Convert the wattages to amps on your lights: watts = amps x volts (household circuits are 120, as your generator probably will be as well)... so amps = watts / volts.

You can then add all of the amperages to see what your need will be and add a bit so you're not running over 85% capacity of the generator for surge safety (blown fuses/circuits will stop your shoot dead).

Find someone who owns one and ask to borrow it/ rent it from them, much cheaper that way. Lots of small generators will be enough for a bunch of lights and a coffee machine/ small fridge. You could even look into purchasing one so you won't ever have to worry about it again... it would be more of an investment, but you'd have it whenever you need it in the future... and you can have the production rent it from you for a discount to keep the budget down and offset your cost.

An alternate idea: have you considered calling the power company and seeing what they'd charge for a temporary drop? I've seen this done on construction sites and it may prove to be far less expensive (and less complicated) than buying/renting a generator.

oooh. thats a great idea, paul.