amplifier power consumption

Look on the back of your amp.

The largest your budget allows. Even though the watts may be sufficient to run your amp, dynamics can still suffer.

have you seen this :

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&productId=100625870&langId=-1&catalogId=10053&ci_src=14110944&ci_sku=100625870&cm_mmc=shopping-_-googlebase-_-D27X-_-100625870&locStoreNum=159&marketID=1

I would be hesitant to use even the 540W ups, without knowing the exact power requirements of your particular amp.

Here is a Cambridge integrated amp rated 75W/channel and spec'd for a maximum power consumption of 600W.

Here is a Creek integrated amp rated at 80W/channel and spec'd for 340W consumption at full power.

So the numbers can vary widely.

Also, I would want to derate the ups capability by a factor of 2 or more (meaning its rating should be twice the expected worst case requirement) to allow margin and to minimize the possibility of affecting dynamics.

Keep in mind, too, that some ups's, especially the inexpensive ones, put out quasi-square waves instead of sine waves. You would want to be sure to get one that puts out true sine waves.

Re the wattmeter that Riley linked to, I would question whether or not it will capture and display very brief peaks in power demand, which is what is important for your purposes.

Regards,
-- Al

To repeat Rog's response it should give the info on the back of the unit. Usually near the AC power inlet...

As Al said make sure the UPS puts out a true sine wave. SCRs will give you at best a 12 step wave (12 pulse). Cheapo Battery Back Up UPS a square wave, horrible for audio equipment.

The only true UPS units on the market today that I know of uses IGBT technology.
*Undistorted waveform (low harmonics)
*Fast switching speeds (100% step loads)
*No ripple DC (long battery life)

A True On Line UPS unit takes the AC incoming power and converts it to DC then converts it back to AC.

A True On Line UPS unit using IGBT technology cost quite a bit more $$ to buy than an SCR based unit.