Some UPSs use a regulating supply to tame incoming spikes and surges, complementing low voltages from the UPS battery. All UPSs have a fast-acting shunt that switches the output supply to the battery in case of main power failure. Some UPS units lack the battery aspect and instead regulate the power directly. Usually these units vary widely in price and features. The capacity of the battery and the amount of filtering the internal circuitry can perform also varies among the available models.
Most manufacturers offer units based on voltage and wattage capacities.
Choosing a unit with too much capacity can result in overspending, but an underpowered unit can cause problems by not providing all units plugged into it with enough power. Workstations and servers can require much more, typically in the 600V, 400W range. When choosing a UPS, total the power supply draws from all the equipment that will be plugged in to the system. The number of output jacks on the rear of a UPS tends to increase as capacity increases. A 200V unit may have four sockets, and a 600V UPS offers six, for example.