Wireless transmission of electricity involves transmitting electrical energy from a power source to an electrical load without interconnected wires. There are three main technologies available: inductive coupling for very short distances, evanescent wave coupling for moderate distances up to 10 meters, and microwave power transmission for long distances. Microwave power transmission has the highest efficiency of up to 98% but requires large transmitter and receiver equipment which poses some health and safety risks. Wireless transmission could help reduce electrical waste and costs of power lines but retrofitting equipment may be expensive.