Solar panels still generate power when it is overcast. Not as much as on sunny days, obviously, but a lot more than most people give them credit for.

On stormy days, also, wind generators will be generating a lot more power than usual. They are more than likely capable of heating your blanket.

Although less efficient, power on days when intermittency is high can also be transferred from pumped water storage and from hundreds of miles away where it might be sunny and windy.

Depends on how stormy, Many wind turbines have a max wind speed rating, above which they will be shutdown in some way.

Are there figures on the actual effects of this? How often it happens/the level by which electricity output is reduced?

I would imagine it is highly dependent on the model of wind turbine. The window of operation like varies widely.

http://en.wikipedia.org/wiki/IEC_61400

I would imagine that high wind rating turbines are more expensive.

>I would imagine it is highly dependent on the model of wind turbine.

This is why I asked for figures on the overall effect - covering all models. I'm not seeing anything so far to convince me that its effect is more than negligible.

Because thorium reactors don't exist?

"On a really stormy day [..] but those solar panels [..]"

Good thing that wind turbine is going to be cranking. :-)

Or disabled due to wind speeds being too high.