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.
For reference, a taser does 50,000 volts, at 3.6mA, for 180 watts. The force of this bullet could power that discharge for about eleven seconds.
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.
In response to comments:
The laws of thermodynamics insist that the energy of deformation can be no greater than the energy introduced to the system by the bullet. This models the bullet as a blunt pushing object, since you can't tell us other properties of the material. Most energy would be lost to heat, unless magic.
Yes, you'd have a net zero electrical balance. They aren't used for electrical generation. Piezoelectric lighters deliver a high-voltage spark by throwing the spark before deformation reverts. I'm not sure where it draws the charge from afterwards. It's probably taking advantage of greater surface area on a non-conductor to allow the charge to revert relatively slowly.
The jolt will actually equalize the charge between the two people. If the other person wears armor, then there will be little or no damage at these levels of current. Incrementally half-strength discharges would occur until the charge is dissipated across enough surface area.
You WANT them to be grounded, so that they deliver the jolt to an opponent instead of to the ground. Yes, shoes would be enough, but high voltage leaks out into the atmosphere pretty quickly. Play with a Van de Graaff generator some time to get a better feel for it.