Watts per hour or kilo-watts per hour is the method in which power providing companies have been tracking and billing for the consumption of electrical power for the last 50 years. All you have to do is go outside to your meter and read the outside of it or simply look at your last electric bill. Most companies will even provide you with the formula they use to calculate the cost per watt used in residential or commercial applications. To put the context of 'Watt-per-hour' in practical terms, in a 120 volt system, Vista consumes about 0.025 amps vs 0.84 amps that XP does. Since most users, personal and business, do not turn off their computers at night or when not in use, that is a very significant savings in power consumption and not an invention of Microsoft's.