Excerpted from Managing Apple Devices: Deploying and Maintaining iOS 8 and OS X Yosemite Devices, Second Edition by Arek Dreyer and Kevin M. White.

You can estimate the power requirements of new equipment by gathering information from Apple’s websites and then using simple calculations. Apple rates all of its devices by the maximum amount of power (watts) used during operation or charging. Most electric outlets and circuits, on the other hand, are rated by the amount of current that runs through the wiring (amps).

If you know the proper equation, you can calculate the amperage based on given wattage and voltage. However, a better practice is to add up your total wattage needs and then let an expert electrician figure out what kind of amperage to supply. In this case, you will need to estimate the total wattage of your systems. At the time of this writing, the power consumption for Apple devices is as follows:

Note that these are maximum-rated loads; in typical use, a Mac desktop computer rarely draws this level of power. On the other hand, power adapters for Mac notebooks and iOS devices almost always operate at their highest power rating when charging the device, but then drop significantly when the device is fully charged.