Homebrewing - Beer, Mead, Wine, Cider

I also did 20L batches when I was brewing. My setup was in my basement which while not actively temperature controlled did tend to maintain a fairly consistent temperature throughout the day and night. That baseline would trend close to 13C in the winter and may as warm as 21C in the summer. You may not need a lot of thermal stabilization depending on how responsive the temperature in your brew house is to the outside air.

The other thing to consider is that the temperature control is most critical during primary fermentation when the yeast are most active. As long as the temperature is maintained to their tolerance, and the whole vessel is neither rapidly heated or cooled then whatever off flavors they produce in primary fermentation they will tend to clean io during secondary, where you can even be more sloppy with your temperature. Even beers brewed with ale yeast benefit from a long secondary rest, and can produce some very clean profiles without strict temperature control.

In the end, it all depends on the kind of beer you want to produce. If you're looking for consistency batch to batch, then you'll definitely was t to control for as many variables as you can including temperature, water chemistry, strike and crash temps, your grains age, grind and whatnot. On the other hand if you're OK with some variability between batches of the same recipe and you take good notes on the process you can start to account for what factors contributed to different elements in the profile. (E.g., low ABV because it was too cold, or popcorn & bubblegum notes from too warm or too short a secondary)

For my own batches I tended not to control for temperatures during primary very much, other than to make sure I kept it within the acceptable range for the yeast I was using. I also tended to get busy and either not get around to bottling/kegging very quickly (or just forget) and so my beers tended to have that longer secondary fermentation I mentioned. (To be clear, it all just sat in the fermenter on the yeast cake for a week or three before I'd rack it off and either bottle or keg it. Didn't move it to a second fermentation vessel or anything.)

Time heals many of the woes of the home brewer. Another week rarely hurts. Just don't let it autolyze.

Anyway, I was always very happy with what I made, and I was tended to stick close to the reference styles. I usually received some great feedback from friends and family on my stuff. The guys at the beer club were less kind because, well, I'm in the US and if your beer doesn't have an oil slick and taste like dish soap because of the pounds of hops they like to use in every batch then they kinda dump on you for it.

Belgian yeast likes to get a bit hot, by the way (and the beer tastes best when fermented on the warmer side of the yeast's preferred range). That might be a decent choice for a test batch once you get an insulated space setup.

I can respect working with no power. I did that for a while too. Everything I mentioned is pretty power efficient though.

i use aquarium heaters but never cared about the energy consumption. my experience is that since fermentation is exothermic they rarely kick in. especially with larger fermenters it's usually more important to prevent it from getting too hot. so maybe running larger batches might be something for you.