This cable is kinda fascinating - it has an integrated boost converter, so you can plug it into any USB port (from a computer, battery pack, etc) and it will give you a higher DC voltage! It's a handy way to power 9V powered devices with a center-positive 2.1mm DC barrel jack from a USB port. We also have a version that provides 12V, and if you just need a passthough cable, here's one for 5VDC.

These are rated at 500mA output, here's our measurements for various current draws when we set our bench supply to provide a steady 5V voltage

•  50mA (9.0V output), 100mA,  90% efficiency
• 100ma (8.95), 205mA in, 87% efficiency
• 200ma (8.9V): 410mA in, 86% efficiency
• 300ma (8.8V): 620mA in, 85% efficiency
• 500ma (8.8V): 1060mA in, 83% efficiency

So, overall there's a 90% efficiency at low current draw that drops do about 80% at high current use. There's also a 3 mA quiescent current. Note that often times the USB port voltage can droop too, and in addition you may have power losses with any thin or low quality cables with high resistivity!