I would think that considerable energy savings could be achieved by having the device active only when the user is engaging with it. Much of the time the device could be in a sleeping mode awaiting incoming data or user input. If the device isn't being worn, it could be in an even deeper sleep mode. Still, there needs to be a recharging plan. Will the devices operate on a long life battery (like an electric watch) or will they have some mechanism to recharge overnight when not in use like a cell phone?