The Emerson Liebert PSA 500 is aimed at business applications rather than home users.

This goes some way to explaining the pricing, which is more than double that of the Zigor and a touch more than the APC unit, despite the Liebert's lower VA rating. It also explains the different form factor.

The PSA 500 is a desktop design with controls placed at the front and sockets hidden away at the rear. It has the advantages of a line-interactive design, and the back-up time is a respectable seven minutes at full load.

To save space on the rear panel, the outlets are IEC rather than 13A sockets and two IEC-to-IEC leads are provided.

However, if you want to use all four sockets you'll need to buy extra leads, and for equipment that doesn't have an IEC socket, such as a printer with a hard-wired 13A lead or a router that's powered by a 13A power supply block, you'll also need to fork out some cash for a converter cable.

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