Wow. this thread has really gone FAR to the technical side. Let me give you a real world example of how to apply the above in an actually entertainment job setting. Also I have included some shorthand for doing all the complex math.
"Hey, Is a 65kw geni big enough for that street fair?"
"hmm. probably, but I think it will be close"
"Okay, I'll just order a 100kw then."
