I did some quick calcs to see what the heat loss for a 3' by 3' by 3' insulated chamber would be so that I could understand how much heating power we really need in typical brewing.
20C interior temp desired (ales)
4C exterior temp (that's cold!)
5 sq meter chamber surface area for 3 by 3 by 3 chamber.
Polyisocyanate foam R = 1.0 sqm K/watt in
Expanded polystyrene (EPS) foam = 0.7 sqm K/watt in
Temp difference is 16 C or K
Watts = (Area x Temp Diff)/(R x foam thickness)
1 inch polyiso loses 80 watts
1 inch EPS loses 114 watts
2 inch polyiso loses 40 watts
2 inch EPS loses 57 watts
As you can see, the heating power does not have to be that high. Since the example above is pretty extreme, I'd expect that most systems would need even less heating power.