If I was to buy an uninterruptible power supply with battery backup I would choose one that has sinewave output. Practically all power supplies in computers, printers, etc have power factor controlled input stages, which means that the ac mains voltage after rectification in a diode bridge is processed by a boost converter which delivers a 400V dc voltage to the transformer stage (which processes this high dc voltage into usable 3.3V, 5V, 12V, etc.). The advantage of using this boost converter is that it can be controlled so that the ac current drawn from the mains is a clean sinewave (has a high power factor), which creates less power loss in the ac distribution (the grid).
This power factor preregulator is designed to operate with a normal sinewave voltage, such as delivered by the mains outlet. If you feed it with a quasi-square wave voltage it sometimes does not behave as intended, especially with cheaper UPS designs where the amplitude of the quasi-square wave ac voltage can vary with battery voltage. The result can be that the PFC preregulator can have trouble regulating the intermediate 400V dc voltage, which can cause irregular operation of the entire power supply in the computer/printer. This is especially likely to happen when the UPS battery is fully charged.
A cheaper UPS with quasi square wave output voltage switches between say +160V and -160V dc output voltage, with a dead time in between. During the dead time, the output voltage is zero. More elaborate designs will regulate the peak voltage accurately, while cheaper designs will put out a voltage that follows the battery voltage, and to keep the ac voltage somewhat constant the dead time is then regulated.
All these square wave output designs have a common issue which is that when the dead time ends and the voltage rises, this voltage rise happens very fast and it will cause large current spikes in the input filter of the computer power supply. These current spikes can cause abnormal electromagnetic noise inside the power supply, and can usually be heard. The stress on the computer power supply components will be higher, which can affect reliability and lifetime.
If the un-interruptible power supply delivers a clean sine-wave output voltage, these high current transients do no appear, and the sine-wave voltage is accurately controlled. It is more pure than the ac mains voltage.
Even if most computer power supplies can operate with the cheaper quasi-square wave UPS designs, there are definitely some equipment that is not functioning well and need sine-wave input voltage.
I would be especially careful with UPS designs that delivers 230V quasi-square wave.
Thanks guys, lots of useful suggestions.
Jeff: Epson not Canon, so not hard drive.
Howard: I need at least one monitor, so that I can do a controlled shut down of any other apps I might be running at the time. I can shut down one of the two monitors once I have moved or closed things to one screen. I had forgotten about the router/modem, but I can't imagine that it would use much power.
Anyone good any info on sine wave cleaness issues?