I dunno, I'm not spending 600 quid on a big bulky box (not to mention installation), and I don't reckon many others will too. The beauty of the electricty grid is the economy of scale. Surely the best way of storing energy is through hydroelectric stations with two levels (there is one in Wales), or a tidal power station that pumps in extra water with surplus power giving more of a boost when the tide switches (like one in France). Another option might be to use surplus power to produce hydrogen. In any case, this will take a lot of investment and won't happen for years.