Influence is difficult to quantify.

It's even tougher to predict. Roll the clock back to the year 2000. At the time, SunPower was not a major player in thesolarmarket. Cypress Semiconductor CEO T.J. Rodgers in fact rescued the company with a personal check for $750,000 that year. It was a personal check because Cypress' board was unsure about investing. The investment (which later was transferred to Cypress) turned into $2.5 billion and SunPower into one of the leading solar panels makers in the U.S.

First Solar? "I don't think a $2 Billion market cap is justified for First Solar Inc. and I am going to look to short it," wrote investor Himanshu Pandya in late 2006 a few weeks after the company's IPO. First Solar's stock would increase by more than ten times in value over the next year. More importantly, First Solar became an icon in solar for its ability to exceed expectations, ramp up manufacturing and expand beyond its core businesses.

Imperium Renewables has become a symbol of overextended ambition in the greentech world when it imploded in late 2007. Ten months earlier, investors put $113 million into the company amid optimism for alternative fuels.

But let's not cast stones. Our 2007 list of the top ten startups included Tesla Motors, which has reinvigorated electric cars, and Serious Materials, which helped launch green building products.  It also included Atlantium, an Israeli water purification founded in 2003 that's still creeping out of startup mode, Mascoma, the cellulosic fuel developer that's been saddled with periodic delays, and A123 Systems, the battery maker that lost the Chevy Volt deal and has seen competition mushroom around it.

In 2008, we selected BrightSource Energy as a top ten startup. The company has since become the success story in solar thermal. The list also included Think, the Norwegian automaker struggling on life support. No. 10 on the list was Silver Spring Networks, a good choice, but it's a company that's been far more successful than most of the other selections in the past 12 months.

So with that in mind, we present our Top Ten lists for 2009. The lists try to predict which companies will have the biggest impact in solar, in smart grid, and as buyers of green technology in the corporate world. We debated corporate strategies, we looked at market share and financial information, and we tried to distill singular personality features that will allow these companies to stand above their peers.

And like any good list, we left room for debate, so please don't hesitate to give us your comments and criticism. Here are the lists: 

Top Ten Solar Companies

Top Ten Smart Grid Companies

1. Itron

2. Silver Spring Networks

3. Tendril Networks

4. Echelon

5. eMeter

6. EnerNoc

7. General Electric

8. IBM

9. NKG Insulators

10. Austin Energy

Top Ten Enterprise Customers

1. The U.S. Government

2. The Tata Group

3. General Electric

4. Walmart

5. PepsiCo.

6. Masdar

7. Procter and Gamble

8. Chevron

9. Google

10. McDonald's