We almost never mention the 2010 Rolls-Royce Phantom here on GreenCarReports.com. That's because--at three tons, with 453 horsepower from its 6.7-liter V-12 engine, and an EPA rating of 13 mpg city / 19 mpg highway--it's [ahem] just not that green.

Well, Rolls-Royce is looking ahead. Not only into a new century of carbon limits and cities that allow only zero-emission vehicles, but also, in the nearer term, to the 2012 London Olympics.

And the latest rumor out of London is that the carmaker that calls itself "the Standard of the World"  will make a small number of all-electric Phantom luxury limousines for VIP transport during the global event.

2010 Toyota Prius high-voltage battery pack

2010 Toyota Prius high-voltage battery pack

Which prompted us to get a pencil, find an old envelope, frown mightily, and start scribbling.

A standard Phantom does 0 to 60 mph in less than 6 seconds, in the smooth, silent mode of travel the company likes to call "wafting". Indeed, an electric Phantom might suit the marque's personality nicely, since electric drive is quiet and it eliminates any rising and falling engine notes.

Ignore top speed for the moment, since Olympic dignitaries aren't likely to travel long distances.

We'll assume the Phantom is about three times the weight of a 2009 Tesla Roadster, which does 0 to 60 in less than 4 seconds. Discount for slower acceleration, scribble, carry 5 ... looks like an electric Phantom needs a battery pack of at least 130 kilowatt-hours.

For perspective, that's two and a half of the $109,000 Tesla's 53-kilowatt-hour lithium-ion packs. Or eight of the 16-kWh packs used in the 2011 Chevrolet Volt extended-range EV.

Or 81 of the 1.6-kWh nickel-metal-hydride packs in your standard 2010 Toyota Prius hybrid.

Like the all-electric Mini E, now being tested in the U.S. and Europe as well as in its home market, electric Phantoms might well only be leased. Which, if nothing else, means dispensing with any awkward warranty claims on really, really huge battery packs.

What would all of this cost Rolls-Royce parent company BMW? As they say about buying one, "If you have to ask, you can't afford it."