The issue with a super-capacitor is that it leaks. If you put X Joules in there and wait a while then you will only have X/2 Joules. Wait a little longer and yo

The issue with a super-capacitor is that it leaks. If you put X Joules in there and wait a while then you will only have X/2 Joules. Wait a little longer and you'll only have X/4. So on the other hand, super-capacitors are great at charging and discharging rapidly with low loss. So if you want to fire a high powered laser for a few femtoseconds then they work great. You can't get power into and out of a battery that quickly because chemical reactions take time, so super-capacitors have their applications. It's just that holding power for more than a few minutes is probably not it.