If this should work on the regional level for countries, we will need group constraints (as was used in the original implementation). @brynpickering , group constraints still exist in 0.6.7, right? Would you advice against using them at this point in Calliope's development cycle?

An alternative and simpler implementation could allow net self-sufficiency only on the unit level (i.e. countries on the national level and regions on the regional level). It would not require group constraints. As mentioned by @brynpickering below, this is incorrect. We will need group constraints either way.

We'd need group constraint either way though to limit net import, right?

In terms of development cycle, group constraints will be deprecated from v0.7; they're just too fragile and difficult to test as-is. However, they still exist in v0.6.7 and are (I hope) more robust. I would be fine with sticking with dependence on them for Euro-Calliope <v2.0 as it would fit with the general timeline for v0.7 (earliest end of year).

I took the implementation from https://github.com/timtroendle/geographic-scale and adapted it to the current version. Still I cannot get this to work and I am not quite sure why. I pushed my work in progress into a draft PR and I propose to push this to 1.2 because it requires thorough testing. Any objections, @brynpickering ?

v1.2 works for me. I'll take a look next week at your PR to try and troubleshoot.