How much power is required to run a PC and how much energy does it consume? This may look like the same question written in two different ways, but it’s not. To answer it, in terms of physics, we need to think about two different quantities: power and energy consumption – or watts and watt-hours, if we’re speaking specifically about the electricity used by a PC or any other electrical device or household appliance. The difference between watts and watt-hours As we said before: watts measure the power it takes for a device to function and watt-hours measure the energy consumed by that device over a certain period of time. In order to understand the difference between power (expressed in watts) and energy consumption (expressed in watt-hours), we can make a comparison with speed and distance. Think of a car travelling at 50 km/h: its speed is the power with which it’s moving at any moment (metaphorically speaking: the watts), while the distance that it covers in a specific period of time, if it maintains a constant pace, is the speed multiplied by the length of time elapsed. Getting back to our hypothetical PC, the power measures how much energy has to flow through it, at any time, to make it work. Let’s pretend our PC is a watermill: to make the wheel turn, a certain flow of water is required every second. But if we want to calculate how much energy it consumes in one hour, then we need to calculate how much water passes in that period of time, which is the total power used to turn the wheel. A simple equation The energy consumed (the total energy used to run our PC or electrical appliance) is the power multiplied by the duration of use. Actually, in physics the unit of measurement of energy is called a joule. The equation that we should use to calculate power, then, is watt=joule/second. The shorter the time, the greater the power necessary to perform the same task: for example, if I have less time available to travel a certain distance, I’ll have to go faster to get there. On our utility bills, however, we don’t find consumption measured in joules, but in (kilo)watt-hours. A watt-hour is the unit of measurement used commercially because it’s easier to quantify. It’s the equivalent of the total energy supplied if electrical power of one watt is maintained for one hour; that’s equal to 3600 joules. So you’ll usually hear people talk about kilowatt-hours, if we’re looking at a utility bill for domestic use, or megawatt-hours if we’re looking at the consumption of, or energy produced by, a power plant. Sure, but what about in practice? Watt-hours express the electric energy supplied to maintain a certain power in watts, and to run our electrical device. In the case of a PC, if the monitor measures 14”, then average consumption is around 35W per hour (30W if the monitor is on standby). Watt-hours reflect consumption – or waste, in some cases. In the case of a PC, its energy consumption depends on the level of the brightness of the screen, the size of the monitor and the use of the standby function. When we say “it consumes more” When discussing energy consumption, it’s sometimes said that using the washing machine in the evening (as long as it doesn’t bother your neighbors) consumes less energy. Actually, that isn’t correct: what changes is simply the cost of energy at different times of day and at the weekend. The reason for that is that during the day (on weekdays), industries and businesses are more active and so consumption is at its highest. In contrast, in the evening or at night, consumption is lower because offices, shops and many industries are closed. It’s fair to say, however, that an open oven or fridge consumes more because they have to work harder to maintain the desired temperature and to keep the lights on. Linguistics apart, though, both power and energy consumption are useful pieces of information that we should keep in mind.
