What does it mean that a UPS is sine-wave or square-wave?
A. The sine-wave or square-wave refers to how the power that the UPS provides looks when on battery. Normal AC power from a wall outlet is sine wave. However, manufacturing a UPS which provides sine-wave power is more expensive, so lower end UPS units will provide square-wave or modified square-wave power. Since computers convert AC power into DC, the type of wave usually will not cause problems for computers, monitors or other non-motorized electronic devices, although they will cause electronics plugged into them to emit a buzzing noise when on battery. Square-wave power will cause damage to motorized loads such as pumps, motors, etc. and should not ever be used with them. Sine-wave units produce cleaner power which is healthier for computers and can also provide power for motorized loads. Smart-UPS units over 700 VA all have built in automatic voltage regulation as well which cleans up AC power even when the unit is not on battery.
