A megawatt is 1000 kilowatts (also equal to 1341 horse power) or 1*10^6 watts, a gigawatt is 1000 megawatts. A watt is equal to one joule per second and a joule is defined as the amount of work done...
It's unfortunate that both of these units include the same word "watt". That gives the impression that one can be converted into the other. These two units are used for measuring different...