Not sure why you're looking for a 1/2 HP unit (seems rather stronger for a 90g tank), unless your home is incredibley hot. I have a 1/4hp Pacific Coast on my 120 and it's more than sufficient to keep the tank comfortable.
I have to agree that 1/2hp is alot of chiller for a 90g. A chiller is one of those things where bigger is not better.
JBJ Chillers have a size calculator on their web site, and the people at Tradewind will take all of your info and then tell you approximately how much wattage each size chiller will use to keep your tank cool. While often it pays to step up the chiller by one size, even this is not always necessary or desirable.
Once you size your chiller, Tony from Reef Exotics has very good prices on chillers and he is local (Nassau County) so you could pick it up and get it even faster (and he is a sponsor here).