USB 1.1 and 2.0 devices use power up to 4.4 volts and deliver up to up to 500 mA for configured devices and 100 mA for unconfigured and suspended devices. USB 3.0 can provide 50% more power (150 mA) for unconfigured and suspended devices, and 80% more power (900 mA) is allotted for configured devices. The voltage consumption in USB 3.0 is now 4v, down from 4.4v of USB 2.0.

When you connect USB devices to USB ports, the connected device draws power from the USB ports itself. There are two types of USB devices: low-power devices and high-power devices. A low-power device draws 100mA or less with minimum operating voltage of 4.4v in USB 2.0 and 4v in USB 3.0. High-power devices draw the maximum number of unit loads supported by the standard, more than 100mA from the USB power line. By default all devices are low-power and it’s the device's software which may request high-power as long as the power is available from the providing bus.

Every peripheral attached to a computer requires an amount of power to operate. When you connect peripherals like USB flash drives, USB keyboards and USB mice, you notice lights indicating that power is being drawn on the device. They don’t require extra power to function. When you connect a digital camera, it receives the required power and lets you transfer files. USB ports can also charge your handheld devices like mobile phones, smartphones, etc ...

There are however many uncommon devices which can also fetch power from a USB port, like a USB fan or table lamp.

The older USB flash drives were based on EPROM and EEPROM technologies and had a limited capacity. They were very slow in terms of reading and writing data and required high-voltage circuitry. Thankfully the latest USB flash drives are now much more power efficient and they offer larger storage capacities and higher write and read speeds. Ontop USB flash drives take this one step further. With new TurboJet technology, Ontop USB flash drives deliver reading speed of over 22MB/s and write speeds of over 18MB/s.