If NRG Energy Inc. (NYSE: NRG) were an athletic prospect, scouts would rate it as a “triple threat.” That’s because the Princeton-based wholesale power generator is involved in all three of the key energy sources of the future: Solar, wind and nuclear.

And that’s only part of the reason I like this stock.

Growing profit margins and earnings momentum add to the energy company’s appeal – and a rebound in U.S. economic activity hasn’t even begun in full.

When NRG announced its second-quarter results a few weeks ago, the company said that its profits tripled from a year ago – eclipsing Wall Street estimates and setting a new record. It also boosted its earnings guidance for all of 2009, and increased its stock-buyback target from its previous $330 million worth of its shares to $500 million.

Income from continuing operations was $432 million – a marked improvement over last year’s $41 million loss.  And its recent acquisition of the Texas retail-energy business of Reliant Energy Inc. [now RRI Energy Inc. (NYSE: RRI)] is starting to pay off.

In two months the tie-up has already delivered $200 million of the planned $400 million in adjusted earnings before income taxes, depreciation and amortization (essentially a cash-flow metric that professional investors refer to as “EBITDA”) gains for the year.  With disciplined management this acquisition should outperform its estimated gains.  This analysis is being recognized as we speak by the market, with unusual January call option activity in RRI stock last Friday.

NRG has interest in 44 power plants with 24,005 megawatts (MW) net ownership, most of which is in the United States. Plants in Texas and the Northeast account for almost 18,000 MW, giving the company positioning in fairly strong markets where environmental, but NRG also has operations in Australia and Germany.

The company distinguishes itself by having operating margins that are roughly double that of its peers – the product of its efficient fleet composition and prudent active energy price hedging policies. The hedges NRG currently has in place are likely to outperform analysts’ estimates, as well.