Not All Megawatts are Created Equal

Yes, if the intermittent nature of wind and solar is not accounted for in the generating mix.

When the market penetration of these intermittent sources is low, there is little risk, however when solar and wind begin to make up a significant portion the generation mix, then stability issues must be addressed. (Not sure where grid stability issues will come into play but I suspect once solar and wind get to 10% or greater, we will need to be careful. Others in the power industry will have a more accurate number then mine I am sure.)

We will need base load generation that can quickly ramp up and ramp down power to accommodate the varying generation from wind and solar. Other possibilities would include utility scale storage with fast response times and with Smart Grid technologies, the utility may 'throttle' the output of these distributed generation sources to keep the grid stable. Folks hoping to sell 100% of their "excess energy" may not be too happy about that.

This topic is largely ignored by all the renewable energy cheerleaders. People talk about storage, but not that much is being done about it. There are a number of pilot installations but I am sure there will still be many issues to work out on the storage side.

Here are some links alluding to these kinds of issues. (You may need to create a login to see some or all of these. It's free.)

http://tdworld.com/generation-renewables/solar-will-shine-brighter-smarter-inverters

http://tdworld.com/generation-renewables/grid-friendly-utility-scale-pv-plants

http://tdworld.com/generation-renewables/can-smart-solar-keep-sun-shining-pv

http://tdworld.com/generation-renewables/wind-power-making-it-play-nice-no-breeze