Estimate Power Usage

Next, determine how much power each of these typical units use. For simplicity sake, use the Idle mode power rating. For a desktop PC, this is typically 85 watts plus another 25 watts for an LCD monitor (or 75 watts for a CRT) for a total of 110 watts with an LCD monitor (or 160 watts with a CRT) consumed per hour. Although the power consumption rate of desk top units will vary, using a single value to represent all units will provide a reasonable approximation.

To estimate the total amount of power consumed by desktop units, multiply the number of computers by 110 watts, by 24 hours (in a day), and then by 365 days per year. Multiple this product by the number of years between equipment refreshes. Typically this is between three and five years.

Finally, to convert this hours-used figure to money, multiply the cost of a kilowatt-hour by the energy consumed by the desktop unit over its useful life. Together, this indicates the amount of money the company has or will spend on this desktop computer over its useful life.

▲ KWH used = Number of units multiplied by the watts used per hour multiplied by 8760 hours in a year multiplied by the years of useful life

▲ Cost to power the unit while in the company = KWH used during its useful life multiplied by the company's KWH rate


Notebook PCs are designed to be more energy efficient than desktop units, so that they can run longer on battery power. They consume about one-half the power of a desktop unit when in Active mode. They are typically turned off when not in use and not attached to a "dock." They also spend a portion of their work life powered by a battery instead of a wall outlet. The topic may be worth discussing with company leaders.

Repeat this same exercise of estimating the number of units and their power usage for the many laser printers and inkjet printers sprinkled throughout the company. Laser printers can be big power users, as they must heat their internal fuser to heat the toner so it will adhere to paper. Inkjet printers use much less power. Servers, storage area networks, UPS units, and printers vary widely in power consumption. Some units, such as UPS systems and network firewalls, are always on-line. For those devices present in small number, use the same value as a similarly sized component of a different model.

Finally, double this number. Cooling typically consumes one watt of electricity for every watt used by an electronic device. Computers and printers add to the around-the-clock cooling load of offices. Of course, the cooling in the data centers is more intense and concentrated.

Network devices should be noted but kept separate from the data center and desktop numbers. They are a part of the total cost of powering desktop computing, but their shared nature makes them more difficult to include in power management. However, an understanding of their energy consumption is valuable when comparing energy efficiency of future replacement equipment.

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