OP
twistypencil
Contributor
One reason is that I want a tank at home of clean air so I can work on and check out my gear.
I may also be driving somewhere so that the logistics are simplified with several of my own tanks.
You apparently dive and fill locally. Depending on where you live and how much you dive, that can get expensive compared to having your own tank(s). The fundamental idea is that filing is cheaper than filling plus renting. Example: suppose you do N dives per year, and the cost of a fill is $F, and the cost of a rental is $R, and the cost of an annual visual tank inspection is $V. Then your total cost for the year is
$T = N($F+$R) + $I
Typical numbers are $F=$8, $R=$10, and $I=$25.
For N=10 you spend $180 if you rent, $125 if you do not.
For N=100 you spend $1800 if you rent, $1025 if you do not.
For doing not much diving, it takes a while to amortize tank ownership.
For a lot of diving, it take just a few months.
I think you replaced $V with $I ini your example