Typically all usb ports use 5volts which breaks down to 500milliamps per port. for which any ipod will need.
As it turns out, depending on your PC, not all USB ports are created equal. Yes, they all offer 5 volts. But where they often differ, is on amperage which could be equally important to the peripheral people are looking to power — particular power hungry devices like hard drives...look at it this way
[The first issue] is Amps vs Volts. Amps is volume, volts is pressure. The best example is a garden hose. Volts is the pressure of the water coming out of a hose. You can raise the pressure by putting your finger over the end of the hose (increasing the pressure and making the water go further). However, the actual amount of water coming out of the hose doesn’t change. The amount of water being supplied through the hose is amps. Amps or volume doesn’t change as the pressure increases or decreases. Watts is actually the measure of amps and voltage combined, but it is not applicable to the discussion we’re having.
Where as all USB ports support the 5 volt requirement in the USB specification, amperage varies from one system to the next. 500 milliamps (ma) appears to be the standard with many notebooks supporting more. You have to remember that originally, USB was designed for mice, flash memory, and other low power devices. Any device you plug into a USB port on a computer and do not plug into an AC adapter is running at 5v and generally low amperage.
Devices like USB keychain flash drives and mice run at 5v but only require limited amounts of power (or amperage). However, larger devices like 3.5″ USB Drives, external speakers, etc. require significant power and often need to be plugged into the wall. The USB cable on those types of devices just uses the 5v USB as a communications protocol to send data, not to try and power the device.
A USB hard disk drive literally has a hard disk drive (HDD) inside of it. A HDD itself has a motor which spins the platters as well as another motor that moves the heads around the platters. This requires a lot of power since things are moving. A 2.5″ USB HDD (portable HDD) is one of the few devices that has moving parts that can still be powered by the limited amperage of many USB ports.
Every computer has different amperage at their USB ports. For example, even in some of the new Macbooks, we’ve observed a low amperage of 500ma. USB HDDs happen to be the main device that really becomes a problem on low amperage USB ports. Today, most computer manufacturers realize that USB HDDs will be used and are taking measures to make sure there’s enough amperage in their USB ports. [But, there are still some systems out there (new and old) whose USB amperage is too low to power USB HDDs.] Many older laptops for example were designed before USB HDDs were widely used.
All USB HDDs require different amounts of power. The variable here is the actual HDD inside. Generally, larger drives require more power because there are more platters inside. When there are more platters it takes more power for the motor to spin them. Also, more platters = more heads for the platters. Those heads are on their own motors as well.
Finally, faster RPM drives require more power to spin their motors faster (just like running a car engine faster uses more gas). There are also slight variations in the amount of power between similar drives. For instance, a Western Digital 80GB 2.5″ HDD may use more or less power than an almost identical Seagate 80GB 2.5″ HDD.
hope this helps