Battery? You do it on a big scale, using the erratic power to pump water up a mountain, then let it down again to turn turbines when you want it back as electricity.

This works amazingly well... they built one in the UK in the 80s to deal with demand peaks - it uses spare grid capacity at low load times (eg from nukes) to pump the water up and can generate something like 1.3GW within 18 seconds when required. No reason why the same theory couldn't be used with wind, tidal or solar power to help fill in the gaps.

http://home.clara.net/darvill/altenerg/pumped.htm