The energy sector can be one of the most stable places to invest money over the long term. Oil, electricity, and natural gas are not items people are willing to quickly cut back on, even in a recession. But in such a stable business, finding growth stocks isn't always easy. Here are a few stocks I'm watching that might give investors some growth in energy.

Natural gas vehicles
Clean Energy Fuels (Nasdaq: CLNE) is providing the infrastructure for natural gas fleets around the country. The company is spending its money on expansion, so it's running around breakeven -- but revenue is quickly rising. In the third quarter, revenue increased 46% to $45.7 million. Those sales should continue to grow as natural gas stations become more available. For its next expansionary move, Clean Energy Fuels will own and operate facilities at Flying J stations throughout the country, giving it a strong foothold.

Power generation
SmartHeat (Nasdaq: HEAT) is a fast-growing clean technology and energy-saving company based in China. Its heat exchangers are used to make power plants more energy-efficient. Given China's growing power needs, who wants to bet against that market? Revenue was up 36.1% in the third quarter, but a higher diluted share count led to a drop in earnings per share. SmartHeat is growing, but watch the share dilution, or you might take a sucker punch like the one Advanced Battery Technologies (Nasdaq: ABAT) recently dealt to investors.

A-Power Energy (Nasdaq: APWR) builds power plants in China, and now it's getting into the wind turbine business. Its stock has taken a beating this month on a weak earnings report and some negative comments about the General Electric gearboxes it will be using for wind turbines. But that might mean a buying opportunity, given the stock's 12.3 P/E and China's growing power.

Save a little power
Any talk of power needs to include the emerging rechargeable battery market. Whether you're driving an electric car, steering a scooter in China, or using electricity at home, you may soon be using battery power on a mass scale. Advanced Battery Technologies is the profitable (albeit unpredictable) player in this growing field. But two American companies, A123 Systems (Nasdaq: AONE) and Ener1 (Nasdaq: HEV), are making moves in grid storage and electric vehicles. Both companies expect big-time growth in 2011 as electric vehicles start hitting showrooms.

What stocks will be powering your portfolio for 2011? Leave your pick in the comments section below.

Interested in reading more about Clean Energy Fuels? Click here to add it to My Watchlist, which will aggregate all of our Foolish analysis on this stock for you.