heater wattage for fermentation fridge?

[brewbrew]

Just putting together a controlled temp chamber for fermentation - based on a mid sized larder frigde (150cm tall). Does anyone have any advice on the heater wattage that I will need to get to 18 deg C in a UK winter with 25 litres fermenting in it?
Thanks

[sonicated]

Any 1ft greenhouse heater from eBay will be more than enough - I heat a FC with 3 FVs in it with one. You could even use a lesser powered reptile bulb (they don't emit light) but that would be more expensive. I would stick a fan in there as well for good heat distribution - computer fans are great as they have bearings that will last for years.

This is my setup

[wmfd]

I've just checked and I have two 60W tubular heaters in mine, although I think that was to be able to get to 36 degrees for honey warming.

I have a vague recollection that one was enough to heat the fridge to fermentation temperatures!