Most DC appliances use from 5 to 48VDC. Going from one DC voltage to another is difficult. A friend of mine has a pure solar/battery house wired for 12VDC, 24VDC, and 120VAC. It is complex and a little daunting for the average visitor.
A simple low-voltage 48VDC source (like POE 802.3af) would be a nice standard, but for really powering bigger things, you need 380VDC (not much safer than 120VAC). This report shows only 5-7% efficiencies are achieved by going to DC in a data center: http://hightech.lbl.gov/documents/data_centers/DCDemoFinalReport.pdf - Brad
