Not enough info. How big is the HP? Is it an inverter HP? (They start with lower inrush and can run at a, low, constant speed.) Do you actually want to use the water heater or just keep it above freezing? How big is the water heater? You might consider running a point of use, demand heater,but there is no way you can run a 25 kW WH with a 17 kW genset. That size genset might not even handle your heat pump.

Honestly, powering a big demand water heater is going to be very expensive for you because it requires a big power source on demand. A tank type can take a little energy whenever it is available and keep you in warm or hot water. It is like a battery in some respects.

You have a third alternative. You can install a system with more sophisticated load controls. That allows you to prioritize what will run. For example, you could make sure that the refrig runs when ever it needs to cool, but the water heater takes priority over the heat pump. It still won't help much with a big, demand water heater. In order to make it work, every time you turn on a hot water faucet, everything else will need to shut off. (You might not be standing in the dark, but dinner prep will be interrupted, the TV will shut off. It won't be good for the heat pump to shut down a few times a minute either.

@Ionized: More info for you. Feedback?

1) HP is 3 tons Trane XR17 2 stage - not sure if it's an inverter.
2) Tankless WH
(Navien NPE-240A) has a recirculating pump that keeps hot water freeze protection but requires electricity (so unit can periodically fire to keep warm).
3) Not sure if the tankless specs lead you to other conclusion?

Oh, it is a gas water heater. That changes everything. Consider editing your initial post to indicate that clearly.

Sorry I should have been more clear that the electric is to keep provide an ignition & to keep the supply & inlet pipes from freezing by sloshing the water around essentially.

So how does this change your recommendations?

That generator should power a 3-ton heat pump-- but it may or may not power the auxiliary electric strip heat which will operate for lengthy periods of time when outside temperatures drop below about 28 degrees and will operate intermittently at higher temperatures.
Your gas logs may suffice for emergency space heating.
The date for the sale to end has already passed.

Thanks bus_driver for feedback. Interesting note by you.

It's a 17KW rebadged Generac so I am not too worry about finding something around same price. ~$3500.

However taking it up to 20KW - would that be able to handle the heat strip & Tankless you think?

If the tankless is gas then its electrical requirements will be rather small. The heat strips could be 10Kw-15kw just by themselves. Without lookiing, the HP might be 4kw-5kw when running but its start up load could be significantly higher.

If by 240v microwave you mean an advantium, those things are around 6Kw-7Kw by themselves (30 amps).

Posted by weedmeister (My Page) on Thu, Feb 27, 14 at 1:10
If the tankless is gas then its electrical requirements will be rather small. The heat strips could be 10Kw-15kw just by themselves. Without lookiing, the HP might be 4kw-5kw when running but its start up load could be significantly higher.

Yes, I gather this will be a crucial talking point with the HVAC & Electrician. However, skimming online I think there are ways to wire the generator or heat pump to handle those hard startup loads: 5-2-1 Hard Start Kit Capacitor. Not sure what it is but...

If by 240v microwave you mean an advantium, those things are around 6Kw-7Kw by themselves (30 amps).

Similar it is a Miele Convection Microwave.

When we do our hierarchy of what gets the load first, we can always sub out this for that - in this case we can use our gas stove oven to heat stuff or even toast bread :)

If it is a simple micro-convection oven and not a speed oven then it might not use that much power. You just need to check the manual.

A hard start kit will only make a small dent in the starting load for the HP. Wiring it differently may help by keeping the heat strips out of the backed-up circuits, but you may notice their absence. An HVAC consult is not likely to help you much unless you are considering a HP replacement. In that case, you will need to find a detail-oriented HVAC pro to help you choose a system (inverter-driven, perhaps) that will play best with a relatively small power source, and have better low-temperature performance so it can heat your home without the need for resistance heat back-up if you get a cold snap. (For all we know, you already have that.)

In the end, you have three choices: First, dumb load shedding (simple transfer panel with auto switch) with a relatively small genset, second a big, dumb genset, third, intelligent load shedding with a relatively small genset.

The first gives you no versatility powering a limited number of loads, limits fuel consumption and limits your investment in expensive genset and switching hardware. The second will have higher fuel consumption because the genset will be running at a low load a great deal of the time, and higher (genset) hardware costs. The third is more expensive from an auxiliary hardware (controls) point of view, but less into a genset. Fuel consumption will be lower. There is a question about the reliability of automatic load shedding vs. a big dumb generator or a dumb auto transfer switch, but it might be only in my mind. Read up about it.

Your decision might depend on how long your outages are. If they are short, fuel is not so much of an issue. Start putting numbers together to see what works best with your needs in mind.

There is a 4th alternative that might or might not make sense from a money standpoint. Maybe someone with more hardware experience can be a guide. You get a relatively small genset with an automatic transfer panel supplying your critical loads for when you are absent, heat pump, water heater and refrigerator). Have a second genset attachment to your main panel with a manual transfer switch or lock-out. When you are home, you manage your loads yourself after disconnecting the genset, and reconnecting at the main panel. That might mean turning off your heat pump when you want to do much of anything else.

Thank you ionized. You've given me a lot of good discussion points to speak to the sales & install team of my choosing.

I think if push comes to shove, we may be able to live without heat, since we have pretty good gas fireplace that can stand-in until power comes back up (albeit a short term).
But in the summer A/C is a deal killer if it is not on.

Anyways - I understand dumb load shedding but intelligent load shedding seems like the best way forward & will be a main driver in my research & execution of this project.

Thanks again for the tips!