Standard efficiency for piezoelectric materials is 6-12%. Your typical bullet has about 2000 joules, so you'd generate about 2 kilowatt-seconds at 100% efficiency. Enough to fully power an outlet for about 15 seconds. You will want to decide how quickly it was discharged.

There are problems. Piezoelectric materials create juice when you stress them, but then suck that back in when they snap back. This means that, if the person's grounded, you won't be able to use it as a weapon. If they aren't grounded, then the same electrical zap that they hit the other person with will be returned upon him the next time anyone hits him.