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Which UPS to buy?

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There is a bit of confusion over what is a UPS (un-interrupible power supply) and what is a SPS (stand-by power supply).
The SPS normally runs the load directly from the mains while charging or trickle-charging the battery. Only on a supply side fault or interruption, the unit in a finite time switches over to the battery until the mains supply is restored.
The UPS maintains only the battery charge from the mains supply. The load takes it power directly from the battery via the inverter always. Therefore there is no changeover or switching of the power supply and any transients or brownouts on the supply side does not effect the load directly as they are separate systems connected only through the battery.
The UPS is not as efficient in energy management as a SPS, but it is much better in maintaining a clean power supply to the load(computer etc).