Its not quite as simple as how many watts per outlet of course. The first issue is how many outlets might be fed from the same circuit breaker. Your first step is going to be to identify as many independent circuits as possible. Hopefully its all neatly labeled at the circuit breaker panel for the building. And someone hopefully knows where that is. And hopefully there’s a key.
While at the circuit breaker panel trying to figure out what is what outlet/circuit wise, you’ll also be able to see what size breakers are on each circuit. These are ususally labeled on the end of the throw switch for each breaker, typically 15 or 20 amps. If there’s anything over 20 there I’d suggest staying away from it, since its usually a dedicated circuit for air conditionion or something else large.
The easy way to figure out how many amps each light will use is to divide the number of watts by 100. (To be technically acurate you’d divide by the exact voltage, somewhere between 110 and 120, but 100 is easy to do and build in a safety margin). So a 750 watt lamp will need 7.5 amps.
If you’ve got 20 amp circuit breakers, you can add lights untill you run out of amps. Then move on to the next circuit.
And of course keep in mind things like coffee pots, hair dryers, office eqiupment, etc. that might be in operation while you’re shooting. Those need to be added in to the total as well. In fact I usually run a dedicated line to the makeup room just for hair dryers, and another to craft services for the coffee pot, both of which use WAY more amps than you’d think.
Hope some of that was helpful,
M.L.