How Long Will a Propane Tank Run a Generator?
When the power goes out, the big question is how long your propane will last. Pick your generator and your tank below and you’ll get a plain answer in hours or days, plus what it will cost to run.
Fuel use: Generac and Champion spec sheets · Prices: EIA · By Nathan Brooks · Reviewed Oct. 15, 2026
Work out how long your propane will last
On a big tank’s gauge, “full” reads about 80%, and a reading of 40% means about half full.
Default: latest U.S. home delivery average. Tank exchanges at a store usually cost more per gallon.
These are estimates. Your generator’s own manual or fuel label is the final word, and real use goes up and down all day as things switch on and off.
What “load” means, in plain words
A generator doesn’t burn the same amount of fuel all the time. It burns more when more things in your house are drawing power from it, just like a car uses more gas going uphill. That “how hard is it working” is called the load.
Most people never run a generator at full load for long. During a typical outage the fridge cycles on and off, the lights are on in a few rooms, and the furnace fan or well pump kicks in now and then. That’s why the half load figure is the most realistic starting point for most homes. If you turn on the electric oven, the dryer and central air at the same time, you’ll be much closer to full load and your propane will go a lot faster.
How long a home tank runs a standby generator
This is the question most people with a whole-house generator are asking. Here’s how long a full tank lasts if the generator runs nonstop, using fuel figures published by Generac for its Guardian home generators:
| Generator | 250-gal tank, half load | 500-gal tank, half load | 500-gal tank, full load | 1,000-gal tank, half load |
|---|---|---|---|---|
| 10 kW | 8.6 days | 17.2 days | 11.3 days | 34.4 days |
| 14 kW | 4.6 days | 9.2 days | 5.4 days | 18.4 days |
| 18 kW | 4.9 days | 9.8 days | 5.5 days | 19.6 days |
| 20 kW | 3.5 days | 7.0 days | 4.7 days | 14.1 days |
| 22 kW | 3.3 days | 6.6 days | 4.3 days | 13.2 days |
| 24 kW | 3.3 days | 6.6 days | 4.3 days | 13.2 days |
| 26 kW | 4.0 days | 8.1 days | 4.6 days | 16.2 days |
Usable gallons: 200 (250-gal tank), 400 (500-gal) and 800 (1,000-gal), because tanks are filled to about 80%. Fuel use from Generac Guardian spec sheets. Newer and older models can use more or less, so check the fuel label on yours.
Notice something surprising? A bigger generator doesn’t always burn more. Generac’s 26 kW model uses less propane at half load than its 22 kW model, because it has a different engine. That’s why it’s worth looking up the real number for your model rather than guessing from the size.
How long a 20-lb tank runs a portable generator
If you’re running a portable generator off the same kind of tank you use for your grill, you’re looking at hours, not days. Here are the run times one major manufacturer publishes for its dual-fuel generators on a 20-lb tank:
| Generator (Champion, on propane) | Run time on a 20-lb tank |
|---|---|
| 2,500-watt inverter | up to 25 hours at 1/4 load |
| 3,500-watt portable | 10.5 hours at 1/2 load |
| 7,500-watt portable | 5.5 hours at 1/2 load |
Source: Champion Power Equipment product pages. Other brands are in the same general range, but check yours.
In practice, that means a big portable generator can empty a grill tank before the end of a single evening. If you expect outages that last more than a day, a couple of spare tanks or a 100-lb cylinder (about 23 gallons) makes a big difference. For where to fill them and what it costs, see where to refill propane.
Why your generator might use more (or less) than this
- What you plug in. Big electric appliances, such as an electric range, dryer, water heater or central air, use a lot of power and push the generator toward full load.
- Your tank isn’t really “full.” Big tanks are only filled to about 80% so the propane has room to expand, and a 20-lb tank holds about 4.7 gallons, not 20.
- The weekly self-test. Home standby generators start up on their own for a few minutes each week or so to stay ready. It’s a small amount of propane each time, but it adds up over a year.
- Very cold weather. Propane has to turn from liquid into gas before the generator can burn it. In deep cold, a small, nearly empty tank may not be able to keep up with a big generator. This is one reason installers size the tank to the generator, not just to how long you want it to run.
What size tank do you need for a whole-house generator?
Start with how many days you want to be covered without a delivery, then work backward. For example, a 22 kW generator at half load burns about 2.5 gallons an hour, which is about 61 gallons a day. For three days you’d need about 180 gallons, which a full 250-gallon tank (about 200 usable gallons) just covers. For a full week, you’d want a 500-gallon tank.
If the same tank also heats your home, count that too. In a cold snap, your furnace and your generator are pulling from the same tank at the same time. Our propane heating cost calculator shows how much your home uses for heat in a winter month. Your installer and propane supplier should make the final call on tank size. For sizes and what each is used for, see propane tank sizes.
Plan for the cost, not just the hours
Most people think about run time and forget the bill. A 22 kW standby generator at half load burns about 2.5 gallons an hour. At $2.60 a gallon, the U.S. average in early October 2026, that’s about $6.50 an hour, or roughly $156 for a full day of nonstop running. A three-day outage at that pace would use about $470 worth of propane.
That’s not a reason to skip a generator; it’s a reason to run it smartly. I’d keep the big electric appliances off, check the gauge before storm season and top up the tank in the fall, when deliveries are easier to get. — Nathan Brooks
How to make your propane last longer during an outage
- Run only what you need. Keep the big electric appliances off, and don’t run the dryer, oven and air conditioning at the same time.
- Fill up before storm season. Deliveries slow down or stop during big storms, so a tank that’s already full is your best backup plan.
- Check your gauge before a storm hits. If it reads 30% or less, call your supplier right away.
- With a portable, stagger your loads. Run the fridge for a while, then switch to charging phones or running a space heater, instead of running everything at once.
- Keep a spare. An extra full 20-lb tank, stored outdoors and upright, can keep a small generator going through a second night.
Stay safe: carbon monoxide is the real danger
Generators make carbon monoxide (CO), a gas you can’t see or smell that can kill in minutes. Running on propane doesn’t make it safe indoors. The U.S. Consumer Product Safety Commission (CPSC) says:
- Run a portable generator outside only, at least 20 feet away from the house.
- Never run it inside a home, garage, basement, crawlspace or shed, even with the door or windows open. Opening them doesn’t let in enough air.
- Install battery-powered CO alarms (or alarms with a battery backup) on every level of your home and outside sleeping areas.
Quick answers
How long will a 500-gallon propane tank run a generator?
A full 500-gallon tank holds about 400 usable gallons. That runs a 22 kW home standby generator for about 6.6 days nonstop at half load, or about 4.3 days at full load. Smaller generators run longer. Use the calculator above for your model.
How long will a 20-lb propane tank run a generator?
A 20-lb tank holds about 4.7 gallons. Depending on the generator and how much you plug in, that’s roughly 5 to 25 hours: a large portable at half load uses it up in about 5 or 6 hours, while a small inverter at light load can run about a day.
How much propane does a 22 kW Generac use per hour?
According to Generac’s spec sheet, the 22 kW Guardian uses about 2.53 gallons an hour at half load and 3.90 gallons an hour at full load on propane.
How much propane does a whole-house generator use per day?
Running nonstop at half load, a 10 kW standby generator uses about 23 gallons a day and a 22 kW model about 61 gallons a day. At full load, the 22 kW uses about 94 gallons a day.
Is propane or gasoline better for a portable generator?
Gasoline usually gives a little more power, but propane stores for years without going bad and a tank is easy to swap. Many people with dual-fuel generators keep propane as their backup fuel for that reason.
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Sources
Generac Guardian 10/14/18 kW spec sheet · Generac Guardian 20/22/24 kW spec sheet · Generac Guardian 26 kW spec sheet · Generac: how much fuel does my home standby generator use? · Champion 2500W dual fuel inverter · Champion 3500W dual fuel generator · Champion 7500W dual fuel generator · CPSC: winter storm generator and CO safety tips (2026) · EIA weekly residential propane prices
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