Despite today's more efficient electrical generators, modern electric-only power plants waste more of their fuel sources potential energy than did Thomas Edison's power plants of the late 1800s. Edison used cast-off steam from his generators to warm nearby homes and factories. But few modern power plants use this residual heat, instead venting it into the air. When newer, larger plants required more real estate, they were built farther from customers. Moreover, because electricity travels easily, plants were located wherever they could tap the energy of a river or where local coal was especially cheap. The heat generated during the production of electricity does not travel far, however, so when power plants moved out to the horizon, the steam went to waste.