i had a smallish honda generator...which was quiet,
and it had an auxilliary fuel tank made from a boat
fuel tank, so it could run an extra long time before
needing to be re-fuelled. I do not know how to calculate
size needed, but the best thing to do is convert as
much as possible to propane...cookstove, water heater,
fridge and freezer, and also heat with wood, that way
the only electricity you need is for lights, and such.

Is there also a water pump needed, or do they have
water supplied by a utility company or gravity flow?

I would think that if they can isolate part of the
house that would be more efficient...also you need
to dis-connect the main power line if you run a
generator..so the power does not feed back into the
grid.

some systems are wired so that they come on automatically.

some generators are diesel, and these are more heavy
duty and provide more power for larger homes...

i remember when we used alternate power, for instance,
it was not possible to use washing machine and t.v.
at the same time, for instance. or power tools
and computers...or vacuum cleaner.

this is different than generator, this was solar panels
with an inverter, but the same principal in that the
available energy is finite.