You can order a 22KW, with transfer switch from Home Depot and have it delivered for $6200. Delivery time is sitting at about a week right now. The smaller sizes are out of stock.
If you are going to run it off of natural gas, then bigger is usually better.
I installed an 18KW last year. While every Generac Authorized Installer was quoting > 5 months, the truck delivered min in less that a week. I DYI'ed everything except burying 500 gallon propane tank. The installed cost including the propane tank purchase and install was $4000 cheaper that just what I was quoted for the Authorized Installer to provide and install the genset. I also did some HVAV and water heater load management that would have cost me another $400 - $500 dollars.
There was a thread hear about it here, but i could not find it. There is a thread over at the Texas Hunting form:
Texas Hunting ForumQuite a bit of discussion there about sizing. I had to go propane and I was looking for the 'perfect' balance of maximum runtime vs convivence. I travel a lot for work and needed any management to be totally automated for the wife. Just last week I was in California when had an outage (Oncore claims it was "equipment related", but it happened as system load peaked on Tuesday.) The wife did not know there was a problem until the neighbor call asking "Is your power was out too?' NOPE
Full disclosure...Complete DYI is probably not right for most folks. In my younger days, I did heavy industrial electrical and I've installed 1MW standby/peak shaving units, so rerouting the "little" 200 amp incoming service to my house was no problem. Still, there is a lot you can save money on doing the physical mounting of the generator, any hand trenching, and the like. If you get a local electrician to install the transfer switch, then the rest is very doable. The instructions for Generac were straight forward. Search YouTube