How do you know how much you need as far as watts?
A few ways. You could measure with your trusty amp clamp and a dmm for volts/amps. volts x amps=watts. You likely don't need to measure volts since you already know roughly what is coming out of the wall. (it does make your calculations more accurate since actual voltage may vary quite a bit from place to place, and influences current draw)
Some devices have actual ratings on them, especially smaller transformer powered devices.
Others are naturally rated in watts. Lightbulbs.
Motors require additional headroom for start up inrush.
It would be easy to say "just go overkill" but in the real world that is wrong answer for multiple reasons. Noise, fuel consumption, cost, portability, ultimate engine life are all negatively affected when a generator isn't loaded down sufficiently.
I used to do minor generator repair. Constant calls about poor running generators in middle of night. 60,000watt diesel towed generator powering 2 drop lights and a phone charger tends to "wet stack". Gasoline isn't prone to that condition, but it is still bad to run them under such light load. Think of it like stop and go driving for short trips, not enough heat.
Deep cycle batteries and an inverter may be a quieter option, depending on what you want to run. A small engine/alternator can provide occasional recharge if absolutely necessary. Batteries are heavier than a generator, but may not need to be moved around much.
