45W? must be a really rotten fridge to need such a large heater. I used 8W and fed it through a energy meter to see how much power it needed, in heart of winter with fridge/freezer in my integral garage unheated ambient temperature around zero and set temperature of 20úC I required an average of 5W to maintain temperature, this is in a large fridge compartment which has been condemned because the insulation has failed. So from my experience I would say 10W is ample. The fish tank heater however has two advantages, it has a small mass so heats up and cools down quickly, and the thermostat is built in so the mark/space time is short. So a 40 or even 60W fish tank heater is OK, but any other heater needs to be around 10W maximum easy simple way is a bulb, then you can use larger bulb (10W) in winter and small bulb (2W) in summer.
If you want to keep price down the
STC-1000 comes in at around ���ã14 but requires wiring and housing, depends on fridge or freezer used, it may fit in the fridge. I looked at the twin sockets, and socket box with extender with STC-1000 in the bottom and likely cost near the same as InkBird by time I added all the bits.
There are other similar looking thermostatic controllers, the STC-100 looks same as a glance but has a single change over relay rather than two relays, the MH1210A also looks nearly the same but has a single relay which can be used for heat or cool but not both together like the STC-1000 but are cheaper, if I was building it into the fridge I may use separate units one for heating and one for cooling. But in the main the STC-1000 is the unit to get if making your own controller.
I was surprised, an 18W underfloor heating tile would cause the fermentor to over shoot, but the 80W fridge/freezer motor caused not even 0.1úC of over shoot. I think three reasons.
1) Fermenting makes heat so over heat could be due to fermenting not size of heater.
2) The tile was heavy so had quite a large mass storing the heat so even when switched off it was still heating.
3) The freezer has a fan to move the air from the evaporator to the freezer compartment, so when running it removes heat from fermentor better, and once it stops it stops moving the air as well so even if the evaporator is cold that lack of heat is not drawing heat from the fermentor.
Since I use a frost free freezer I can't comment about a simple fridge, but I would think having a fan in the fridge would help.