Power-management does not reduce the performance of a computer, but simply adds features to reduce their power consumption when not in use. These energy-efficient machines save money on electricity bills and reduce pollution from power plants. Most power management savings come from reducing power when the machine is not fully active by adding low-power or "sleep" modes that kick in when idle. "Sleep" modes usually involve slowing the clock rate of the central processing unit (CPU), and may include spinning down the hard disk. While it is possible to reduce energy use in PCs while they are active, the ENERGY STAR program and this technical guide are concerned with power management during idle periods.

Power management in monitors is accomplished during times of system inactivity (usually defined by mouse or keyboard activity) by dimming or blanking the monitor and reducing or eliminating the beam control power. Because some of the electronics are kept warm in the low power mode, monitor recovery is faster than from a full power-off mode.

Power-managed computers are available at no additional cost, and a single power-managed computer and monitor may save anywhere from $5 to $50 per year in electricity bills, depending on how many hours per day the computer is left on and on how much of the time the computer is in active use. The amount of electricity savings from power management is highly dependent on user operating patterns-if the computer and monitor are often left on at night and on weekends, savings will be toward the higher end of this range. For a typical office environment with 100 computers, using power-managed PCs and monitors instead of non-efficient equipment could save $2,200 per year.

Figure 2.1 shows the total power used by an active computer (PC and monitor; typically about 120 W), one that just meets the ENERGY STAR standard (30 W each for the PC and monitor), and a "best case" scenario for a desktop PC. Some manufacturers have gone well beyond the ENERGY STAR level, however, so that it is easy to purchase a PC and monitor that together use only 30 W in low-power modes (with most of the extra savings from the monitor). Laptop PCs can power down to much lower levels than desktop PCs, but in the future we may see desktop PCs with energy performance comparable to laptops.

Figure 2.1: PC and Monitor power use in suspend mode for different computers.

Table 2.1 shows a typical PC operating pattern, derived from observations of PCs in actual use. The typical scenario has 9.5 hours of on-time per day-four hours of active use and 5.5 hours of idle time (low-power for power-managing PCs). The user is away one weekday each week (for vacations, travel, etc.), with 20% of machines left on during these days and weekends. A typical year has about 880 hours of active time, another 2280 hours of low-power time, with the remaining time off (see Appendix E for a more detailed summary). Power management savings are a function of the low-power time percentage and the difference between the electricity use at full-on and low-power modes for the particular PC or monitor. A machine on continuously, but power-managed, would save about three times as much as the scenario below shows.

The following formula shows how to calculate the electricity savings for a PC or monitor, with a sample calculation for a typical system.

    Annual Electricity Savings = Hours-in-year   x Low-time-% x (Active-power - Low-Power)
    for example,               = 8760 hours/year x 26%        x (165 W        - 30 W)
                               = 307 kWh/year

Table 2.2 shows the energy and dollar implications of power managing several typical PC and monitor types, according to the typical scenario above. As can be seen, power management of monitors has higher energy savings potential than it does for PCs alone. There is a considerable amount of variation in energy use in PCs and monitors, so while these values illustrate typical use, yours may differ. Note that PCs and monitors vary considerably in both their active and low-power energy consumption, which, in addition to the operating pattern, greatly influences the energy use and savings you can expect.

In addition to direct electricity savings, power-managed computers generate less heat, and since most offices have to cool the air more than they heat it, for every four kWh of energy saved by the computer, an additional kWh is saved in the cooling and ventilation system. The reduction in waste heat can also increase comfort levels in the office building, and the reduced electricity use can reduce the risk of building wiring overloads.

Power-managed equipment also may actually last longer than conventional products. Because most such equipment will spend a large portion of time in a low-power sleep mode, mechanical wear on disk drives and heat stress on other components can be reduced. Other potential benefits from using power-managed office equipment include reduced electromagnetic field emissions from monitors (because there is less radiation in low-power modes) and reduced noise (since some ENERGY STAR computers have no fans or can turn them off, and hard drives are silent when not spinning).

Continue to Chapter 3: How Does Power Management Work?

Return to the PC User Guide table of contents and home page.

Return to Chapter 1: Introduction to Power Management