I am building a house that will have a 4 ton GSHP.  We will have 320 amp service.  I will be in a rural subdivision, but we will have natural gas service.  I am interested in installing a back-up generator, so if we lose power, we will still have heat, hot water, etc...

What size of back-up generator should I get.  If you have experience installing, designing, or using these systems, please let me know what you have and what you like/dislike about it.  If there is a brand/model you can recommend or one you suggest I stay away from, please let me know. 

Thank you. 

Briggs & Stratton 40236 15,000-Watt Bi-Fuel Standby Generator with 100 Amp Automatic Transfer Switch

<script language=javascript> <!-- var scaleLevels; scaleLevels = new Array(4); scaleLevels[0] = new MediaServicesZoomScale(600, 297, 600); scaleLevels[1] = new MediaServicesZoomScale(1210, 600, 400); scaleLevels[2] = new MediaServicesZoomScale(1816, 900, 400); scaleLevels[3] = new MediaServicesZoomScale(2421, 1200, 400); DynAPI.addZoomViewer("http://z2-ec2.images-amazon.com/images/R/B000OAJPEI.01",420,420,2421,1200,21185478,"amztile",scaleLevels); //--> </script> Only one I could find that will even handle a 4 ton without going to gasoline , good or bad couldnt tell you

If you can, avoid getting a Gas or Diseal system. Chances are you'll use it so infrequently, you'll never use up the gas before it goes bad.... Unless you want to get into the habit of removing gas from the generator tank to run your car and replacing it with new gas every 6 months. I found this one without too much trouble, runs around 4k, installation is another 2k, Not sure it's it air cooled or water cooled, but I would recommend a Water cooled system over a air cooled one. But of course you should read up on them before making any purchases. 
 
Guardian 20kW (Aluminum) w/200 Amp SE Automatic Transfer Switch
Natural Gas or LP Gas Operation

 

You need to be certain that the generator's surge rating or KVA matches the geo unit's nameplate Locked Rotor Amps, typically abbreviated LRA.

There are many more options in NG than the B&S unit shown above - check out Generac / Guardian.

While that B&S is 'rated' for 4 tons, IMO it'll be marginal and hard on the geo unit's compressor. If any other significant load (such as water heater) happens to be on when the geo compressor attempts a start, it'll fail.

Natural Gas, since it contains quite a bit less energy than propane, often results in a generator being somewhat derated while burning it.

A marginal generator is a foolish economy given the repair / replacement costs of a house's typical connected loads.

I have a 15 KW Guardian standby generator. It was installed in 2004. The steel housing started to rust at age 3. Aluminum lasts longer. It has operated as long as three days nonstop, reliably powering most of the house, except for stove, oven and dryer. The generator is attached to a 500 gal propane tank filled to 85%. This is enough fuel for about 6 days of run time at full load. I looked for help on this forum and got good advice regarding the power needs of my Tranquility 27. On the start-up day of my new 3 ton geo, the generator performed fine, there was slight quick dimming of the basement lights, but not much more noticeable a dip than when the geo starts on the utility power. There has not been a real life power outage, since the new heat pump was installed in October this year.

I have a contract with the generator installer to maintain it. So far it has been annual oil and filter changes, and a checkup visit six months in between. The total install cost was $5200. Maintenance cost has been $500 for a two year period (4 visits in 2 years).

Regards,
Masoud

That's good info.

I have a 25 KW Guardian liquid cooled 1800 RPM generator. House has 3 ton Envision HP, which starts fine but with noticeable flicker of lights. LRA on the Envision is 83 Amps.

The upfront cost was higher than the minimum unit required, but it is much quieter, robust, and more fuel efficient than smaller 3600 rpm units. If we ever get a major hurricane here in Jax, FL, my location at the end of a single line down a wooded road suggests I could be without power for weeks instead of days, so that was the basis for my planning. I'd also like to be able to spare some power for my aged next door neighbor.

One night I was out of town and wife considered calling county sheriff since she thought she heard a truck idling in driveway. Fortunately she didn't - power had gone out and house was on generator.

Neighbor 5 houses away has one of the smaller 10-15 KW high speed units on our side of his house approx 400' away. His at that range is louder than ours at 50', and it won't run his whole house.