Network operators in Denmark and northern Germany (where they have reached this level for real) and elsewhere (in studies) have stated that 20% of electricity (in kWh terms, not in capacity terms) can be generated by wind with the existing networks being able to cope with minimal investment. More than that would start to require some investment, which has already been assessed.

20% of kWh means a lot more in terms of capacity (MW) as windfarms typically produce one third or less of the kWh of the baseload plants per unit of capacity due to their intermittent nature (i.e. a 200 MW wind farm produces one third of the actual electricity of a 200 MW gas powered plant).

So the cost of intermittence on the system is NOT a valid argument today and will not be for a long time to limit the construction of windfarms.

In the long run, we're all dead. John Maynard Keynes