my buddy needs a bass amp but he dont know how many watts he needs to do a gig could you tell me
I read a good line the other day on here actualy about amps. "Amp wattage is like a Ferrari versus a Ford. Both cars will do 60 MPH, but the Ferrari will do it better and easier"
Basicaly, I'd tell him to get the most amp he can afford and just keep it low if it's too much better to have too much than not enough, to a certain point. Meaning don't tell him to like get a 1,000 watt amp.
"This is The End, beautiful friend, The End"
Bass amps are way different than guitar amps. Guitar stuff is aimed directly at the mid frequencies, which are easy to produce. Not much power is needed for it. But bass stuff is obviously aimed at low end/sub frequencies which takes a lot of power and super tough speakers. Most 'affordable' ones are SS. So the more watts the better. Also the more speakers the better.

A general rule of thumb for bass speakers:
10" speakers for punch
15" for rumble

For gigging...he should be looking at at LEAST 2x10"s at around 300watts. More or less..