Chillers: What HP to Get?

[Enigma]

I've just been surprised with a one week vacation to Kelowna in August. I am mortified! (Though I tried to hide my horror from hubby)

I figured i would battle out my first summer reefing with fans and bottles of frozen water (to float in the tanks). My 18-year-old son (my critter sitter) can't be asked to do that. I'm going to need a chiller, and everything will be moved into one tank well beforehand.

The new tank is a 40B and the sump/fuge is around 15 gallons. So, I need to plan for +/- 55 gallons.

I have no idea what temperatures to plan for. We live in a 1960's bungalow, with crummy insulation and windows, and no a/c.

The EcoPlus 1/10hp would fit in the stand beside the sump. I don't think the 1/4hp would fit very well.

What kind of temperature spike should I plan for?