My take on this is a small SmartOS cluster running on 3 x Thinkpad T430s - each with the DVD drive removed and two SSDs installed as a ZFS mirror.
The whole cluster idles at 150 watts and each node is backed by it's own dedicated battery. Upstream I have about 4 hours on an APC unit which also protects the router and modem from outages.
My own little cloud for under $1,200.