Price, of course. Prices are expressed in cents/kilowatt hour. Plan types include fixed rates and variable rates. Fixed rates offer consistency throughout the plan term, which can run from six to 36 months. Variable rates can change monthly - they're great when prices go down, but not-so-great when they don't. Finally, if you care about your carbon footprint, you may choose a plan sourced by solar or wind energy.
Twenty-nine states have deregulated electricity, natural gas or both. That allows you to shop for the supply portion of your bill from alternative providers who may offer rates lower than the default supplier – usually a utility. Delivery services and billing will remain the responsibility of the local utility as they own the power lines and wires that keep the lights on.