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MIT scientists propose power storage using cement blocks
(www.datacenterdynamics.com)
This is a most excellent place for technology news and articles.
I've only done solar + battery setups on RVs and found 8kWh to be the probably the minimum if you have to run the AC at 100% duty cycle. Realistically at night that probably won't happen but I haven't had a chance to test the current one where I'll actually he doing that with a kill-a-watt to get a real world number.