What Size Propane Tank for a Generac Generator?

The right propane tank for a Generac generator depends on the type: a home standby unit needs a tank of at least 250 gallons, while a dual-fuel portable runs on a 20 lb or 30 lb cylinder. Below are runtime tables, the 80% fill math, and the reason cold weather can make a big tank act small.

What size propane tank does a Generac standby generator need?

Generac’s support documentation puts the minimum tank for a home standby generator at 250 gallons. Treat that as a floor, not a universal answer. Your exact model, local code, climate and how long you want to run all affect whether 250, 500 or 1,000 gallons is right. Check your owner’s manual and installation guide for your unit.

Tank size (water capacity)	Usable propane at 80% fill	Typical fit
250 gal	about 200 gal	Minimum for standby; short outages, mild winters
500 gal	about 400 gal	Multi-day outages, colder climates, larger units
1,000 gal	about 800 gal	Long outages, remote homes; needs more space and permits

Tanks are filled to about 80% so the liquid has room to expand. A "250-gallon" tank therefore holds roughly 200 gallons of fuel you can burn.

250 vs. 500 gallons: a simple rule

• 250 gallons fits if outages usually last a day or two, winters are mild and your propane company delivers quickly.
• 500 gallons fits if you get ice storms or multi-day outages, live where deliveries are slow, or run a large unit near full load.
• 1,000 gallons is for people who want a week or more without a delivery, and have room and permits for it.

Buried tanks cost more to install than above-ground tanks and need permits. Leasing versus owning also changes your options, which the FAQ below covers.

How long will a 250 or 500 gallon tank run a 22kW Generac?

Generac’s published spec for its 22kW standby generator lists LP use of about 2.53 gal/hr at 50% load and about 3.71 gal/hr at full load. Other documents for similar units may show different numbers, so use the fuel consumption table for your exact model. The math below assumes a tank filled to 80%.

Tank	Usable gallons	Hours at 50% load (2.53 gal/hr)	Hours at full load (3.71 gal/hr)
250 gal	200	about 79	about 54
500 gal	400	about 158	about 108
1,000 gal	800	about 316	about 216

These are best-case numbers. Few tanks sit at 80% when the lights go out. A 250-gallon tank at 40% on the gauge holds about 100 gallons, which is roughly 40 hours at 50% load. Also, don’t plan to run a tank to empty. Pressure and vaporization fall as the level drops.

For 10kW, 14kW, 18kW or 24kW units, consumption is different. Use the 50% and 100% figures in your manual and the same math. Don’t rely on rule-of-thumb charts from aggregator sites. They often disagree with Generac’s own sheets.

How do you calculate propane runtime from tank size and load?

You need four numbers: tank size, gauge reading, your generator’s gallons per hour, and how much of the unit’s capacity your house actually uses.

1. Find gal/hr at 50% and 100% load in your generator’s spec sheet.
2. Estimate your load. Most homes sit well below full output. Use the 50% figure for planning, and the full-load figure for a worst case. The steps in how to size a generator for your home show how to total your circuits.
3. Calculate usable gallons: tank size x gauge percentage. A 500-gallon tank reading 60% holds about 300 gallons. A full tank reads 80%.
4. Divide: usable gallons / gal/hr = hours.
5. Keep a reserve of 10 to 20% so you never run the tank dry.

Worked example (hypothetical numbers): your unit burns 2.0 gal/hr at your typical load, and a 500-gallon tank reads 60%. That is 300 gal / 2.0 = 150 hours, or about 6 days. With a 20% reserve (240 usable gallons), plan on about 120 hours.

Decision rule by outage length

Multiply the hours you want to cover by gal/hr. Using the 22kW figures above:

Outage	Gallons at 2.53 gal/hr	Gallons at 3.71 gal/hr
24 hours	about 61	about 89
72 hours	about 182	about 267
7 days	about 425	about 623

As a rule of thumb, if the result is under about 150 gallons, a 250-gallon tank has a comfortable margin. Between 150 and 300 gallons, choose 500. Above that, even a 500 will need a refill during the outage, so plan a delivery or step up to 1,000 gallons.

Why does cold weather change the propane tank size you need?

Gallons are only half the story. Propane has to boil from liquid into vapor before the generator can burn it, and the tank supplies that heat through its walls. The amount of vapor a tank can produce is its vaporization capacity. It drops when the air is cold and when the tank is low, because less of the tank wall is wet with liquid.

A 22kW unit at full load burns about 3.71 gal/hr, or roughly 340,000 BTU/hr (propane has about 91,500 BTU per gallon). A small tank in freezing weather, with a low gauge reading, may not vaporize that much. The symptom is falling pressure, a generator that stumbles or shuts down, and a tank that still has fuel in it.

What to do:

• Ask your propane supplier or installer for the tank’s vaporization rate at your coldest expected temperature and at about 30% full.
• Count other gas appliances on the same tank. A furnace, water heater and range all draw from the same vapor supply.
• Go up a size if you live where winters stay well below freezing and you run a large unit. A bigger tank has more wetted surface.
• Keep the tank above about 30% in winter, not just above empty.
• Keep snow and ice off above-ground tanks and regulators.

A buried tank has steadier ground temperatures, which helps in some climates. Your installer can tell you what applies locally.

What propane tank works with a Generac portable generator?

Portables are a different case. For its multi-fuel portable generators, Generac recommends standard 20 lb or 30 lb cylinders. It notes that larger cylinders or stationary tanks often need different regulators, fittings or piping. A 250-gallon standby tank is not the answer for a portable.

Cylinder	Approx. propane	Notes
20 lb (grill size)	about 4.7 gal	Works if your model is dual fuel and the manual allows it
30 lb	about 7 gal	Listed alongside 20 lb by Generac for multi-fuel units
100 lb	about 23.6 gal	Needs the correct regulator and hose; check the manual first

Gallons come from pounds divided by about 4.24 lb per gallon. To get runtime, divide gallons by your model’s LP gal/hr. For example, a portable burning 1.0 gal/hr at half load (an illustrative figure) would run a 20 lb cylinder about 4.7 hours. Check your model’s spec sheet for its real propane runtime.

Can you use a 20 lb BBQ cylinder?

Yes, if your Generac is a dual-fuel or multi-fuel model and the manual approves it. Use the regulated hose and fittings specified for that model. Set the cylinder upright outdoors, away from the exhaust. A gasoline-only Generac cannot run on propane without a manufacturer-approved conversion, so don’t improvise one.

If you’re still choosing a unit, compare Generac portable generators or broader propane-capable portable generators. For long runtimes, a pair of 100 lb cylinders on the right regulator beats swapping 20 lb tanks, but only where the manual allows it.

Who decides tank size and placement: code, installer or supplier?

Local code and your installer do. Tank setbacks from the house, openings and property lines follow NFPA 58 and local rules, and they depend on tank size. The generator has its own clearance rules from windows, doors and vents. A larger tank may need more distance, a permit or an inspection.

• Have a licensed propane or gas installer set the tank, piping and regulator, and size the line for the generator’s full-load demand.
• Ask your propane company whether it leases tanks. Leased tanks may come with delivery contracts and placement rules.
• Never overfill a tank. The 80% limit is a safety rule.
• Run any generator outdoors only, well away from windows and doors, and keep working CO alarms inside the home.
• If you smell gas, shut off the supply, leave the area and call your propane supplier or 911.

Propane or natural gas?

If a natural gas line runs to your house, a standby generator on natural gas needs no tank and no refills. But output ratings can differ between fuels on the same model, and supply can be interrupted in a big disaster. Propane stores on site but needs deliveries and the sizing above. For a fuel comparison, see gasoline vs. propane vs. diesel generators.

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Frequently Asked Questions

Can I use a 100 lb propane tank with a Generac standby generator?

Not as the main supply. Generac requires at least a 250-gallon tank for home standby units. A 100 lb cylinder holds about 23.6 gallons, which would run a 22kW unit only a few hours. Portable multi-fuel models are the ones designed for cylinders.

Should I lease or buy the propane tank for my generator?

Leasing usually costs less up front, but the supplier owns the tank and often requires you to buy fuel from them. Owning lets you shop for fuel and choose a buried or above-ground install. Terms and costs vary, so compare quotes.

Why does my generator shut down when the propane tank isn’t empty?

The tank may not be vaporizing enough propane. Cold air and a low fuel level cut vapor output. Ask your supplier to check the tank’s vaporization capacity and regulator, and keep the level above roughly 30% in winter.

Is it OK to fill a propane tank past 80%?

No. Tanks are filled to about 80% so the liquid can expand as it warms. Overfilling can send liquid into the lines or cause a release through the relief valve. Only a trained propane supplier should fill a stationary tank.

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