By saying that it cost around 5€ I meant that the material to build this cost me around 5€. I would happily set them up for free if one gave me the material for free, but the microcontroller and so on do cost some money.
The API and (of course) the source code are free (as in freedom, being AGPL licensed). And since I did my best to document it well and provide easy to follow instructions in the README, I do think I managed to make it free as in freedom.
In my understanding a problem often is that electricity isn't used when renewable electricity sources can provide it. One solution to this is storing it, another (complementary) one is to adjust the usage to times when there is a lot of renewable energy available. This especially makes sense for big consumers (like factories), but a lot of washing machines also sum up to a lot of electricity demand. So we have to adjust to mainly use electricity when it's available via renewable sources, and this thing can hopefully help with that.