have to drive the Volt for 13.5 years longer than the Aveo to justify the price difference at $1.65 per gallon.

So if you want to invest in an industry that strictly caters to people worried about their carbon footprint, well, more power to you.