Good question. It depends on how that would work. Meaning can I offset against the whole year and just get less money back per kWh or will it become seasonal or day pricing. Day pricing would be the most impactful as during summer time prices are cheaper and I deliver back the most.

If I assume that 2/3 of my usage will be in Winter and I can offset just 200kWh (there is always a bit of sun) then it look like this:

1390.5 * 2/3 = 927 kWh 927kWh - 200 = 727 kWh 727 kWh * 0.40 = 290.80 / year (0.40 is price per kWh)

So I would spend 290 per year so about 25e. Which is 75% less then I do now.

This excludes any other cost / benefit.