The U.S. Department of Energy has concluded that the existing U.S. electrical grid — without additional construction for generation or transmission — could accommodate 180 million cars that draw their power from night-time (off-peak) electricity. That’s about 85% of the cars in the country.

The full report is not yet available, but based on the press release, there’s lots of good news — with some bad news mixed in.

First, the good news: aside from dramatically lowering U.S. oil imports and keeping that money inside U.S. borders, grid-connected cars would marginally improve air quality. Even though no new coal plants would be needed to accommodate the cars, existing coal and natural gas plants would be run at greater capacity. This means what should be an easy plus for the environment is, at best, a marginal gain. Instead of the double-digit declines in sulphur and CO2 emissions, we get a measly 5% CO2 decrease, and an actual increase in sulphur.

Still: air quality in urban areas would improve substantially from the lack of tailpipe emissions.

One researcher is quoted saying that because electricity sales would increase without additional capital expense, prices per-kilowatt-hour could go down. I’m skeptical on that last count.

Something the press release itself doesn’t deal with is the potential for vehicle-to-grid services. This is where electric cars (either hybrids or full EVs) have the potential to be a real-life silver bullet. Anyone who advocates for increased use of renewables is inevitably confronted with the problem of intermittency. With wind, the rule of thumb is that if grid energy supplied by wind grows to more than 25-30%, utilities need to spend prohibitive amounts on “spinning reserve” to even out supply.

Well, a nation driving plug-in hybrids makes for a spinning reserve of amazing proportions — according to one estimate (PDF), the U.S. fleet would power the U.S. electrical grid seven times over.

This means plug-in hybrids (or eventually full-blooded EVs) finally eliminate the fundamental barrier to solar and wind electricity. They will make wind more competitive than it already is. And as the cost of solar comes down (see Bradford, Travis), it will have a massive reservoir to dump electrons into, without the actual unsightly reservoirs we currently build for hydro dams.

All of this requires the kind of smart grid Al Gore and David love so much, as well as a great deal of foresight by policymakers. Luckily, the utility lobby is actually on the right side of the issue for once, and The Mustache is on our side too. Even GM and Ford are promising plug-ins.

Environmentalists, even more than most groups, have good reason to be suspicious of technical breakthroughs that purport to solve our problems. But between rapidly declining solar and wind costs, a new crop of electric vehicles entering the market in the next few years, and a strong consensus in the U.S. today that things need to change, I’m thinking the V2G concept is a no-brainer.