I just came off of a 7-day power outage and keep our house at 73 degrees with a 15 KW generator and a 4-ton geothermal unit. I went through $295.00 in LP gas, but you can't put a price on living a half way normal life, with a power outage. I thought it would get its first test last winter but it was very mild.
I'm in Southern MD and in storm afterthought mode am purchasing a 10 kW generator. My calcs show me able to run some basic refrigerator and outlets as well as my well pump and my 2 ton geo and heat pump water heater in e-heat mode.
I had planned on 10KW too but ended up going with a 15KW. Price them both because you will never have too much power in an outage. The generator made a believer out of me the way it ran for all of those days. I was concerned about the noise being a problem, but did not notice after a few hours.
I agree that extra capacity is better than insufficient capacity. A larger generator (as compared to a smaller generator) may require greater fuel flow, larger gauge wire, and possibly more labor from an electrician depending on how the larger generator is to be wired to the electrical service in the house. My point - check on the total installed cost, not just the generator cost. That said, we opted for a 20kW unit. To help keep generator size down, consider transfer switches with load shedding capability, soft start units for geothermal systems, and lock out some or possibly all electric heat strips if you can make due without them. Glad we had a generator during our recent 44 hour outage. Many people around here were without power for 4-6 days, had no AC with highs in the upper 90's to low 100's, and had no well water.
I had considered larger than 10 kW, but not up for spending too much on an insurance policy. This is to make due in a pinch. Basics and temp comfort is it. I do not have propane or NG on property, so my solution will require trips to the gas station. Too large and those trips become more frequent or larger. My all in cost for the genny, 25' cord, recptacle and breakers is less than $1300.
Beware attempting to operate a heat pump, central AC, geo system of any description on a marginal generator. Repeated compressor starts on sagging line voltage won't do the system any good, and the money saved on the smaller genny may well be spent several times over on costly system repairs. If nothing else, consider retrofitting the compressor section with a soft starter such as Intellistart of Securestart. These reduce inrush current to Emerson / Copeland equipped systems by 50% or more.
FWIW, in a recent discussion on a generator forum, a 20% capacity cushion (running at no more than 80% capacity) was suggested by generator techs. This was a discussion related to longevity of air-cooled generators.
That's good advice. However, any discussion of sizing of a generator intended to operate heat pump or AC will almost certainly center upon handling startup current. My own 3T Envision has an LRA / inrush of 82 amps per nameplate but typically operates at 5.5 - 6.5 amps in low stage, 10 or so in high stage. Rumor has it Intellistart / Secue Start or similar soft starter cuts inrush by half or more, but I can't swear to it
