[240glt]

^ just make sure you are complying with local fire and building codes

I polish my diesel tanks once per year. That involves filtering and treating the fuel. Untreated diesel can have microbial growth that rots tanks and reduces the efficacy of the fuel. I'm currently holding about 5000 litres of fuel for four diesel generators and two diesel fire pumps.

Natural gas storage is possible if you have the real estate to do it, but you'll have a hard time getting permission to put a natural gas vessel inside or in top of a building

Not sure if cost is all that big a factor. You'll only run your generator during power outages, at intervals as directed by code, and for your annual load bank tests. As noted, we have six inline 6 turbocharged diesel engines, and I only have to top up the tanks once per year, if that

Contamination is a concern but in our jurisdiction we require spill containment and leak alarms. My latest generators have belly style day tanks with all that built in. Not sure if you need a day tank in your jurisdiction, but in ours you do, and code will never allow a natural gas day tank within an enclosed space. At least not without an evacuation system and probably some other infrastructure, if at all

Just be careful. And get an engineer.