Thunderstorms are constantly being produced around the globe, and it's recommendable to know as much as possible about these singular atmospheric phenomenons. They are dangerous of course, and it's not the first time that critical problems arise. Fortunately the survival rate in terms of strikes is more or less 90%.
A ray could achieve an enormous length, near 200 km but usually they don't surpass 2 km. And trying to answer the initial question, it's also obvious that not every ray carries the same power although an average estimation might be a gigawatt. So ten to the power of nine, with a billion volts!
If in a usual thunderstorm dozens of thunders strike ground, the energy starts to exist in great proportions. Several methods try to catch it but that's a very difficult idea because as anyone knows the probabilities of a strike in the same place or near a dedicated tower are low. So with static technology is hard... right now impossible.
So, the potential difference (voltage) from a reference point (ground this time) has been estimated in a billion volts and the electricity uses air to create an improvised wire. So, how much current in amperes are we talking about? Other number that is surprising and worrying at the same time: even near 200.000 amperes.