Let's assume the calculation is correct and you actually earn back in 11-13 years. You are better off investing the money in something like the S&P 500 where you'd double your investment in about a decade without the hassle.

Things will start to get interesting where €1600 don't buy you 5kWh but double or triple the amount.

I can imagine most people are better off either investing like I said or in insulation.