The entire state of Victoria in Australia once had a 2 week long natural gas network outage following the Longford gas plant explosion.
Given it's for occasional outage backup, one of the assessments to be made is what exactly does one need to operate during such outage periods?
If you want to run all those high powered / high energy consumption appliances, then yes you will need to spend the money on a large system.
But with some strategic energy management choices, one can have very functional backup for a lot less money.
We made the decision to forego the use of high powered appliances during grid outages, and have a system which can run most things but we do not run large ducted aircon system, nor use our oven if we are running from backup. The second dwelling can run their small split system AC and we can get by with fans if we need, and when we complete a renovation we'll also have a smaller split AC unit we could use if needed at one end of the house. If it happens to be Winter and an unusually cold night, then I can always use the fireplace.
As a result of choosing to moderate energy consumption during outages, the outlay for a backup system is about 20% of what would be required to enable the property to operate as if it were on grid, for what amounts to an average of 35-40 hours of outage per year.