As an Amazon Associate, I earn from qualifying purchases. Some links in my guides go to Amazon, and buying through them costs you nothing extra. Read the disclaimer.
A typical air-cooled home standby generator burns roughly 2 to 4 gallons of propane an hour while running under load. Generac’s spec sheet for the 22 kW Guardian lists 2.53 gallons an hour at half load and 3.90 at full load. Smaller units use less. The 16 kW model is listed at 2.03 and 2.94 gallons an hour.
Burn rates by model
| Model | 1/2 load | Full load |
|---|---|---|
| 16 kW | 2.03 gal/hr | 2.94 gal/hr |
| 20 kW | 2.37 gal/hr | 3.56 gal/hr |
| 22 kW | 2.53 gal/hr | 3.90 gal/hr |
Source: Generac Guardian 16–22 kW specification sheet. Other brands publish similar tables. Check your model.
Hours per tank size
| Tank (water capacity) | Usable at ~80% fill | Hours at 2.5 gal/hr |
|---|---|---|
| 120 gallon | ~96 gallons | ~38 hours |
| 250 gallon | ~200 gallons | ~80 hours |
| 500 gallon | ~400 gallons | ~160 hours |
| 1,000 gallon | ~800 gallons | ~320 hours |
Generator-only, at the Generac 22 kW half-load rate. Shared tanks last less.
How long your tank lasts
Propane tanks are normally filled to about 80 percent of capacity to leave room for expansion. A 500 gallon tank holds about 400 usable gallons.
- At 2.5 gallons an hour (a 22 kW unit at half load), 400 gallons lasts about 160 hours, roughly 6 to 7 days of continuous running.
- A 250 gallon tank (about 200 gallons usable) lasts about 80 hours at that rate.
- A 100 pound cylinder (about 23.6 gallons) lasts only around 9 hours. That’s why cylinders aren’t used for whole-house standby units.
If the same tank also feeds your furnace, water heater or range, the runtime is shorter. Many households also don’t run at half load all day, so real runtime is often longer.
A three-day ice storm, in gallons
A house with a 22 kW Guardian on propane loses power for 72 hours in January. The generator averages about 40 percent load, with the furnace, fridge, freezer, lights, well pump cycling and some cooking. That’s a little under the half-load burn rate, call it about 2.3 gallons an hour. Over 72 hours that’s about 165 gallons. If the same 500 gallon tank also feeds the furnace, which might use several gallons a day in that cold, the total could reach 180 to 200 gallons. Starting at 60 percent full (300 gallons), you’d finish the storm with about 100 gallons left. Starting at 30 percent, you’d run dry.
Propane used between outages
Standby units run a short exercise cycle on a schedule, often weekly, sometimes less frequently on newer controllers. These runs are at light load and short, so they use only a little fuel, but it adds up over a year. Check your controller settings.
Cold weather and vaporization
A tank must turn liquid propane into vapor fast enough to feed the engine. Smaller tanks in very cold weather may not keep up with a large generator at full load. Your propane supplier can size the tank for your generator and climate.
Planning your propane supply
- Keep the tank above about 50 percent going into storm season.
- Ask about automatic delivery, or a tank monitor that alerts you and the supplier.
- Know where the tank shutoff is.
- Clear snow from the regulator and tank.
Plans usually fall short in one of these ways:
- Sizing the tank for normal heating use only.
- Assuming the generator always runs at full load. Plan for typical loads, but have a margin.
- DIY regulator or line changes. That’s licensed gas work.
Using less during an outage
- Avoid running big electric loads you don’t need. Load-managed systems help automatically.
- Lower the thermostat a few degrees.
- Batch cooking and laundry so they don’t overlap with other heavy loads.
Propane use in brief
Is natural gas cheaper than propane to run a generator?
Often per unit of energy, but it depends on local prices. Natural gas also removes storage worries.
Can I switch my standby from propane to natural gas later?
Many models are field-convertible using the manufacturer’s procedure.
What does a tank monitor do?
It reports the tank’s fill level to your phone and often to the supplier, so deliveries happen before the tank runs low.
Plan run time with the fuel run-time calculator. See also how big a standby to buy, install costs and the fuel hub.
