I also have a tonne of stuff running on a single, fanless server via docker compose. It is amazing, barely breaks 20% CPU, better uptime than aws ;-) ... One overlooked aspect is that if you use something like my Streacom db4 case, beautiful, fanless, noiseless, you can put it in a living area. Depending on where you live, the power usage can be fully offset against heating costs for your home, making it very efficie…
Heat pumps can achieve greater than 100% efficiency relative to electric resistive heating, because they move heat, rather than do work to create heat. These can achieve equivalent heating at 50% of the electricity utilization compared to resistive heating.[0]
Local fossil fuel heating (e.g. natural gas furnace) is much more efficient than electric resistive heating. The efficiency of local fossil fuel combustion is close to 100% of the energy content of the fuel, as it is turned directly into heat and that heat is dissipated into the home. More than 60% of the energy content used to generate electricity is lost in the generation process in the US.[1]
So, the heat from a computer does offset heating costs, but at less than 100% efficiency. For a small fanless machine, this is not likely to be a large expenditure, but it is valuable to keep in mind that heat from electronics is not "free".
[0]: https://www.energy.gov/energysaver/heat-pump-systems [1]: https://www.eia.gov/todayinenergy/detail.php?id=44436