> But these kits only cost a few hundred euros. So why not? If it saves me 10 euro per month or so, it would earn itself back in 3-4 years under ideal circumstances

I'm not saying this is an unreasonable position, but the net present value of an investment that pays 10 euros a month for 4 years is also a couple of hundred euros. So in the end it might be a wash (or might not beat out investing in higher yield projects)

https://www.calculatorsoup.com/calculators/financial/present...