While I haven't mentioned it much, one of my favorite sectors to invest in is energy.

1. And when it comes to energy ETFs, the 800-pound gorilla is XLE, the energy-select sector SPDR, which trades over 20 million shares on a typical day. XLE’s holdings are heavily tilted toward the major integrated oil companies, with ExxonMobil (XOM) and Chevron (CVX) accounting for slightly more the one-third of the ETF's holdings, followed by ConocoPhillips (COP), Schlumberger (SLB), Occidental Petroleum (OXY), and so on.

With cap-and-trade legislation passing the House over the weekend, investing in the energy space is getting even more interesting. XLEs are up this morning.

2. Up, too, are the popular oil services ETF, OIH (top 5 holdings favor drillers, and include RIG, SLB, DO, BHI, and NE), and the exploration and production ETF, XOP (top 5 holdings are XEC, PXD, EAC, INT, and HK).

3. There are a variety of ETFs out there in the clean/green space. Perhaps the best known of these -- and certainly the most popular -- is PowerShares WilderHill Clean Energy (PBW), whose largest holdings include a healthy dose of solar companies (top 5 holdings are FSYS, VLNC, SOLR, ESLR, and SOL).

4. Among the more interesting alternatives is a sibling ETF, PowerShares WilderHill Progressive Energy (PUW), which places more emphasis on energy efficiency and nuclear power, and has a list of top holdings that includes MX.TO, ES, PX, USU, and CCO.TO.

5. For a solar-only ETF play, Claymore/MAC Global Solar Energy (TAN) is an excellent bet. Note that many of the holdings of TAN are not traded on US exchanges. The current top 5 holdings are MBTN.SW, FSLR, S92.BE, CTN.DU, and SWV.BE. Also in the top 10 holdings are 2 Chinese solar companies whose ADRs are available in the US: STP and TSL.

6. In the chart below, I've highlighted my favorite all-purpose clean energy ETF: PBW, and have included a ratio of PBW to XLE in order to get a sense of the relative performance of clean energy with respect to the broad energy sector.

While PBW has pulled back with the broader market during the past 3 weeks, it's continued to perform strongly against the broad-energy-sector ETF. As the ratio chart suggests, pair trades involving clean energy ETFs -- such as PBW, PUW, and TAN, versus XLE, XOP, and OIH -- are one way to play the Washington-energy-legislation game going forward.

                                   
Click to enlarge