PhotoWhat's the biggest energy hog in your home? Your refrigerator? Your microwave? Not even close, says the Natural Resources Defense Council (NRDC).

In a report, NRDC says the cable TV set-top box, present in about 80 percent of U.S. homes, consumed approximately 27 billion kilowatt-hours of electricity in 2010. That's equivalent to the annual output of nine average coal-fired power plants.

If you get your television signals from a cable, satellite or telephone company, you probably have a set-top box. There are about 160 million of them in use in the U.S., according to NRDC and its partner, Ecos.

'Startling' findings

The groups say their findings were startling. The electricity required to operate all U.S. boxes is equal to the annual household electricity consumption of the entire state of Maryland, results in 16 million metric tons of carbon dioxide (CO2) emissions, and costs households more than $3 billion each year.

How can such a relatively small appliance use so much electricity? Today’s set-top boxes operate at near full power even when the consumer is neither watching nor recording a show. As a nation, we spend $2 billion each year to power these boxes when they are not being actively used.

Boxes equipped with digital video recorders (DVRs) use even more electricity. DVRs typically use around 40 percent more energy per year than their non-DVR counterparts, the study says.

How to reduce energy consumption?

Is there any way to reduce this electricity consumption, aside from unplugging set-top boxes when they aren't in use? The study says there is.

Better designed pay-TV set-top boxes could reduce the energy use of the installed base of boxes by 30 percent to 50 percent by 2020, the groups say.

The big opportunities include: a) shifting to whole-home solutions that include a main box connected to the primary TV with either TVs specially designed to access the video content stored on the main box or low-power thin client boxes that serve the same function, and b) having the boxes automatically power down to much lower power levels when not in use, much like a programmable thermostat controls heating and cooling costs.

How likely is that to happen? The cable and satellite providers control set-top box installation, configuration, software updates, repair, refurbishment, retirement, and resale. The consumer, who pays the electric bill, has little choice about what box the service provider installs and how much energy it uses. Since the cable company doesn't pay the electric bill, they have no incentive to make the boxes more energy efficient, NRDC says.

New TV sets that are able to stream video content directly from the Internet may be one solution. According to the study, streaming devices use significantly less power than set-top boxes. The most efficient of the streaming devices studied is AppleTV, which drew just three watts in “on” mode and less than one watt in “sleep” mode.