The formula for storage (E=.5CV^2) is for total energy storage in the capacitor. Time does not enter into it. The unit is J, or, if you prefer, Ws (Watt x second; NOT watt per second!)
So if your cap holds 15Ws, it could deliver 15W during 1s, or 1W during 15s, or .1W during 150s, or... All this is assuming that there are no losses etc. Plus, as the capacitor discharges, voltage drops. It's got to do with the time constant of the system and the time:
Vt = Vo x (e^(-t/(R x C)))
Where V0=initial voltage
t=time (seconds)
R=resistance of the load
C=capacity of the capacitor (F)
Vt=voltage after t seconds.
(time constant, tau = R x C)
Yesterday I saw similar capacitors to yours: 2600F, 2.7V; apparently they're used as a starter aid for starting the motor of trucks. I was amazed when I heard this, and saw these capacitors. If I hadn't seen them myself a few days ago, I would have thought you had made a typo. 2600F... And I was impressed by those 'tiny' 1F/5.5V supercaps. Wish I had a few of those 2600F ones too.
BTW, as far as energy storage goes, nothing so far beats good old accus for powerdensity. But those capacitors sure can be good fun. BTW, don't charge them by correcting straight to a voltage source; they will be like a complete short, till they get charged. Add a resistor or lightbulb in series to limit current.
I still remember when my high-school physics teacher told our class that 1F capacitors don't exist nor ever would. And now (15 years later) we have 2600F ones. I feel old.
Peter.