Hi have a 40kW standby generator with a 500 gal. underground propane tank. Its been in service more than 4 years. I’ve averaged about 20 hours a year of power outages and it automatically runs for 15 minutes every week to keep it healthy. My tank is now down to about 60% - I haven’t refilled the tank since I installed it...
You want enough fuel storage to go at least days and ideally a week or so in a real emergency.
For diesel, if you really have enough to last awhile, you won’t burn through it fast enough to keep it from degrading. And the legal storage requirements for hundreds of gallons of diesel/gasoline these days is daunting - even on a farm.
Sorry, the first word was supposed to be “I”...