Electric cars are not the best way for society to green transportation, famed greentech venture capitalist Vinod Khosla said Wednesday.

"Forget plug-ins," he said during a keynote address at ThinkEquity Partners' ThinkGreen conference in San Francisco. "They are nice toys. But they will not be material to climate change."

In particular, he pointed to battery problems.

Some analysts and companies have said current lead-acid and nickel-metal hydride batteries can't store enough energy for their size to drive far enough -- on electricity alone -- for mainstream drivers.

Not everyone agrees. But lithium-ion batteries, often found in consumer electronics such as cell phones, have caught the interest of the plug-in and electric-vehicle community because they are more powerful for their size.

But they are expensive and also have the problem of "thermal runaway" -- overheating and even catching fire. A number of researchers and entrepreneurs are working to try to solve those problems (see Batteries Key to Plugging in at Electric Vehicle Symposium).

Still, Khosla said the battery problems are harder to fix than those of cellulosic ethanol, which he thinks makes more sense than plug-in hybrids.

"Are we more likely to get a [fivefold] reduction in cost in cellulosic ethanol than a [fivefold] reduction in cost of batteries?" he asked in his keynote.

Cellulosic ethanol is made from nonfood biomass like switchgrass, wood chips and corncobs. Companies developing alternative fuel haven't yet been able to produce it on a mass scale, or at an affordable price.

It's no surprise that Khosla supports biofuels, of course.

He has invested in starch-based ethanol companies such as Altra and Cilion, as well as cellulosic-ethanol companies like Range Fuels and Mascoma, among many others (see portfolio chart here). Although Khosla is not a big fan of corn-based ethanol, he said he views it as a step toward cellulosic ethanol.

He also previously said he thought plug-in hybrids were too expensive (see In Depth: Ethanol Shmethanol).

The point of view is controversial. Plenty of other entrepreneurs and analysts like the technology, including Earth Policy Institute President Lester Brown.

For example, Ford Motor Co. (NYSE: F) and Toyota Motor Corp. (NYSE: TM) are testing plug-in hybrids (see Who's Reviving the Electric Car?).

Khosla added that, although he doesn't view electric vehicles as a climate-change solution, it doesn't mean such efforts won't go unrewarded. "They will be great investments," he said.

Plug-in hybrids weren't the only transportation technology Khosla disparaged during his keynote.

He also gave biodiesel a thumbs down, saying it requires too much land.

The fuel will be permanently dependent on subsidies, and has relatively poor technical performance and prospects, he said.

"It's not such a great idea," he said.