Help with Walk Ins

[DonaRose]

Hi,
I've got to be honest, I'm not a refrigeration engineer, I'm a subzero chemist with a sudden interest in the area. I also own a Subway restaurant and we are building a walk in cool room/ walk in freezer as soon as we can hire someone.

Trouble is the quotes so far are very vague and conflicting on specs so I'm looking for an answer from an engineer rather than a salesman. Specs are 4.3m x 1.5m x 2.4m for each, temps are your usual -20 and 0. Or thereabouts. Floor is concrete in the fridge, something else in the freezer (this bit also conflicts), walls are to be 75mm/150mm polystyrene.

The big conflict is the wattage and the power. Is 1.5KW for the freezer going to be enough (I mean, really?), what's ideal for both of them? And, do we need three phase power?

I know isn't really what the forum's here, but I'm getting completely desperate for someone who actually knows the technical side of this and not just the sales pitch. Any help would be greatly appreciated.

Cheers.

[DonaRose]

Can't edit to add, but know it's important. We're in Melbourne, so outside temps do get up to 44 degrees and the structure will be built outside, in some sunlight.
The door will be opened a few times a day but for no more than a half hour per 24 hours.
All stock entering will already be at the right temp.