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Summary: Vampire devices such an awful lot of power, often for no good reason.

I've Googled extensively trying to find credible numbers that indicate how much power is wasted in wall-warts that are plugged in but not connected to anything. For example, a cell phone charger is usually idle but consumes some power regardless. I could only find information that appears to be self-serving or seems to lump disparate things together.

The Energystar site ( says that vampire devices suck 100 billion KW-hrs of power every year in the US, but I really don't know what this data means. Does it include TV cable boxes? Pull power from most of them and then you'll need a long reboot cycle; even when "off" they consume power. It's not unlike old-time CRT TVs which sometimes kept the tube's filament powered up to give the illusion of "instant on."

I measured a half dozen wall warts around the house and found quite a range of results. Four, when the devices were in off-states, drew less than the 0.1 watts I could measure with my Belkin tool. One sucked 0.6 W, and the charger for the house phone needed an astonishing 8.1 W with the load disconnected.

If - and these are big ifs - the average house has 10 wall-warts plugged in at a time which are not powering anything, and the average power draw for these vampires is, say, 0.2 W, and assuming 100 million households in the US, that's on the order of 1.5 billion KW/hr per year of waste, two orders of magnitudes lower than the Energystar figures.

But a billion+ KW/hr is a lot of waste.

Then there are the devices that are sort of on. Like the microwave oven. Ours takes 1.5 watts when it's just displaying the time. The cable box takes 24 W when "off." But I was quite surprised that our new smart TV takes too little to measure. Considering it boots in six seconds, and has wireless connectivity, games, a web browser, etc., one would think some slow-booting OS is lurking under the hood, and that "off" meant a low-power sleep mode.

In the dim past when we used linear power supplies, wasting power was the only way to regulate voltage. A series transistor turned the excess input into heat. Efficiency could be astonishingly low; it was all a matter of how much power one chose to lose in the transistor. But modern switching power supplies are much more efficient.

A company using the now mandatory lower-case "i" prefix has an interesting part that promises to cut that waste power pretty close to zero. iWatt's iW1700 ( is a controller that could be the heart of a wall-wart or other switching power supply. The data sheet is a little sketchy but it seems to be good for an amp or so at 0 to 18 volts. In an off-state, a supply built from this part would consume just 4 mW at 230 VAC. As my dad drilled into our heads "waste not want not."

iWatt sent me a complete module, which is about a cubic inch in size (the chip itself is a six pin SOT-23 package). One solders the mains to the board and gets power out of a USB connector. I'm a 5V kind of guy and try to keep my fingers away from 110VAC, so did not hook it up. But the module is a lot smaller than most of the wall warts around here. A notable exception is the charger for the iPhone, a tiny little thing about the size of the iWatt module.

Seems to me the iWatt device could be embedded into a power cord. That bump on the end of the cord which goes into the wall is just about the right size to house one of these modules.

The ENERGY STAR standards continue to tighten. And, the International Energy Agency's One Watt initiative demands standby power in appliances to drop to 0.5W by 2013. A solid decade ago President Bush issued an executive order that limits standby power to a watt for all federally-purchased products, if such products are available.

Clearly, the standby squeeze is on. Do you constrain standby power in your products?

Published October 19, 2011