Propane generators burn fuel at different rates depending on load and engine efficiency. This calculator estimates your runtime based on tank size, actual power draw, and generator efficiency — not the misleading "up to X hours" on the marketing label.
Propane Fuel Consumption by Generator Size
| Generator Rating | Approx. Load (50%) | Propane Use (lbs/hr) | Runtime on 20lb Tank |
|---|---|---|---|
| 2,000W inverter | 1,000W | ~1.0 lb/hr | ~20 hours |
| 3,500W portable | 1,750W | ~1.5 lb/hr | ~13 hours |
| 5,500W portable | 2,750W | ~2.2 lb/hr | ~9 hours |
| 7,500W standby | 3,750W | ~3.0 lb/hr | ~6.5 hours |
| 12,000W standby | 6,000W | ~4.5 lb/hr | ~4.5 hours |
These figures assume 50% load, which is where most generators run most efficiently. At 25% load, fuel consumption drops but efficiency also drops — you burn more fuel per watt produced.
How to Estimate Your Propane Generator Runtime
- Find your propane weight. A standard BBQ tank holds 20 lbs of propane when full. Larger tanks for whole-house generators hold 100-500 lbs. Weigh the tank and subtract the empty weight (stamped "TW" on the collar) to find remaining fuel.
- Add up your loads. List every device you will run simultaneously. A refrigerator draws 150-400W (check the compressor rating, not the running average). Space heater: 1,500W. Sump pump: 800-1,500W starting, 400-800W running.
- Check your generator efficiency. One pound of propane contains roughly 21,660 BTU or about 6.35 kWh of energy. Portable generators convert 15-25% of that to electricity. Inverter generators hit 20-30%. Whole-house standby units reach 25-35%.
- Run the calculator. Enter your values above. Then add a 15% safety buffer — generators lose efficiency as altitude increases and temperatures drop.
Why Generator Efficiency Varies So Much
A propane generator is a heat engine with inherent thermodynamic limits. The small single-cylinder engines in portable generators convert about 15-25% of propane energy to electricity. The rest becomes heat and exhaust.
Inverter generators do better because they adjust engine speed to match the load. At half-load, a conventional generator still spins at 3,600 RPM burning nearly the same fuel as at full load. An inverter generator throttles down to 2,000-2,500 RPM, cutting fuel consumption by 20-40%.
Altitude matters too. For every 1,000 feet above sea level, expect a 3-4% power loss. A 5,500W generator at 5,000 feet effectively produces about 4,700W. If you live in the mountains, size your generator accordingly.
Frequently Asked Questions
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Generator runtime specs from manufacturers assume ideal conditions that rarely match the real world. Use this calculator with your actual load and tank size, then keep a 15% fuel reserve for cold snaps and altitude effects. For extended outages, track your actual consumption during the first few hours and adjust your rationing plan accordingly.
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Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.