USB is a standard interface that lets you connect a computer and devices such as printers and keyboards. USB devices might come with their own power supply, but most of the time, they don't. If the manufacturer can pull enough power from your notebook's USB port to operate its product, it can boast a power-brick-free device and save money by not supplying a power supply with the product. The USB bus allows up to five "unit loads" (100 milliamperes [mA] each at 5V) or 2.5W maximum of power per "Vbus." Many USB accessories pull 90 to 95 percent of that 2.5W maximum by themselves. Although Intel recommends over-current protection, this isn't meant to police the 500mA-per-port maximum. In fact, Intel recommends setting the trip level of such devices to 2.5 to 3 times the maximum allowable port current, up to a maximum transient peak of 5A.

Some notebooks provide multiple powered USB ports, and people just can't wait to start plugging in all those wonderful flat-bed scanners, external hard drives, backup units, and DVD devices. When you have a fully charged battery and your notebook is plugged into the wall, your power supply is buffered against surges by tapping power from the notebook's batteries. What happens if the batteries aren't charged? Not only will the power supply be facing its own demons (hottest CPU, biggest and brightest TFT display, and high RPM hard drive), but it will also be tasked with charging dead batteries and trying to supply all those USB devices. At that precise moment, your power supply may be well beyond its maximum rating, and you might find out just how good your full-coverage warranty really is (you do have one don't you?).

My newest brand-name notebook lists a 90W maximum draw (all by itself), yet it comes with a 64W power supply. Like most manufacturers, this one relies heavily on the batteries to handle short-term overages on the supply. With three fully powered USB ports allowing up to three 2.5W devices (and already planning on a 26W deficit), I'm approaching non-powered USB devices carefully. So should you. At the very least, I'd recommend running heavy-draw, non-powered USB devices only after the batteries have been sufficiently charged.

And then there's the full-coverage warranty on my machine that includes the statement, "[Brand name deleted] hereby disclaims any representations or warranty that this product is compatible with any combination of products you may choose to connect."