To get 1,000 megawatts it actually uses 3,000 megawatts from the fuel. The leftover 2,000 megawatts is inevitably produced as heat in as shown by the second law of thermodynamics. A variety of units are used for power and energy. Kilowatts and megawatts are a thousand watts and a million watts respectively. A horsepower is 746 watts by an optimistic definition, but few horses can put out that power for long.
Energy is measured in kilowatt-hours.
A kilowatt-hour is therefore 3,600,000 joules. Another unit of energy is the British thermal unit or BTU, sometimes used in engineering. It is the amount of energy needed to raise one pound of water one degree Farenheit. This is a very sketchy exposition. An elementary physics book will do much better.