Imagine a computer system having 450W SMPS . It have 65W CPU and 4GB RAM. The computer is a server which frequently uses RAM and CPU.
How much power does it will consume ?
(I asking this to calculate the electric expenditure for my server )
Without knowing exactly what hardware you have or having actually measured the power, the best we can do is guess. There are around 720 hours in a month, so if it draws 200 Watts, it will consume 144 kilowatt-hours per month. This is usually what people want to know because they are billed monthly and are billed for each kilowatt-hour consumed.
You can purchase a watt meter at electronics stores for $30 or less. With a watt meter, you can measure your actual wattage. Multiply the watt measurement by .72 to get kilowatt-hours per month.
Watts are already a measure of the rate at which energy is consumed, so "watt per hour" wouldn't make any sense.
Since so many components can influence the final wattage a computer system uses the most accurate answer is to use a wattmeter between the computer and the power mains.
See one in action here.
Typically, the newer the machine the lesser the power it will use to perform the same task. That is why you just can't compare apples to oranges.