You can probably estimate your maximum power consumption by adding up the rating for each device on your system, and then adding some overhead (maybe 25%?) for expansion or misestimation gives you a baseline for the minimum you'd want, but when you've got a generator, maybe you want to keep other things going (fridge? freezer? wifi?), so figure out what the goal is and look for that.
Beyond just the generating capacity, you may prefer a specific fuel for ease of storage or operation (propane vs. gas vs. multifuel), and you may want to consult an electrician about connecting it to house circuits or similar should you want a more permanent solution. There are also natural gas whole house generators which will automatically switch themselves on, but they tend to be a fair bit more pricey and require an installation. More recently, there's also some reasonable battery back up options, potentially with solar charging too.
As a baseline, no-frills power generator, though, you need a unit that can cover the power requirements you have, a set of cables with enough current rating to connect it to your system's power, an idea of which cables need to actually be exchanged in advance (keeps you from guessing in the dark), a way to get and store fuel, and a schedule where you test the generator - most standard units need to be run once every few months to keep things lubricated and operating properly. You'll also want a good outdoor location (covered is often fine, but ventilation is necessary) to keep the generator while it's operating.
