There is a similar pattern in Powershares Cleantech ( PZD), one of the other top performers over the past three years that's not water-specific in its approach. The top two holdings in this fund are broad sector plays, Borg Warner ( BWA), an automotive sector giant, and Schneider Electric, which along with other of the Cleantech ETF's top holdings -- Corning ( GLW), Siemens ( SI), Johnson Controls ( JCI), ABB ( ABB) -- are conglomerates touching everything from power transmission, water services, mining services and oil and gas, to electric car seats, solar, and commercial building florescent and LED lighting applications. Morningstar's Burns said that while these broader funds avoid volatility, the most important question an investor needs to ask is why invest in a clean energy fund that holds a GE-type stock as a top holding, if that stock is already in your core equities portfolio. To the extent an investor wants to buy and hold these ETFs as a view that clean energy is the future, it's a matter of timing relative to the rest of the world making this decision. "We use clean energy ETFs when we are on the road visiting clients to make the point that if the world is running out of energy and the traditional energy grid needs a massive investment in solar, that's great, but if the whole market figured that out three months ago or three years ago, you end up buying at a high, and overpaying as the last sucker in," Burns said. With the solar ETFs down more than 60% over the past three years, though, overpaying isn't the current problem. -- Written by Eric Rosenbaum from New York. >To contact the writer of this article, click here: Eric Rosenbaum. >To follow the writer on Twitter, go to Eric Rosenbaum. Follow TheStreet on Twitter and become a fan on Facebook.