Out where the gas mains end — Harvest, New Market, Hazel Green, most of rural Limestone County — your propane tank is your fuel supply, and its size decides how many days of outage you can ride out. The runtime math takes exactly two numbers: usable gallons in the tank, and the generator’s consumption at half load from its manufacturer sheet (about 2.53 gallons per hour for a Generac 22kW). Everything else on this page is context for those two numbers, starting with how the sizing method end to end treats propane differently from utility gas.
Why does tank size equal runtime?
Because unlike a gas meter, a tank doesn’t limit flow — it limits total supply. A 500-gallon tank feeds a hungry generator at full demand just fine; it simply runs out sooner. So where in-city buyers ask “can my supply feed it?”, you’re asking “for how long?”
Two corrections people miss. First, a “500-gallon” tank holds about 400 usable gallons — tanks are filled to roughly 80 percent of rated capacity to leave expansion room, per NFPA 58. Second, the generator rarely runs at full load, which is why manufacturers publish half-load consumption and why that’s the honest planning figure.
There’s a cold-weather wrinkle too: a tank vaporizes liquid propane into usable gas more slowly as temperatures drop, and a small tank has less wetted surface to work with and can struggle to keep up with a large generator’s demand on the coldest nights. One more reason tank sizing isn’t just a runtime calculation.
250 or 500 gallons — who needs which?
The 250 suits smaller generators, essential-circuit plans, and households that treat the generator as bridge power for a day or two. The 500 is the whole-home workhorse: bigger units burn more per hour, and multi-day events are precisely when you don’t want to be watching a gauge.
Splitting the difference by intent works better than by budget. Ask what outage you’re actually planning for. If the answer is “the 2011 kind — most of a week,” the math almost always lands on the 500 for a whole-home unit. If it’s “overnight, a few times a year,” the 250 with a modest unit and managed loads to stretch runtime is a defensible, cheaper answer.
And the unit size itself moves the math more than the tank does — a right-sized generator from your load calculation burns meaningfully less than an oversized one doing the same job.
Above ground or buried?
Above-ground tanks cost less to install and are easier to service and inspect; you also see them, and so does your HOA if you have one. Buried tanks disappear from the yard and hold temperature more steadily, at the price of excavation and a slightly more involved service life.
Around here both are everywhere. Lot layout, setback rules, and covenant language tend to make the call — the tank also has to sit where a delivery truck can reach it, which on long rural driveways is a real siting constraint, not a detail.
What’s the refill reality in the county?
Propane arrives by truck, and after a widespread storm event every truck in the valley is running the same list. Rural routes get reached when they get reached. The planning consequence is simple: size the tank so the event doesn’t depend on a mid-outage refill, and keep it decently full going into storm season rather than gambling on a spring top-off.
That on-site independence cuts the other way too — no utility to fail, no meter to outgrow. How that trade nets out against natural gas service is the full fuel comparison, and if you’re close enough to town that both are live options, start there.