Resistive loads are easy to measure. If you want to light four 100W light bulbs, you will need a generator that is rated at 400W. Reactive loads are more difficult, because these require varying degrees of power. An 800W generator should be enough to operate most modern ventilators, and if you are looking for an emergency backup generator, this is probably the minimum size you should consider. One major disadvantage to this size generator is that it is available only with a recoil starter. You must pull on a cord to start it, like a lawn mower, and this requires upper arm strength.
Recoil starters are standard on less expensive generators, even if they have a higher-rated output.
Common brands are Coleman and Generac. Higher-rated and more expensive generators can be started with the push of a button. A good choice might be one of the units made by Onan or Honda. These are built better, run more quietly, and will last longer over time than an inexpensive unit. Gasoline-fueled units are more common and have a larger choice of watt ratings.