I mentioned a figure of about 10 - 15 kWh/kg for combustion energy from most hydrocarbons and about 40 kWh/kg for Hydrogen. The best reasonably available rechargeable batteries at present only manage around 500Wh/kg - or about 5% or less of what is available thermally in fuel of the same mass. Russell --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .