On Big vs small Gensets.
The biggest difference in consumption comes at no load, as the loads go up consumption is relatively equal with equal fuels.
It take X amount of a given fuel to produce 3KW, regardless if your geneset can make 7 or 20.
The largest consumption variable is the load you put on it even with a fixed speed genset. (see attached)
The 20KW genset isnt using 20KW of fuel to make 3KW.
When pricing the gensets by KW, a 20K home unit is way less money than a Honda 7K.
It isnt a universal given that " you can get away with a portable unit" Many can, many cannot. I did for decades, then I changed locations and it became impractical to do so.
Lets just look at heating and cooling.
Heat
If one has a gas furnace you can probably heat the house with the a honda 2K because all you need to run is the Tstats and blower.
If your house uses a 3 ton heat pump its going to take about 7KW to start it and 5KW to run it.
Need to run a well pump on top of this? Even the biggest portable is going to struggle.
Cool
IF you have a 10KBTU hour window AC unit you can cool a single bedroom with a Honda 2K, but not reliably start a fridge at the same time.
IF your house has a single 5 ton AC and you cannot place a window unit than its going to be really hard to cool you house.
DO Need to run a well pump on top of this? Even the biggest portable is going to struggle.
Its much easier to go without power in the city where water and sewage are handled somewhere else than when you have to pay for the watts/energy to run them.
On ROI, thats going to be a more complex answer thats different for everyone and it matters tremendously what ones personal formulas look like.
Heres one example of an ROI calculation. It isnt just about the replacement cost of watts -
With so many working at home if you have a 2 day power outage and two people work at home, thats 4 lost days of work.
Whats the cost of that if one party makes 60k a year and the other 40?
Whats the cost of that if each party makes 120K a year?