a) it is very rare for our power to be out more than 2 hours
b) mainly to keep it running when the power goes out and to protect it from the lightning surges that come with the concomitant thunderstorms,
The one above has some pretty scathing reports.
What do you think of this one?
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=2956468&CatId=233
I bought a bunch of them on sale somewhere. They're fine for the money, but like Skip said, you need to know the power consumption of the machine you are protecting.
Also, standard battery backups and UPS's (there actually is a difference, though I doubt it matters in this case) are designed to work with computers. Other devices may or may not have signaling ability, or may only be able to interface with a particular make/model of battery backup / UPS. You should check with the clock system vendor to see if a particular model of battery backup is specified.
Whether or not this is actually important depends upon whether the equipment being protected requires a graceful shutdown. An increasing number of microprocessor-controlled devices actually have a shutdown routine that happens when you push the power button, though it may not be obvious to the user. These devices don't like having their plugs pulled because they don't get the chance to execute whatever routines they need to run in order to start up smoothly the next time.
I would suspect that a time clock system falls into this category. A forceful shutdown due to loss of power could possibly cause data loss, not merely the clock showing an incorrect time.
All but the very cheapest battery backup / UPS devices have signaling ability that allows
a computer to gracefully shut itself down in the event of a prolonged power failure that exhausts the backup battery. They are able to report their status to the device, so the device will be able to perform a graceful shutdown on itself when the juice gets critically low. But not all devices recognize all battery backups; so I suggest that you check whether a particular make or model of power protection is specified for your device.
If the device you are protecting has no signaling ability, then my suggestion is that you calculate your device's power consumption, anticipate the longest power failure you can imagine in your area (or at least, the longest you can imagine before the failure of the device you are protecting becomes the least of your problems), and then buy a battery backup / UPS sufficient to power your device for that amount of time.
Rich
