It's an understatement to say that investors are excited by the opportunities in green tech.

John Doerr, one of the world's most successful venture capitalists, called cleantech the "biggest economic opportunity of this century." One of the best-performing stocks of 2007 was none other than First Solar (Nasdaq: FSLR), up an incredible 795%. And recent data from the Cleantech Group (reported by CNET News) showed that VC investment in green tech increased 44% in 2007 -- from $3.6 billion to nearly $5.2 billion.

Triple back-up-the-truck booyah, right?

Not so fast

A centerpiece of the recent Roth Capital investment conference in California was an "Investing in Green tech" expert panel. Its goal was ostensibly to reveal how to make obscene profits by investing in green tech stocks.

But it did the exact opposite.

As the panel went on, it became clear that even these experts -- people who now devote their careers to advancing "green" technologies -- weren't quite sure what the perfect green tech policy, incentive, initiative, or technology looked like. But who could blame them?

Wrap your head around this...

First, there's significant government involvement in the sector that distorts market forces. That should be an immediate red flag for prospective investors. Whenever the government is involved in something, there can be no certainty.

Second, green tech development cycles are becoming increasingly rapid. That means that what seems like a great idea today could be obsolete tomorrow. For an investor in an early stage company, that means your product may never get to market -- so you're staring down a significant risk of total capital loss.

Finally, we still haven't decided what the goal of green tech is. Is it to increase efficiency and reduce demand? If that happens, energy prices will drop and consumption will just rise again. Is it to build cleaner generation and consumption technologies? Unfortunately, every alternative solution has a shortcoming. For wind, it's the fact that wind tends not to blow during hot days when demand is highest. Plus, windmills aren't always welcome additions to a community's skyline. And then there's the equipment bottleneck along the solar supply chain. There just doesn't seem to be enough silicon or tellurium to go around.

Buyer beware

Yet investors continue to throw money at the sector and be optimistic about current investment opportunities. Just look at the analyst ratings for a few well-known green tech stocks:

Company                           No. of Buy                  No. of Sell
                                    Recommendations      Recommendations

First Solar                               21                             0
Comverge                                8                              0
Fuel-Tech                                 8                             1
Fuel Cell                                   6                             1
Canadian Solar                         3                             0
Energy Conversion Devices       10                            0
Evergreen Solar                       13                            1

That overwhelmingly positive analyst sentiment could very well have you considering entering the sector. This comment from Lisa Bicker of CleanTech San Diego, however, should send you running: "The capital markets for these types of investments are very frothy right now...yet there are few productive investments available."

A case study

For evidence of what happens when frothy markets meet a lack of productive investments, take a look at what's happened to ethanol stocks over the past two years. Once thought to be a product that could make the United States both greener and more energy independent, recent research has revealed that ethanol production may actually offset or, even worse, supersede the greenhouse gas reductions causes by ethanol use. What's more, the combination of rising corn prices and farmers growing more corn and less of everything else has led to higher food prices across the board.

Of course, demand for ethanol wasn't necessarily stoked by market forces. The government, politicians who coveted the Iowa primary, and several powerful interest groups were very much involved in making it a green tech priority.

All of this combined to make ethanol stocks a very bad investment back when they were being touted in the spring of 2006. For example, on April 5, 2006, analyst Michael Brush wrote about a few "ethanol stocks to get revved up about." Here is the performance of those picks since his article was published:

Company Return since 4/5/06
Green Plains Renewable Energy (75%)
Pacific Ethanol (81%)
Archer Daniels Midland +28%
MGP Ingredients (63%)

Another high-profile ethanol play, VeraSun Energy, IPO'd in June later that year with shares trading at $25 per share. It now trades for $7.25.

I am not against saving the world

This is not to say that energy companies pursuing green solutions are bad companies or that they're misguided. The world is clearly pursuing cleaner energy solutions even as the demand for energy around the world rises. A company like Fuel-Tech, which can help power plants reduce their emissions, will clearly benefit.

The fact, though, is that investors can turn even the best company into a bad buy by paying the wrong price. That's a real risk in the green tech sector where outcomes are uncertain and valuations "frothy."

But if you do it, do it right

This all said, there clearly is a wide market opportunity for green tech companies in today's economy -- and a wide market opportunity is one of the core traits we look for in the small companies we recommend to investors in our Motley Fool Hidden Gems service. So while we're somewhat wary of the sector, we're also taking a long hard look at it.

Governing that research are a few tips gleaned from those Roth Conference panelists:

Focus on green initiatives that offer immediate return on investment to customers. Those are the ones most likely to be widely adopted.

Peak demand for electricity continues to be an enormous challenge, which makes distributed generation, energy storage, and advanced metering technologies extremely interesting to the large utilities that will be making many of the spending choices going forward.

Hybrid vehicles. They have clear consumer appeal and are one of the few ways for the individual to participate tangibly in emissions reduction. Don't overpay.

So while we're looking hard at green tech at Motley Fool Hidden Gems and have Fuel-Tech on our Watch List, we won't recommend any stock at the expense of a compelling valuation. And when it comes to buying green tech stocks, you should do the same.

After all, that focus on valuation in the small-cap space has our picks more than 20 percentage points ahead of the market. You can check out what we're recommending today and read all of our notes from the Roth Conference by joining Hidden Gems free for 30 days. There's no obligation to subscribe.