I did the math so long ago I've forgotten it but after all the ciphering I ended up with a 5kw unit to run our operation the couple times a year we usually need to.
I avoid the max draw scenario of fridge/freezer/water heater/well pump/heater fan all firing up at the same time but that's easy to manage.
Well it depends as regards easy to manage. Depends on what you have. When looking at a backup generator you have two ways to go about it: install it with a transfer switch to completely take the place of utility power or just use it to supplement. The transfer switch comes in two models: automatic and manual. Manual one you have to do; automatic ones are a lot more expensive since they are set up to start the generator up as soon as the power fails; there is a pause that can be programmed if you live in an area that has fairly frequent short power outages that will not keep turning the generator on and off when you do not need it.
To just supplement basically means you are using extension cords. Now you can do that with a microwave, or a refrigerator and some lights but not really with a furnace or installed equipment like water pumps.
As regards the size of your backup system that obviously varies immensely. The last two houses my mother had were equipped with back up generators with a manual switch in the first and an automatic one in the second. The first one was sort of jerry built; we did the wiring ourselves and then got an electrician to approve it. The second one was professionally done. I have lived in an for several years and also for a few years in a house with a backup generator so I have a fair amount of experience in all of this.
The vast majority of backup generators professionally installed use natural gas; hooked up to the system. They are fairly expensive to buy and install. Upwards of $10,000 and above. Most of them are set up so that once a month they start up and run for a few minutes; and if something is wrong they have either an alarm or a flashing light. Usually one inside the house as well.
You can do your own a lot cheaper of course if you are willing to do more of it yourself. The biggest thing is wiring it up; you need a electrician to either install it or approve it because if you don't and there is a fire the insurance company will not pay out. You can get a good quality portable generator of 7500 running watts for around $1000. Dual fuel is the way to go. If not that then frankly go with propane since as I earlier posted it does not deteriorate as gasoline does. In one year gas will start to lose its octane power and in two years its not going to be worth a whole lot. Staballoy will give you an extra year but that is all. A 7500 watt generator will use when under a big load around half a gallon an hour of gas and a little more of propane.
Oh and a side note: those propane bottles you see used in barbecues are typically what are called 20 pound containers. Why that is who knows? they have when full around 4.5 gallons of propane. The next size is 30 lbs and have around 7 gallons; then comes 40 lb bottles that have about 9 gallons in them. then comes the big 100 lb bottles that have just under 25 gallons in them. Needless to say the bigger they are the heavier they are and the more they cost. a 20 lb bottle you can exchange for $20 or so; to buy them is a far amount more expensive. 30 lb bottles are usually around $70 or so; and 40 lb around $100. 100 lb are $150 and above.
Propane can be gotten - tanks refueled- at a fair number of places. Expect to pay around 50 cents a gallon more than regular gasoline.
The only thing to think about as regards your professionally installed system running on natural gas is what to do if that runs out. Does not happen often but it has; if the gas company cannot keep the system going and pressure drops too low. the really cautious have their backup system set up to run on either Propane or Natural gas and you can switch back and forth.
Most of this comes down to how concerned you are and how much you want to spend.
As the saying goes about any fool can solve most problems if you throw enough money at it. The smart ones figure out how to do it with less.
