For the natural-gas home, generator fuel is somebody else’s infrastructure [→ /fuel-running-costs/natural-gas-vs-propane-generators/]. For the propane home, it’s a tank in the yard — and the size of that tank quietly decides how long your backup power lasts, how it behaves in an ice storm, and how often the delivery truck visits mid-crisis.
The short answer this guide will earn: 500 gallons is the standard for whole-house standby generators, and for good reason — it carries a 22 kW-class unit through roughly 5–8 days of substantial use. A 250-gallon tank is the “short outages only” compromise (2–4 days), and 1,000 gallons is hurricane-country’s long-siege answer. But the real sizing decision involves three twists most guides skip: the 80% fill rule that shrinks every tank on paper, the cold-weather vaporization limit that shrinks small tanks in exactly the weather that matters, and the shared-tank question when the same propane also heats your home. All three below, plus the buy-vs-lease math and the pre-storm rules that make any size work.
First, the Two Rules That Shrink Every Tank
Rule 1: The 80% fill rule. Propane tanks are never filled full — safety codes cap fills at ~80% to leave expansion room. So the working capacities are:
| Nominal tank | Actual usable propane |
|---|---|
| 120 gallon | ~96 gallons |
| 250 gallon | ~200 gallons |
| 500 gallon | ~400 gallons |
| 1,000 gallon | ~800 gallons |
Every runtime calculation on this page — and every one you do — starts from the right column. (And in practice you’ll call for refills well before empty, so the planning capacity is smaller still: the gallons between “full” and “call the truck at 30%.”)
Rule 2: The cold-weather vaporization limit. Your generator burns propane vapor, and the liquid in the tank must boil off fast enough to feed it. Vaporization rate depends on liquid surface area (bigger tank = more), fill level (fuller = more), and temperature — and in deep cold, a small or low tank may physically not vaporize fast enough to feed a hungry 22 kW unit at full draw, causing pressure starvation, stumbling, and shutdowns [→ /installation-costs/natural-gas-line-sizing-standby-generators/]. This is the quiet technical reason cold-climate installers push tank sizes up: the 250-gallon tank that “has enough gallons” for your outage can still starve your generator at 5°F when it’s half empty. Bigger tanks aren’t just more fuel — they’re more deliverable fuel in exactly the ice-storm scenario propane homes buy generators for.
The Runtime Math by Tank Size
Using a 22–24 kW-class standby at realistic mixed loads (~1.7–2.2 gal/hr at half load; more in hard weather, less duty-mixed [→ /fuel-running-costs/whole-house-generator-cost-per-day/]):
| Tank (usable) | Continuous heavy use | Realistic mixed use | With thermostat/load discipline |
|---|---|---|---|
| 120 gal (~96) | ~1.5 – 2 days | ~2 – 3 days | ~3 – 4 days |
| 250 gal (~200) | ~3 – 4 days | ~4 – 5 days | ~5 – 7 days |
| 500 gal (~400) | ~6 – 8 days | ~8 – 10 days | ~10 – 14 days |
| 1,000 gal (~800) | ~12 – 16 days | ~16 – 20 days | 3+ weeks |
Planning figures — your unit’s consumption curve (in its manual) and your load profile sharpen them. Smaller generators (10–14 kW managed systems [→ /standby-generators/whole-house-vs-partial-home-backup/]) stretch every row meaningfully.
Three readings:
- The 500’s case makes itself: hurricane and ice-storm outages routinely run 5–10 days, which the 500 covers with margin and the 250 covers only with discipline and luck — and mid-crisis propane delivery is exactly as reliable as the roads and the queue [→ /outage-prep/hurricane-power-outage-prep/].
- The 250 is honest for short-outage regions: where multi-day events are rare, ~4–5 realistic days plus a refill call covers the mission at a smaller footprint and price.
- The 1,000 is a siege answer: remote properties, long-restoration rural grids, and households that refuse to depend on the delivery queue — priced accordingly.
The Shared-Tank Question (The Sizing Trap for Propane Homes)
Most propane homes don’t buy a generator tank — they connect the generator to the tank that already heats the house, the water, and the range. Reasonable, common, and the source of the classic sizing miss:
Your outage scenario is the generator AND the furnace drawing together. A winter storm outage means the generator at load plus the heating system it’s powering, both drinking from the same tank — total draw far above either alone, and the vaporization limit stressed hardest [→ /fuel-running-costs/how-much-gas-does-a-generator-use/]. The planning consequences:
- Size (or upsize) for the combined winter draw, not the generator’s solo math — the shared 250 that penciled for a generator alone becomes a 3-day tank when the furnace joins.
- Regulator and line sizing must handle the combined demand — the propane cousin of the gas-verification rule this site staples to every install [→ /installation-costs/natural-gas-line-sizing-standby-generators/]: demand calculated, supply confirmed, in writing.
- Some households split the difference: a dedicated generator tank (even a modest one) alongside the house tank buys independence and simpler math — at the cost of a second tank’s price and pad.
Buy vs Lease (The Ten-Year Math)
| Buying | Leasing | |
|---|---|---|
| Upfront | $1,200 – $2,500 (250 gal) / $1,800 – $3,500 (500 gal), installed; buried adds $1,000+ | $0 – $300 setup |
| Annual fee | None | $50 – $200 |
| Propane pricing | Shop any supplier — the real lever | Captive to the tank owner’s rate |
| Maintenance | Yours (modest: valves, gauges, recertification cycles) | Supplier’s |
| Switching suppliers | Trivial | Means switching tanks |
The honest math most homeowners discover late: the lease’s real cost isn’t the fee — it’s the captive propane pricing, commonly $0.30–$1.00+/gallon above shopped rates. At generator-plus-heating volumes (hundreds of gallons a year), owning typically pays for itself in 3–7 years and then keeps paying [→ /standby-generators/whole-house-generator-cost/]. Leasing’s genuine case: low usage, short ownership horizons, and letting maintenance be someone else’s job.
Buried vs above-ground: burying adds $1,000–$2,500 but wins on aesthetics, HOA peace [→ /portable-generators/quiet-inverter-generators-hoa/], and temperature stability (the ground moderates the cold-vaporization problem — a real technical point, not just curb appeal). Above-ground wins on cost, inspection ease, and relocation.
The Pre-Storm Rules (Any Size)
- Enter storm season above 60–70% — both for gallons and for vaporization headroom; hurricane-country propane homes top off in May.
- Know your gauge and your supplier’s crisis behavior: how fast do they deliver at 30% in a normal week? During a regional event, multiply that.
- Set the refill trigger at 30–40%, not 20% — the buffer is the queue.
- Winter addendum: keep it fuller in deep-cold months (vaporization again), and keep the tank area and regulator clear of snow and ice [→ /portable-generators/run-generator-in-rain/].
- Duty discipline stretches every table above: the thermostat and load-batching levers cut propane’s premium daily cost hardest [→ /fuel-running-costs/whole-house-generator-cost-per-day/].
Frequently Asked Questions
What size propane tank do I need for a whole house generator? 500 gallons is the standard answer for 18–24 kW standby units — roughly 8–10 realistic days of mixed use (usable capacity ~400 gallons after the 80% fill rule). 250 gallons serves short-outage regions (~4–5 days); 1,000 gallons serves long-restoration and remote properties.
How long will a 250-gallon propane tank run a generator? With ~200 usable gallons: roughly 3–4 days of continuous heavy use, 4–5 days realistic mixed use, and up to a week with strong load discipline — if it isn’t also feeding the furnace, which winter outages guarantee it is on shared tanks.
How long will a 500-gallon propane tank last with a generator? Roughly 6–8 days continuous heavy use and 8–10+ realistic days for a 22 kW-class unit — the margin that covers most hurricane and ice-storm restoration windows without betting on mid-crisis delivery.
Can a generator and house share the same propane tank? Commonly yes — with the tank, regulator, and lines sized for the combined peak draw, verified in writing at installation. The classic mistake is sizing for the generator alone and discovering the furnace’s share during the first winter outage.
Is it better to buy or lease a propane tank for a generator? At generator-plus-heating volumes, buying usually wins within 3–7 years — the lease’s hidden cost is captive propane pricing ($0.30–$1.00+/gallon above shopped rates), not the annual fee. Leasing fits low usage and short horizons.
Why does my propane generator stumble in very cold weather? Likely vaporization starvation: cold slows the liquid-to-vapor rate, and a small or low tank can’t feed a big generator’s draw — pressure sags exactly at peak demand. Fixes: fuller fills, bigger (or buried) tanks, verified regulator sizing — not engine tinkering [→ /maintenance-repair/generator-repair-cost/].
The Bottom Line
The 80% rule shrinks every tank, the cold shrinks the small ones further, and the furnace shares the straw — which is how “what size tank” resolves for most standby homes to 500 gallons, owned if the ten-year math allows, entering every storm season above 60%. Take the 250 where outages are short and honest, the 1,000 where the grid restores slowly, and whichever tank sits in the yard: verify the combined draw in writing, set the refill trigger early, and let the thermostat do what discipline does best — stretch the table [→ /fuel-running-costs/natural-gas-vs-propane-generators/].