How Long Does Propane Last?
Real runtime estimates for heaters, BBQs, and generators, with the math behind the numbers and the factors that change them.
Quick answer
A standard 20 lb tank holds about 430,000 BTUs. A 10,000 BTU/hr heater will run it for roughly 43 hours. A 20,000 BTU/hr heater gets about 21 hours.
How propane energy content works
Every propane tank has a fixed amount of energy in it, measured in BTUs (British Thermal Units). One gallon of propane contains approximately 91,500 BTUs of energy. Knowing a tank's gallon capacity and your appliance's BTU rating is all you need to estimate runtime.
Note that propane tanks are sold by weight (pounds), not volume. A 20 lb tank holds about 4.7 gallons of liquid propane. The math: 4.7 gallons × 91,500 BTU/gallon ≈ 430,000 BTUs total.
Common tank sizes and their energy content
| Tank size | Approx. gallons | Approx. total BTUs | Common use |
|---|---|---|---|
| 1 lb | 0.24 gal | ~22,000 BTU | Camp stoves, torches |
| 20 lb | 4.7 gal | ~430,000 BTU | BBQ grills, small heaters |
| 30 lb | 7 gal | ~645,000 BTU | RVs, medium heaters |
| 40 lb | 9.4 gal | ~860,000 BTU | Larger heaters, forklifts |
| 100 lb | 23.6 gal | ~2,160,000 BTU | Home heating, generators |
| 500 gal (buried) | ~400 gal usable | ~36,600,000 BTU | Home standby generator, whole-home heating |
Runtime by appliance, 20 lb tank examples
| Appliance | Typical BTU/hr | Est. runtime (20 lb tank) |
|---|---|---|
| Camp stove (one burner) | 7,500–10,000 | 43–57 hours |
| BBQ grill (medium heat) | 18,000–25,000 | 17–24 hours |
| Portable heater (small) | 9,000–12,000 | 36–48 hours |
| Portable heater (large) | 18,000–30,000 | 14–24 hours |
| Torpedo/construction heater | 60,000–150,000 | 3–7 hours |
| Dual-fuel generator (3,500W, ~50% load) | ~90,000 | ~5 hours |
| Dual-fuel generator (7,500W, ~50% load) | ~130,000 | ~3 hours |
Generators are the most propane-hungry appliances on this list by far. A 20 lb tank is not a realistic long-term fuel source for generator use, a 100 lb tank or a larger buried supply is a much more practical choice for backup power.
What affects how fast propane burns
BTU ratings on appliances are maximums at full output. Actual consumption varies quite a bit in practice:
- Thermostat cycling: A propane heater with a thermostat doesn't run continuously. It fires at full BTU until the space is warm, then cycles off. In a well-insulated room, a 30,000 BTU heater might only run 30–40% of the time, tripling the effective tank runtime.
- Load level: For generators, fuel consumption scales with load. A generator running at 50% of its rated wattage burns roughly 50–60% of the fuel it would at full load. Running lighter loads extends runtime significantly.
- Cold weather: Cold temperatures reduce the vapor pressure inside propane tanks. Below about 0°F (-18°C), the tank may struggle to maintain adequate pressure for high-BTU appliances, limiting output. Keeping tanks in a slightly warmer location (like a shed rather than outdoors in bitter cold) helps, but never bring them indoors.
- Wind: Outdoor heaters and camp stoves lose significant efficiency in wind. A windscreen can meaningfully extend tank life for outdoor cooking.
- Appliance efficiency: Older or poorly maintained appliances often burn more fuel than their rated BTU suggests due to incomplete combustion or heat loss.
- Tank fill level: A tank that reads "half full" on a gauge is actually not always exactly half, propane gauges are notoriously imprecise. Weigh your tank for an accurate reading (tare weight is stamped on the collar).
Propane for generators: planning a multi-day supply
If you're using a dual-fuel portable generator on propane for emergency power, a single 20 lb tank won't last long. Here's a practical planning framework:
- A typical 5,500–7,500W dual-fuel generator at moderate load burns roughly 1–1.5 lbs of propane per hour
- Running 8 hours/day × 1.25 lbs/hr = 10 lbs/day, or roughly one 20 lb tank every two days
- For a 7-day outage at 8 hrs/day runtime, you'd need approximately 3–4 full 20 lb tanks
- A 100 lb tank (23+ gallons) provides far better value and far fewer refill trips, roughly 10–14 days at moderate generator use
Propane stores indefinitely when properly sealed, unlike gasoline, it doesn't degrade. This makes it an excellent long-term emergency fuel choice if you have the right tank size and safe outdoor storage.
Checking how much propane you have left
The most reliable method is to weigh the tank. The tare weight (empty weight) is stamped on the collar. Subtract the tare weight from the current total weight, then divide by 4.2 (lbs per gallon of propane) to get remaining gallons.
Gauge-based readings on portable tanks are rough approximations at best. For small tanks like the 20 lb grill size, the warm-water-and-hand method works as a quick field check: pour warm water down the side of the tank and feel where it transitions from warm to cool, that boundary is approximately where the liquid propane level sits.
Simple takeaway
Propane runtime comes down to one equation: tank BTUs divided by appliance BTU/hr. A 20 lb tank is great for grilling and short-term heating but burns through fast on a generator. For emergency preparedness, a 100 lb cylinder or larger buried tank gives you practical multi-day capacity. And unlike gasoline, propane stored properly never goes stale, what you buy today is just as good years from now.