Local electrical codes can come into play and complicate the planning for your automatic generator. We ran into this.

Although we identified all of the critical items, the way the code requires the installation is for each electrical panel circuit that item was on to be routed to the generator sub-panel in its entirety, and must pass inspection by the County inspectors (both electrical and gas, 2 different guys). So, if, for example, the stair lift was on one 15 amp circut, that whole circuit had to go to the sub-panel. We could not pick and choose what plug outlets where routed to the subpanel. Nor does the County electrical inspector give you any grace if you argue "But sir, there is only one thing on this 15 amp circuit, and it doesn't draw 15 amps." The inspectors assume that when the power is out, every live circuit will be plugged into up to the max. They simply will not approve a sub-panel that draws anything more than the generator is able to output.

This caused us to max out the 8KW before we maxed out all our needs, but only because of the numerous under used circuits that were routed into the sub-panel. We were able to make do by doing just what the inspectors expected---running an extension cord into one of the live circuits via a neaby wall plug that was not normally used, but included in one of the sub-panel circuits.

I asked about the possibility of re-wiring the house circuits to pick up just the critical items. My electrical contractor said it was theoretically possible, but in an already built house, would be way too expensive.

So, although I had been using a 5KW generator with heavy duty extension cords to cover everything we needed, an 8KW automatic standby generator was only adequate with a bit of fudging with a single short extension cord. My best estimate is that if everything is running at once (unlikely), we would sill be drawing just under 5KW.