If the designation "MW" first letter is uppercase, the problem is how to convert kilowatts to megawatts. One kilowatt is equal to one thousand watts, and one megawatt is a million watts, and that means one thousand kilowatts. Thus, to convert power expressed in kilowatts, to megawatts, divide the desired value by a thousand, for example:15 kW=(15/1000) MW=0.015 MW.
If the designation "mW" first letter is uppercase, the problem is how to convert kilowatts to milliwatts. One milliwatt is one thousandth of a watt, so that the capacity, expressed in kilowatts, convert to milliwatts, multiply the required quantity by one million, for example, 15 kW=(15*1000000) mW=15000000 mW.
Express power (and other physical quantities) in the wrong units without the need. Considered inappropriate units when expressing values which are too small or too large. Such numbers are inconvenient to carry out mathematical operations.
If the value still need to be expressed in the wrong units, use the exponential method of representing numbers. For example, the number 15000000 the previous example can be expressed as 1.5*10^7. In this form, the meaning of power or other quantities it is convenient to perform computations using a scientific calculator, which, in contrast to the conventional adapted for the representation of numbers.
If you solve a problem where at least some of the quantities (voltage, current, resistance, power, etc.) expressed in non-si units, first put all the data in SI (in particular, power in watts), and then solve the problem, and then translate the result into convenient units. If this is not done in advance, the determination of the order of the result and the units in which it is expressed, is much more complicated.