It's almost like the other battery powered unit that I bought at the sporting goods for fishing, except it has the 'on' feature, when there is no main power.

It puts out as much air as the other little D powered units. In regards to 'how big a tank'.... food chain is right...lots of variables. It's a matter of...any air is better then 'no air.' It buys you a little time, till the power comes on again.
For less the $20 US dollars, it's not bad for a 'little' back up.

It has a 2 postion switch...one is 'off', the other is 'stand by'. It only 'runs' in stand by if there is no main power.
Shas, It does make some noise.

Penn-Plax Silent Air B11
http://www.petsupermarket.com/Products/ ... 29045.aspx