"The average UK domestic household uses around 4.6kWh per year." Shouldn't that be "The average UK domestic household uses around 4.6MWh per year."?
Also liquefcation and gasefication of air looks to be a promising power storage technology (http://highview-power.com/wordpress/?page_id=1405). It would be interesting to compare costs and efficiencies (especially on a domestic scale).