Creating the power utility's equivalent of an organic food store, a Connecticut company is to announce Monday that it will begin selling what it calls "green" electricity, generated not from oil or nuclear reactors, but from renewable sources like wind, water and methane gas captured from garbage dumps. Naturally, it will be more expensive-about $5 to $6 more a month. But the company, the Connecticut Energy Cooperative, is betting that some customers will be willing to pay a premium for it.