Would that be 1987 Pirelli?? I've heard that sometimes FAI Tan is
comperable. I seem to recall a man-carrying rubber powered plane
making an attemp in the late 1980's or early '1990's.

Good quality rubber strip stores around 3000 *ft/lb per pound of rubber..
Sad to say, I've completely forgotten how to convert that to something we
could compare with a battery, say to kW/kg.