Power supply is a device that provides the necessary electrical energy to power electronic devices. The rating of a power supply is important for choosing the right power supply for a device. Power supplies are rated by voltage, amperes, wattage, and joules.

Voltage
Voltage is the measure of electrical potential difference between two points in a circuit. It is measured in volts (V). The voltage rating of a power supply determines the amount of electrical energy that can be delivered to the device. The voltage rating of a power supply must match the voltage requirement of the device it is powering.

Ampere
Amperes (A) is the measure of electrical current flowing in a circuit. The amperage rating of a power supply determines the amount of electrical current that can be delivered to the device. The amperage rating of a power supply must match the amperage requirement of the device it is powering.

Wattage
Wattage (W) is the measure of electrical power in a circuit. It is calculated by multiplying the voltage by the amperage. The wattage rating of a power supply determines the maximum amount of power that can be delivered to the device. The wattage rating of a power supply must be equal to or greater than the wattage requirement of the device it is powering.

Joules
Joules (J) is the measure of energy in a circuit. It is calculated by multiplying the wattage by the time. The joule rating of a power supply determines the amount of energy that can be delivered to the device over a specific period of time.

In conclusion, power supplies are rated by voltage, amperes, wattage, and joules. It is important to choose the right power supply for a device by matching the voltage, amperage, and wattage requirements.