The need for a Pi UPS
If the Pi looses power whilst writing to it's SDHC card (or a USB Memort Stick), there is a very good chance that the card (or stick) will be corrupted. So, to ensure your Pi CCTV (or Pi based NAS / Music 'ripper' etc) keeps working during (and after) a power cut, you need an Uninterruptible Power Supply (UPS).
After seeing the 'joke' that was available from CPC for 'only' £26 and similar overpriced garbage (all non-rechargable kits with 'batteries not included' - and no USB power-block either) from other vendors, I decided to build my own
NiMH rechargable UPS
It's quite possible to build a NiMH rechargable UPS, including both a recharging cirsuit and voltage regulator, for less than half the cost of the 'joke' UPS from CPC or 'hats' offered by Pi distributors. To see how I did it, click on the title (above left)
My NiMH design has the disadvantage of requiring a +12v (or higher) power supply. For an alternative UPS that continues to use USB 5v power, see below
The PowerBank UPS
Since I completed my Pi UPS project with NiMH cells, LiPO 'Power Bank' units have become unbelievably cheap. The 'low end' (single cell, 1700mAHr) units can be had for as little as £1 (from that well known UK High Street store) and, with the addition of a few diodes, can be pressed into use as a Pi UPS.
The Super Capacitor UPS
Another power storage device that has also become significantly cheaper is the Super Capacitor. Whilst not quite as cheap as a LiPO PowerBank, they do have the advantage of an indefinite life (where-as any battery system will gradually loose capacity to the point of uselessness after 2-5 years).
UK power-cuts rearly last more than a few tens of miniutes. The low power requirements of the Pi Zero makes a 1 Hr. Super Capacitor UPS 'affordable'