I bought a pallet of UPS's from a surplus College Auction and needed a way to test their capacities to see if I got any good ones before I scrapped them. To do this, I bought a cheap AA powered wall clock from a thrift store and converted it so it was powered by a 120v wall wart (Wall Transformer). Then, I hooked a 250w load to the UPS, set the wall clock to midnight and plugged it in too, then you pull the plug on the UPS, go away and take a break, and when you get back, the clock will have stopped when the UPS ran out of juice, you can take the time in minutes, divide by 60 multiple by your load (250) and then divide by the voltage of the batter (almost always 12), and then put an 80% effeciency factor in, and you have the approx capacity of the battery in Ah.
i.e. 250 * 1min / 60 / 12 = 0.347Ah (pretty lame for an 8Ah battery.
The following Steps show how to convert the clock.
Step 1: Voltage Divider 5v Transformer
While the clock may have run straight from 5v, I didn't want to fry it, so I used a 5 pack of 150Ohm resistors to make a voltage divider. Voltage dividers are insanely simple, and because of the low current draw of the clock (I mean, come on, a AA battery powers it for years), you could probably use any size resistors from 10Ohm to 100kOhm,
First, you put all the resistors in series. Twist the leads together, solder if you can. Then, you cut the end off of your 5v wall transformer (or whatever kind you have, the only important thing is that it is dc) and attach one lead to one end of the resistor chain, and one end to the other. This is a voltage divider. Use a volt meter and you can measure the resistence at the different junctures between transistors in the chain to get different voltages. Because of the magic of electronics, the voltage will stay the same regardless of load (Assuming the power supply can supply enough current). And you've also built a current limiter now, since the max load possible is Transformer Voltage / (your resistence value)
My clock would only run on 2v, not on 1v, so I attached the negative side of where the AA goes to the negative end of the series of resistors, and attached the + side of where the AA goes to the spot between the 2nd and 3rd resistor.
The ghetto voltage divider serves a second purpose, which is as a drain resistor to drain off any capacitive energy in the clock and power-supply when the UPS turns off, ensuring that the low power clock stops exactly when the UPS is out of juice.
Total Cost - $5 ($2 clock, $1 Power Supply, $2 - 5 pack of 150Ohm resistors from Radio Shack).