"Usable energy" refers to the energy that can be easily and effectively used to power devices or perform tasks. It is the energy that can be readily converted or harnessed to perform useful work. Full definition
Scientists discovered a way to turn carbon dioxide into usable energy source through the use of sunlight. (natureworldnews.com)
Forty percent more coal will have to be burnt to get the same amount amount of usable energy. (johnquiggin.com)
Animal based fats and proteins are digested and absorbed, turning into usable energy for your dog to function, play, work, or walk with you! (notinthedoghouse.com)