APC Smart-UPS 700
The Smart-UPS 700 is an old (but robust) 450W uninterruptible power supply with true sine-wave output.
True sine-wave output is a good thing to have, even with modern computer power supplies, since the sharp transitions present in output waveforms of lesser UPSes with square-wave or stepped square-wave inverters cause increased stress on the input capacitors of the power supplies of the equipment connected to the UPS. If you value the equipment connected to your UPS, get one with a true sine-wave output. Used Smart-UPSes are really cheap on eBay (just replace the batteries) and better than new non-sine-wave-output UPSes.
The most common failure mode of this UPS is that it refuses to power up at all if the batteries inside have died (it doesn't respond to buttons and shows no lights). You can replace them with a pair of 12V sealed lead-acid (SLA) deep-cycle batteries (6.5x10x15cm WxHxD) and it will happily continue working. Since I've had this UPS, I've replaced the batteries three times — first with two Leoch LPL12-7.0 batteries, second with two Camdenboss BEG120075, and third with two Europower EPL7.2-12. Don't forget to calibrate the batteries after you've replaced them!
The unit is fanless, but the internal temperature can get quite high (45°C and higher!), so I've added an external 92mm fan (it fits perfectly over the ventilation holes on the side). This has increased the lifespan of the batteries inside, especially during summer.
- Construction and repair of APC UPSes (PDF, 1.3MB, in Russian) — includes schematics.
- Power quality from various backup devices — an interesting site showing the output waveforms of various UPSes and power generators (see the difference between the Back-UPS and Smart-UPS units!).