In AC currents, both the current and voltage vary periodically in time. The values $I$ or $V$ correspond to the **root mean square** (RMS).

RMS is a square root of the mean of squares of numbers (see the root mean square calculator for more). The commonly referred voltage of electrical outlets (**230 V** in the EU and Australia, **110 V** in the USA and Canada, **100 V** in Japan) is the **RMS voltage**.

In AC circuits, the current and voltage **might not be in phase**. The maximum value of the current might be ahead of or lag behind the maximum value of the voltage. This makes the transfer of power less effective. In the worst case, when the current and voltage are entirely out of sync, the transmitted power is zero.

The power factor tells us **how synchronized the current is with the voltage**. If they're in sync, the power factor is **1**. Otherwise, it is less than one, reaching zero in the completely out-of-sync case.

The power factor depends on the device. For a device that is **purely resistive**, like an electric kettle or an electric heater, the power factor is **1**. A device with **inductive or capacitive elements** puts the current and voltage out of the phase. This makes its power factor less than 1. Check out the power factor calculator to learn more.