By David Russell, reporter at OptionMonsterNEW YORK -- Electricity stock Calpine ( CPN) has been steadily trending higher for more than a year, and the bulls are looking for the energy to continue. OptionMonster's tracking programs detected the purchase of 7,500 June 18 calls for $1.65. Volume was more than 200 times open interest at the strike. Owning calls locks in the price investors must pay to buy shares, so they can generate significant leverage if the stock continues to rally. If it drops or stalls, however, they will lose all or most of their value. Calpine's stock rose 2.39% to $18.03 Tuesday and is up 19% so far this year. The stock has been gradually working its way higher since the S&P 500 bottomed in March 2009 and as natural-gas generation takes market share from coal-fired plants. Earnings and revenue also beat analyst forecasts by a wide margin in the third quarter. Overall option volume was 14 times greater than average in the session, with call outnumbering puts by 6 to 1. Russell has no positions in CPN.