From looking at the website, they are using hydro power which is pretty cheap in the long run as you don't have to buy fossil fuels.  They may also already sell the excess power to an exchange, who said they don't? It also appears to be a coop, which means they only pass along actual costs and don't charge additional for profit more than they need to run the business.  If they also sell the excess generation, that additional income would also reduce the rate for customers in the coop.