Energy in general is defined as the capacity for doing work. Power is the rate of doing work or the rate of using energy:
P=Work/t=Energy/t , where t is time.

Although casually the terms energy and power are often used interchangeably, we see that technically they have different meanings. The SI unit of energy and work (which are numerically the same) is the joule (J). A joule is the work done by a force of one newton for a distance of one meter. This unit is usually used in physics. Energy comes in many forms, such as heat, motion, gravitational, radiated solar power, and electrical. For different types of energy other physical units are also utilized. For example, the British Thermal Unit (Btu) is often used to measure the heat energy or compare fuels. One Btu is what's needed to heat one pound of water one degree F. The SI derived unit of power is watt (W). Watt is power required to produce or consume one joule of energy per second. This unit and its multiple kilowatt are usually utilized in ratings of various electric loads and sources of electricity, such as residential generators.