What Size Propane Tank for a Generator? (2026)

Choosing the fuel for your standby generator is only half the question if you pick propane — the other half is the tank. And tank sizing is actually two questions: how many gallons you need (runtime), and whether the tank can deliver vapor fast enough in cold weather. Get the first wrong and you run dry mid-outage. Get the second wrong and the generator sputters even with a half-full tank. Here’s the math for both.

(If you haven’t picked a fuel yet, see natural gas vs. propane. For the burn-rate numbers behind this article, see the fuel consumption guide.)

The runtime math (question one)

Propane tanks are filled to 80% for expansion safety — “500 gallons” means about 400 usable gallons. (In some cold-weather regions, fillers go to 90% in winter; assume 80% for planning.)

A 20 kW-class standby burns roughly 2–2.5 gallons per hour at half load and about 3.1–3.7 at full load, depending on the model — for reference, the Kohler 20RESCL is rated 2.2 gal/hr at half load and 3.7 at full, while the Cummins RS20A is 1.8 and 3.1:

Two rules of thumb: size the tank for the outage, not the average — the fuel you have on day one is the fuel you have, because in a widespread outage delivery trucks may not reach you — and remember that bigger generators don’t burn much more at the same load, so this table shifts only modestly with kW rating.

The vaporization trap (question two)

Here’s the part almost no one explains before buying: a standby generator doesn’t burn liquid propane. It burns propane vapor — the gas that boils off the liquid surface inside the tank. The tank’s ability to produce vapor fast enough is called its vaporization capacity, and it depends on two things: the outside temperature and how much liquid surface area is in contact with the tank walls.

That means vaporization capacity drops as the tank empties and as it gets colder — exactly when you need the generator most.

Published field references put a 250-gallon above-ground tank (at 80% fill) at roughly 482,000 BTU/hr of vaporization capacity at 20°F, dropping to about 241,000 BTU/hr at 0°F. Now compare: a 20 kW-class unit at half load needs roughly 213,000 BTU/hr, and at full load — the Kohler 20RESCL’s 3.7 gal/hr times ~91,500 BTU per gallon of propane — about 339,000 BTU/hr.

Do the subtraction: at 0°F, the 250-gallon tank can vaporize 241,000 BTU/hr, and the generator at full load wants 339,000. The generator starves — it stumbles, throws low-fuel-pressure faults, or shuts down — while the tank still holds dozens of usable gallons. Let the tank level drop further and the wet surface area shrinks, making it worse.

This is why the 100-gallon tank — fine for a portable generator or a gas grill — is a non-starter for a whole-house standby. It’s not the gallons; it’s the surface area.

What this means for your tank choice

Adjusting for smaller or larger units. The tables above assume a 20 kW-class unit, the most common whole-house size. A 14 kW unit burns roughly 25–30% less, so a 250-gallon tank stretches to about 4–5 days at half load — but the vaporization constraint doesn’t relax nearly as much, because the tank’s surface area is the same. Going bigger (24–26 kW) pushes full-load propane demand toward 4+ gal/hr, or roughly 370,000+ BTU/hr — which is why 26 kW buyers in cold climates should treat the 500-gallon tank as the minimum, not the default. The fuel guide’s burn-rate table has the model-by-model numbers if you want to run the exact math for your unit.

250 gallons — the budget option. Works in mild climates where deep freezes are rare and outages are short. If your winters regularly hit freezing, treat this as a fair-weather tank for a 20 kW-class unit: fine at half load, marginal at full load when it’s cold and the tank is low. Keep it topped off before storm season.

500 gallons — the default answer for most buyers, and what most installers recommend. About 400 usable gallons gives you roughly a week at half load, and the tank has the surface area to keep vaporizing fast enough through freezing weather at realistic loads. If you’re buying one tank and one generator, this is the pairing.

1,000 gallons — for rural properties at the end of the feeder, regions with multi-day ice-storm outages, or anyone running 26 kW-class units. Some buyers prefer two 500-gallon tanks over one 1,000 for delivery flexibility and redundancy. In a widespread outage, remember the warning from the fuel guide: delivery trucks may not reach you, so the tank is your only fuel.

100-pound cylinders — each holds a little under 24 gallons. Fine for portables, useless as the primary fuel for a whole-house standby: you’d be swapping cylinders in the dark.

Lease vs. own the tank

Most propane companies will happily lease you a tank — low upfront cost, but you’re typically locked to that supplier for fills, and lease customers often pay higher per-gallon rates. Owning the tank (buying it outright) lets you shop suppliers and negotiate price, and propane prices vary wildly (~$2.50–$4.00/gal depending on region, season, and tank ownership, per the fuel guide). If propane is your generator’s primary fuel — and especially if you also heat with it — ownership usually pays back. Ask your supplier for both quotes before you commit.

Above-ground vs. buried, and the rest of the install

• Above-ground tanks are cheaper to install and easy to monitor, but vaporization tracks the air temperature — the cold-weather numbers above apply directly.
• Buried tanks get steadier vaporization (soil temperature stays moderate), cost more to install, and need truck access for fills. Have your propane supplier run the vaporization numbers for your specific tank and climate — it’s their standard calculation.
• The regulator matters: a two-stage regulator setup sized for the generator’s BTU demand is part of the install, not an afterthought. Undersized regulators cause exactly the same starvation symptoms as a cold tank.
• Placement and clearances are code-driven (NFPA 37/58 and local amendments). Your installer and propane company handle this — don’t trench the line yourself to save money. Placement also affects installation cost: a long gas run from a far-placed tank adds real money.
• In hurricane and ice-storm country, consider where the tank sits relative to falling trees and flood lines. A protected tank that survives the storm is the whole point.

The bottom line

For a 20 kW-class whole-house standby, buy the 500-gallon tank unless you have a specific reason not to. It covers a week of half-load runtime, vaporizes fast enough in freezing weather, and costs only modestly more than a 250 to install. Size up to 1,000 (or twin 500s) if your outages run long or your winters run cold — and whatever you buy, keep it full going into storm season. The most expensive propane is the propane you don’t have.