Alternating Current (AC) is the term for direct-current power from the mains. This type of current alternates the flow of current from the wall to the connected device. The other type of current is Direct Current (DC). This type of current always offers the same amount of voltage; hence, it is a straight line with no waves. Most computer power supplies presume the computer will be directly plugged into the wall outlet; therefore, they anticipate receiving AC current. It then converts this AC current into the DC current that the computer’s components require.
Using a stepped sinewave, a less expensive UPS unit will attempt to imitate the sinewave that comes directly from the mains. As the current rises and falls, the graph resembles the steps of a staircase. Superior UPS systems can imitate a pure sinewave or sound nearly identical to AC from the mains, or they can imitate the power coming from the mains exactly.
Why is this essential? Most modern power supplies are far more efficient than their predecessors. This is primarily due to the prevalence of Active Power Field Correction (Active PFC). This ensures that the energy supplied to each component is used efficiently. Active PFC-capable devices dislike being attached to a simulated sinewave UPS. This can result in stress and damage to both the UPS and the linked device. It is highly suggested that if you are using an Active PFC power supply, it should be connected to a pure sinewave UPS.