I would never get electrical water heater, stove, and heater for that reason.
I was thinking the solution for me is to put in a heat pump and then run a backup gas generator. But that solution feels like a significant amount of overbuild. I have 3 minisplits already but they aren't valuable in the winter -- where you install your AC units isn't where you would install your heating units .. I.e. the top of my house for AC, the bottom of the house for heat.
Having multiple energy sources to defray risk is helpful.
If he's building the house new, he should strongly consider investing in designing the building envelope in a way that 20kW heat pumps aren't required. It's very possible, especially with new builds.
The models to follow are Passive House [1] and Pretty Good House [2]. There are many examples of large houses built to those standards in cold climates that use a heat pump that only draws 2kW, which could easily be powered by a generator or backup battery. This assumes that by "big house" we mean something still below ~3500sqft and not some kind of mansion.
The basics are to get the building air leakage below 1ACH50 (3-5x better than most codes, but not hard in a new build), and eliminate thermal bridging by using thick exterior insulation around the building perimeter.