A big generator will run 24/7 non-stop pretty much indefinitely.... you pay more for that industrial build.
A standby generator is for shorter term usage.
Big generators will suck down fuel like crazy.... if you don't have a 250gallon propane tank, I stick with something considerably smaller, and adapt the lifestyle to it for those "temporary" short term outages. 10k, 12k... I wouldn't go bigger unless you can get fuel delivered during icy Texas blackout roads, or after panhandle events like Hurricane Katrina/Michael....
The 120 gallon(
I've run out of propane during blizzard and hurricanes since storm outages have lasted from 1 week to 1 month without the grid, multiple times.
No, I don't recommend a 20kw-24kw generator.
Yes, I do recommend the biggest fuel tank you can fit, hide, bury, store....
No, you won't be able to get an expedited fuel refill after a storm or incident. Yes, watched all the neighbors foolishly leave all lights/luxuries on and run out of fuel in a day or two. Then, clueless why no propane delivery driver existed.
You should consider the smallest fuel sipping generator and adapt to it.
My thermostat is 70F during the winter and 75F during the summer. Love and can afford the grid convenience! My 3.5 ton HVACs, when on generator, has it easy.... winter thermostat down to 55F, that's when the pellet stove or propane fireplace kicks in, to reduce electrical loads... and no AC during the warm days(open windows). I am spec'd to my comfort and convenience level, and below what I can afford(still cheap with lesser used toys).
Convenience and comfort cost plenty of money. If you have the cash and income, go for the industrial 24kw generator. If you need a loan or visamastercard, then use the smallest as possible that'll run the minimal needed to survive.
Since I don't need the excessive comfort or convenience, after buying 1 standby household generator, I will never again. I am happy and capable with my < 10kw gasoline generators in the shed, with 1/2 dozen 5 gallon fuel tanks at both houses. Since they are multifuel, the gas grilles at both houses have 2+ bottles... plenty to cook/grille with for a couple weeks and I can move propane to generator if needed. Nothing worse than driving >75 miles to find a gas station with power that can pump fuel. And, every full propane tank at every store disappeared in a day 1!
I've also been in situations when natural gas/CNG has been turned off, typically when you need it the most(heat/power). So, I don't recommend CNG anything. Its like the grid. It goes down and your out of luck until it comes back on.
For regular short term outages, happens here almost weekly, usually
Battery backup could be as simple as 1/2 dozen APC/TrippLites on your TV/computer/modem/router and a couple battery backup nightlights.
After the 'big one', even my lawn solar lights were borrowed nightly by the neighbors and returned every morning. Its all they had for lighting after dark, for weeks!
That propane/butane camping stove, that garage kerosene/propane heater, are some things to add to your garage/shed, with a few extra bottles of fuel.
BTW, my EU2000i Honda inverter generator is a tank. Had the refrigerators on it for a week. Used a splitter and it ran the toaster, oven, and plenty of lights. It only tripped when the 'fridges kicked in while the microwave oven or toaster were on.... So, pulled the extension cord on the refrigerator for cooking. Doesn't get used much anymore but still there, with fresh stabilized fuel, synth oil, good spark plug/airfilter, no flame arrestor.... waiting to be called into duty. It has more hours on it than I can remember(over a decade old) and never missed a beat.
Comfort isn't wasteful and nor is convenience. People earn their comfort and convenience. I tend to be a the minimal survival level without whining and crying that someone should do something, since I did and for myself! Generator, battery backup, solar, combination,... all cost money and truly takes excessive money to make one tolerably independent for that week or two without utilities.
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Attached is a picture of the local police department after a little storm. Notice the generator that didn't start when needed left side of photo. Notice the solar array on the roof that wasn't working.