I have had a generator backup system in place and in use for almost twenty years. Your needs and your opinions may be different than mine, but I want to throw out a few things for you to consider.

1. Capacity. 17.5kW, or even 10kW, is a lot of juice, and it will need a lot of fuel. What do you really need for an emergency? We did the list then gave it some critical scrutiny. The only things we felt we had to keep powered were the boiler (oil-fired, responsible for heat and domestic hot water) and the refrig/freezers. We added a couple of domestic light circuits and a microwave. Result: we can make do with judicious use of a 3.4kW generator. Advantages: much less fuel usage, much less noise and much less expense. We don't rely on any 220v motors, so this works for us - if we needed to power a well pump, it'd have to be bigger.

2. Fuel and storage. Whatever you decide about size, think about fuel storage. IMO propane is the safest to store, but a big above-ground tank is an eyesore (BTDT). Gasoline, in any useful quantity, is a headache from lots of perspectives: fire safety (including local fire code and insurance issues), fuel rotation (even stabilized fuel doesn't last forever), etc.

3. Operation/maintenance. Unless you buy a commercial unit (and maybe even if you do), you need to make sure you are prepared to deal with maintenance and repair issues. Do you have enough of the right oil? Most small gas engines will use more oil than you'd guess in continuous operation, and you also need to be able to change the oil. Do you have spare spark plugs? Do you have spares for frequently-needed parts, and the ability to identify and replace those parts? Particularly if you buy a generator powered by a small gasoline engine, be aware that even the "commercial" quality units are not made for continuous operation in *critical* applications, and to the extent that you go down the chain in terms of quality that becomes even more true. All kinds of things break or need replacement...spark plugs, fuel filters, recoil starters, diodes in the generator, even the breakers in the transfer switch can go south just when you need them most and can't get replacements.

4. Other electrical issues. Make sure you factor in the current drawn by motors when they start,not just when running...but remember that some of them are (and all could be) in your control, so you can time when they hit the generator. Another issue: not all generators provide safe power for solid-state appliances, including microwaves, computers, etc.

Food for thought.