Each computer has a particular energy requirement because of its component. The CPU and video card are the two main components using energy distributed by the power supply. Not all video cards or CPUs will need the same amount of power and not all the power supply will have the same efficiency.
Computer hardware does not accept 110v directly. The power supply transforms 110V to 12V and 5V depending on the hardware. An acceptable power supply will have an efficiency of 80 percent and a good one will range from 85 percent to 93 percent, meaning for each 100W use only 80 percent to 93 percent are available to the computer. The rest is converted into heat. Low-efficiency power supplies use more power and will produce more heat.
Peak vs. Average
Computer power consumption varies depending on what software is running. The CPU will not run at full speed when writing text or researching on the Internet for example. The video card will also run at a lower speed to save energy when necessary. Playing games and encoding video using GPU assisted encoding are among the rare instances a computer electric consumption will spike.
While the CPU was historically the component requiring the most energy to run, recent offerings are more power savvy. At maximum speed, low-end CPUs can run as low as 35W and the great majority of middle end CPU run at 65w for dual core to 95W for quad core. Higher-end CPUs can read 120-140W but they are mainly designed for extreme users and hardcore gamers. Today's CPUs are more powerful that most non-gaming or imaging application requires. To save energy and lower the amount of heat produced, those CPU scan lower the operating speed or deactivate part of their circuitry. Several of those states are available depending on what CPU power is needed that a specific time. Some of those states can require less than 9W to function properly.
Integrated Video Card
Integrated video cards are integrated to the north-bridge on the motherboard. They are on the low end of the performance scale and usually require little power, from 10 to 30W, depending on the make and the targeted market (laptop or desktop).
Dedicated Video Card
Dedicated video cards have a large-consumption range at full speed, from 30W for a low end card to more than 500W for multi-card high-end configuration. Most middle- to high-end video cards will require a specific power cable as the power available through the connection slot is not enough for normal operation. Just like the CPU, the video card has different states based on current use. The more energy savvy state usually requires two to three times less energy than the full speed state.
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