Uninterruptible Power Supplies (UPSs)

The basic function of an Uninterruptible Power Supply (UPS) is to provide mains-level power to equipment in the event of the mains supply from the Power Supply Company failing (or going outside normal limits).

A UPS essentially consists of an inverter (to convert the low voltage DC power supplied by the batteries into 230/240V AC), rechargeable batteries, a battery charger, and electronics to switch between mains power & battery power.


There are two principal types of UPS: on-line & off-line. Both have a fast switchover device, which switches between the direct mains feed and the battery-inverter feed. The difference lies in the route normally taken by the electric current. An off-line (sometimes called 'standby') UPS feeds the computer load directly from the mains supply, and quickly switches over to battery power when the mains fails.

An on-line UPS feeds the load through the inverter all the time, while the battery is continuously charged from the mains supply. In a power failure, there is no switchover and thus no interruption in the supply feeding the load.

There are many variations on these two basic types of UPS, including some hybrid designs. The term 'line-interactive' is used to describe some hybrids, but not all UPS manufacturers agree on the meaning for this term.


Another key factor to consider (especially for larger UPSs – say 3KVA upwards) is whether the UPS is transformer-based or transformer-less (also known as solid state ).

Transformer-less UPSs (solid state UPSs) are fully electronic with no input transformer on the rectifier side. This makes the units considerably smaller, lighter and consequently much cheaper. Transformer-less units are far more susceptible to voltage variations and transients on their input side and rely on reasonably good quality mains supply. 

Transformer based UPSs are conversely larger, & tend to be more expensive, primarily because of the copper-wound input transformer that acts as a buffer, providing a higher degree of isolation from mains borne transients.

The transformer-based UPS is therefore a more durable design and way more suited to industrial or rural applications where the mains may contain a higher incidence of voltage surges and irregularities.