Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some clean-energy-related stocks to your portfolio, the PowerShares WilderHill Clean Energy Portfolio ETF (NYSEMKT:PBW) could save you a lot of trouble. Instead of trying to figure out which companies will perform best, you can use this ETF to invest in lots of them simultaneously.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The PowerShares ETF's expense ratio -- its annual fee -- is 0.70%. The fund is fairly small, too, so if you're thinking of buying, beware of possibly large spreads between its bid and ask prices. Consider using a limit order if you want to buy in.

This ETF has performed terribly, significantly underperforming the world market over the past three and five years. But the future counts more than the past, and it's been a rough few years for the entire solar energy industry, among others. And, as with most investments, of course, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver. Indeed, stocks that have fallen sharply are sometimes great bargains.

Why clean energy?
Interest in alternative energies and cleaner forms of traditional energy has been around for a long time, but it seems to finally be gaining some traction.

Relatively few clean-energy-related companies had strong performances over the past year. GT Advanced Technologies (NASDAQOTH:GTATQ) plunged 56%, hurt by oversupply in the industry. Some see its long-term prospects as solid, and the stock as undervalued. It's diversifying into sapphire technology, which might supplant Corning's Gorilla Glass in mobile devices, but it faces some competition there, and few are counting Corning out. A recent new contract sent GT Advanced Technologies' stock up 12%.

Brazilian electricity giant CPFL Energia S.A. (NYSE:CPL) sank 20%, and recently yielded 5.9%. Its long-term debt has been rising, largely due to acquisitions, and its free cash flow has been shrinking (and even turning negative recently). But it has been investing heavily in alternative energies, and it serves a massive and growing market in Brazil. The country's growth has been slower than many would like, but that won't last forever.

Power-One (NASDAQ:PWER.DL) shed 9%. Bears worry about its inverters becoming commoditized, despite being quite efficient, and don't like a drop in its net income. But the company is free-cash-flow positive, and is expanding globally, poised to benefit as the industry heats up. It looks attractive to some, who see it as underappreciated, with its profitability and solid balance sheet -- characteristics that plenty of peers don't have.

New to market is SolarCity (NASDAQ:SCTY), which had its IPO late last year, and has behind it Elon Musk, a green-power entrepreneur. Bulls are intrigued by SolarCity's disruptive possibilities, as it helps consumers bypass traditional utility companies with its solar offerings. The company is also partnering with major entities such as carmakers and Wal-Mart to provide solar technology. On the other hand, it's not profitable, and is burning through a lot of cash.

The big picture
Demand for clean energy isn't going away anytime soon. A well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make investing in and profiting from it that much easier.

Longtime Fool contributor Selena Maranjian, whom you can follow on Twitter, owns shares of Corning. The Motley Fool recommends Corning. The Motley Fool owns shares of Corning and Power-One. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.