The manufacturers of Home Standby Generators will give gallons per hour/CFM propane/nat gas consumption, at full load, and 1/2 load.
It's pretty easy to do the calcs.
The 20-22kW home standby generators that I install don't require an upgraded gas line generally.
I try to site the unit near the main service panel and gas meter, and I tee off the main gas line right after the meter. At around 10 feet or so, a 3/4" line works fine.
I just did the calcs for a 22kW with a 500 gallon dedicated propane tank a couple weeks ago. It'll run at 1/2 load for about 10+ days. At roughly 2.39/gal (here) for propane, it'll cost around 5.00/hour to run it.
Pretty pricey for any kind of long term use, but in a disaster situation, invaluable.
On a "basic" installation, where there is not excessive excavation, relocation of utilities, the unit placed far away from the meters, or a main service panel that requires an upgrade, someone could get a standby and installation for somewhere around 12K.
Not that bad really.
When you consider that a good stable 7kW portable from Honda will run you about $4,500, and transfer equipment installed is around 2K, suddenly the standby doesn't look so bad.
