Forget mains power that is to run motors lamps etc.
The scanner / printer will draw some power from the USB port. The ports can only provide 500mA some of whih is lost when powering through hubs.
Some devices are more power hungry than others ie.e printers scanner modems and nedd onnecting direct to the motherboard port or PCI card port. Oters like mice an run from an unpowered HUB.