Blinded by the Light?

When the load of a power supply is not purely resistive but reactive, or worse, non linear, the current through the load may not have the same shape as the voltage across it, and a phase difference between voltage and current will exist. In this case the apparent power drawn by the load is larger than the real or active power consumed by it. The ratio between the real power and the apparent power is called the power factor. When its value drops too low, problems arise.