2 Answers

Supercapacitor cells and modules with an

electrical capacitance reaching thousands of Farads become
attractive components for energy storage, in regenerative
energy based power electrical systems. The characteristics of
high energy supercapacitor cells and modules produced at this
time by leading manufacturers in the field are presented for
organic and aqueous electrolyte technology. A stored specific
energy as high as 5-6 Wh/kg is exhibited by commercial
supercapacitor cells and modules, a value lower than the
corresponding one for lead –acid batteries (30 -35 Wh/kg). The
specific power of supercapacitors reaching 3-5 kW/kg is higher
than that of batteries (limited to 1kW/kg) and this feature is
required in applications. The advantages and disadvantages of
supercapacitors in comparison with batteries, a traditional way
of electrical energy storage are discussed.

Read more >http://www.icrepq.com/icrepq07/329-obrejal.pdf

Some info here

http://en.wikipedia.org/wiki/Electric_double-layer_capacitor