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Do a little research and familiarize yourself with the differences in size, portable vs. permanent and fuel usage. Most sales people know squat about them other than the profit margins. I’ll try to help. First consider what you need to have power for NOT what you want power for, unless money is not an issue. This list doesn’t include any necessary medical equipment in the household that would, of course, be a priority or a fish aquarium that requires a consistently maintained environment.

#1 is refrigerator/freezer.
#2 well pump (if applicable).
#3 septic pump/lift station (if applicable).

Now you can add the conveniences like: 1 or 2 lighting circuits (preferably, the garage circuit if that’s where the generator is so you’ll have light and interior circuits with paddle fans!), microwave or toaster-oven, convenience outlets for TV and such. Then consider the water heater and A/C after all that.

Your choices of connected load items will create the size requirements of the generator as well as the need for a transfer switch. The cheapest way is to have a portable generator just outside the garage or in a carport and then run #12 power cords into the house to plug in lights or appliances as desired. The better way is to install a transfer switch to your electrical panel. A manual transfer switch gives you the option of choosing the load being operated since you can turn breakers on and off as necessary. An example is to run the water heater until the tank is hot then turn it off before turning on an A/C unit. An automatic transfer switch is just that- automatic. Power goes off, generator self starts and the ATS switches the load (house) from utility power to the generator. When the utility power returns the ATS times itself out and switches back. The generator will shut itself down as well.

Permanently installed generators run on either natural gas or propane. Before investing in one of those generators, check if natural gas is available and get pricing on running a gas line for that installation. If not, you’ll need the price for installing a propane tank. They can be installed above ground or buried so consider that. Most homeowner associations frown on permanently installed generators, so be sure you’re not creating a problem for yourself there.

My experience is that people want (a) easy and (b) cheap. (No ex-wife jokes here) so I’ll focus on portable units.

Where are you going to (1) store your generator (2) set up the generator to use it? Remember, it may be severe weather so, where could you put it to keep it safe and be able to refuel it easily? If you’re using power cords, will they be dry and as far out of the way as possible?

Small generators (less than 5500 watts) can only do so much. If you overload the unit and damage it, you won’t have any power at all. So be smart! Portable units are made up to almost 20k watts but they’re not for people on budgets.
The bottom line is to add up the load (watts) you plan to connect and size your generator at least 25% higher. When motors like the compressor in the refrigerator start up there is a sudden draw of power higher than the normal running load and you want to compensate for that.
Small appliances will have the wattage marked on them. Lighting circuits will be less than 1200 watts each. Water heater is 4500 watts alone.

Also,you’ll truly get what you pay for…unless you buy from one of those guys selling from the back of their pick-up truck (don’t do that). Don’t wait till the last minute and you know that bad weather is approaching or you’ll be stuck with whatever is left in the stores. Oh, and the “trick” with connecting the generator to the dryer outlet to backfeed the house? DON’T YOU DARE! It will result in damage and/or injury. I’ve been in home improvement stores and overheard a salesman suggest this method to a prospective customer. At that point, I had to assert myself and step in with more ‘professional’ advice.