Hi,
To prevent fridge freeze up, I'm using a 40 watt "long life" bulb controlled by a Thermocube TC-3 (on at 1.6 C, off at 7.2 c, or 35 f to 45 f) in the exterior compartment.
The ambient temperature is -10 C (14 f) and the fridge is currently *off* (for worst case testing). The temperature at the rear of the fridge is -0.5 C (31 f). I have not blocked off the upper two vents, again for "worst case" testing.
Clearly for my weather temperature range (down to -40), I'll need to bump the wattage up some what on the bulb. Since it is thermostatically controlled, perhaps a 100 watt "rural service" bulb would be best (130 volts).