NEW YORK ( TheStreet) -- PowerShares WilderHill Progressive Energy ( PUW) has led PowerShares WilderHill Clean Energy ( PBW) by a wide margin over the past three months. PUW's return was 19.20% compared to only 0.76% for PBW.

Much of the gap between the ETFs came since the end of July, as PUW's return rose 12.2% while PBW rose 1.1%.

The stocks among PUW's top 10 holdings that provided the lift were Clean Fuels ( CLNE), up about 50%, along with Owens Corning ( OC) and Chesapeake Energy ( CHK), up about 30%. Methanex ( MEOH) also had a portfolio-outperforming return of just under 20%.

PBW's fund had similar returns from its top 10 holdings. Fuel Systems ( FSYS) had return of nearly 50%, while Echelon ( ELON) gained more than 50%. Cosan ( CZZ) and International Rectifier ( IRF) both gained more than 20%.

On the downside, PUW's worst performing top 10 holdings were flat, while PBW saw Applied Materials ( AMAT) and SunPower ( SPWRA) lost ground.

However, the current top 10 holdings don't tell the entire story. As of June 30, PBW's top 10 was dominated by solar stocks, including First Solar ( FSLR), Yingli ( YGE), Evergreen ( ESLR) and JA Solar ( JASO). Over the aforementioned period, JASO lost 20%, FSLR lost nearly 15%, and ESLR lost more than 5%.

Broadwind Energy ( BWEN), the former No. 1 holding on June 30, fell nearly 25% and went from 3.14% of PBW to 2.52% on Sept. 14.

PUW has seen changes among its top 10 as well, but it more closely resembles its June 30 composition. A couple of stocks moved out of the top 10, including Southwestern Energy ( SWN) and USEC ( USU). However, these were more allocation changes than a price decline, as SWN was flat and USU actually gained 30% over the period.

PowerShares ETFs have performed well in many cases, but investors need to pay attention to the holdings in these funds because they will shift over time -- and that will alter results.

Since PUW's inception in October 2006 (PBW was launched in March 2005), the two funds have tracked very closely, with occasional under- and over-performance, until last fall. From its Nov. 19 low, PUW gained 105% compared to a 71% return for PBW.

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