Power Consumption

Contents

• Background
• Computers
  • Table of Power Consumption
  • Description of the Computers
  • Conclusions about Computer Power Consumption
• Other Appliances
• Electrical Measurements
• Converting to Dollars

Background

The recent electricity issues in California have prompted me to be a little more aware about how I use electricity. Unfortunately it can be a bit difficult to know how your habits affect your power consumption around the house. Appliances such as computers, whose consumption of power varies widely and can be reduced with energy-conscious configuration, give you hardly any clue about how much power they consume at any point in time.

To help myself reduce my own power consumption, I have collected some information about how much power various appliances, especially computers, consume. I've collected this information casually; it isn't lab-quality data. But still I think it's illuminating.

Computers

There are a number of factors that affect the amount of power that your computer draws:

• Is the monitor on?
• Is the computer in "Standby Mode"?
• Is the disk active?
• Is the processor active?

While I originally thought that disk activity had a major influence on power consumption, the effect is barely noticable. For desktops, by far the most significant controllable factor is the monitor.

Table of Power Consumption

Measurements of laptops are taken with the batteries disconnected.

Description of Computers

Conclusions about Computer Power Consumption

Although these results may not represent other laptops and desktops, it appears that laptops require far less power than desktops. This is hardly a supprise, given their battery constraints.

It may be worth noting that the CPU and memory, not disk, account for nearly all of the difference between idle and active power consumption levels in the newer computers.

What can you do to most drastically reduce your power consumption, if you feel you must leave your computer on most of the time? The largest and most well-known strategy is to just turn off the monitor while not in use. Another big factor, although less well-known, is to choose a blank screen saver over a processor-intensive one.

With these strategies, a desktop can easily attain an idle power consumption of 60 Watts, or 35 Watts if one can remember to turn off the monitor when getting up, and a laptop can attain 13 Watts.

Of course, if you don't really need to leave your computer on, you can do better by turning it off completely.

Other Appliances

I confess that my primary interest is in the power consumption of computers. However, since I had the equipment, I started measuring the power consumption of other household appliances:

Electrical Measurements

All Wattages are calculated from Amperages using a measured household AC voltage of 122V:

power in Watts = current in Amps * potential in Volts

These measurements are taken with a Sperry Digisnap DSA-500 multimeter. Current measurements are by induction, using split section of extension cord.

According to these methods, I consistently measure a 100 Watt light fixture's power consumption as 94 Watts. I cannot explain the difference except that perhaps our household voltage is a tad low.

For more information about the difference (in the context of Physics) between Amps, Watts, Kilowatts, Kilowatt-hours, current, power, and energy, try the following links:

• Power (and Energy) Defined
• Electrical Power (with equations)
• Measuring Electricity
• AC Measurements (with Calculus)

Converting to Dollars

Although most of the discussion here focusses on saving power, saving power saves money, too.

In our household, we pay between 12 and 19 cents per kilowatt-hour. So if I can trim ten Watts from my power consumption, I save about $15 per year.