This is an excellent question, and the answer might be a little alarming. You see, pokémon are capable of generating not just electricity but full-on artificial lightning bolts that are practically identical to those seen during a thunderstorm (despite the obvious lack of equal distance), which means that at maximum power, under ideal conditions (even without enhancements), a particularly strong electric-type can generate a literally billion-watt strike. Some pikachu are known to exceed even this, especially with the assistance of certain types of Z-crystal, and there is one on record capable of exceeding this naturally. Because of these documented cases, however, researchers don’t entirely know these pikachu are anomalies or actually indicators that pokémon that are traditionally thought to be stronger than that species (fully evolved electric-types and legendaries, to name a few) can wield something even more powerful.
Of course, for human-friendly purposes such as in battle, as generators, and so forth, pokémon are fully capable of holding back. For example, a single voltorb instructed to put out electricity to power a pokémon center might restrain itself to a 80-volt/50 kW state. I still wouldn’t recommend angering one or touching one when it’s working, but it’s enough to power a building if need be.
And to reassure everyone, younger electric-types such as pichu don’t have fully developed electrical sacs and therefore can’t actually wield electricity quite as intense as the above-mentioned pokémon. At most, a pichu might discharge around 1200 volts of electricity into a target. That might sound like a lot, but keep in mind that that’s more or less the voltage you would experience at the wrong end of a Taser.
…or, okay, that’s still a lot.
The point is it’s difficult to say as it depends on the circumstance, the species, and the individual in question, but either way, it’s enough to power a building and to drive researchers to emphasize the usual amount of caution when handling the magical, element-wielding creatures with whom we coexist.