A mid-size portable generator (roughly 5,000 to 8,500 running watts) burns about 2 to 3 lb of propane an hour at half load. That is why a standard 20 lb barbecue-style tank lasts about 6 to 8 hours, and a 100 lb tank runs 30 to 40 hours, before you are back at the exchange cage or the fill station.
Propane consumption is not fixed – it scales with how hard the engine is working, and not quite in a straight line. A generator idling at 25% load sips fuel to keep the engine turning over even when the alternator is barely loaded, so cutting the load in half does not cut consumption in half. Here is the working: propane holds about 21,600 BTU per pound. A small air-cooled generator engine converts roughly 20-22% of that heat into electricity, the rest is lost as heat and exhaust – so 1 lb of propane yields somewhere near 21,600 x 0.21 / 3,412 = 1.3 kWh of usable AC output. That efficiency number comes from published engine-testing standards for small gensets, not a figure we measured ourselves, and it is the reason the burn-rate table below looks the way it does.
Tank size against runtime, by load
The table below uses typical consumption figures published on spec sheets for propane and dual-fuel portable generators in the 7,000-8,500 running-watt class: about 1.7 lb/hour at 25% load, 3.0 lb/hour at 50%, and 5.5 lb/hour at 100%. A smaller inverter generator will sip less; a whole-house standby unit on the same fuel will drink far more. Treat this as a planning range, not a promise for your specific model – check the fuel-consumption table in your owner’s manual before you buy tanks.
| Tank size | Runtime at 25% load | Runtime at 50% load | Runtime at 100% load |
|---|---|---|---|
| 20 lb (standard BBQ exchange tank) | ~11.8 h | ~6.7 h | ~3.6 h |
| 33 lb (forklift/RV cylinder) | ~19.4 h | ~11 h | ~6 h |
| 100 lb (upright cylinder) | ~58.8 h | ~33.3 h | ~18.2 h |
| 420 lb (100-gallon home tank) | ~10.3 days | ~5.8 days | ~3.2 days |
Two things fall out of that table. First, running a generator at 100% load rather than 50% roughly triples the propane bill per hour of runtime, not doubles it – another reason to size a generator with headroom rather than one that maxes out at your expected load. Second, a 20 lb tank at half load gets you through most single-day outages; anything longer than a couple of days points toward a 100 lb cylinder or the 420 lb tank most propane-heated homes already have plumbed in. For what a generator that size can actually carry, see what a 3,500-watt generator will run and the full appliance wattage chart.
Why cold weather starves a propane generator
Propane is stored as a liquid under its own vapor pressure, and that pressure is what pushes gas out of the tank. Pressure falls as temperature falls: a 20 lb tank that reads about 145 psi at 70°F drops to roughly 58 psi at 20°F and can approach 0 psi as it nears -40°F, the point at which liquid propane boils at atmospheric pressure. Draw gas faster than the liquid can re-vaporize – which is exactly what a generator running at high load does – and tank pressure drops further still, sometimes enough to starve the regulator even with fuel left in the tank. This shows up as surging, stalling, or a generator that runs fine for twenty minutes and then dies. The fixes are mechanical, not electrical: keep the tank as full as practical (more liquid surface area vaporizes faster), use a larger tank so the draw rate per square inch of liquid is lower, or run a low-pressure hose from an insulated or heated tank enclosure. This is a propane-specific failure mode – it does not happen with gasoline or diesel, which stay liquid and simply get harder to atomize as they thicken.
Propane does not go stale – gasoline does
This is propane’s real advantage for standby use. Gasoline starts losing volatile components within weeks and is generally considered unreliable after 3-6 months unless treated with stabilizer, which is why a generator that sat all summer often refuses to start when the power finally goes out. Propane is a stable compound in its sealed tank: manufacturers and fire-code bodies do not assign it a shelf life, and correctly stored cylinders remain usable more or less indefinitely, limited only by the tank’s own certification and inspection interval, not the fuel inside it. If your generator sits idle for most of the year and gets called on once or twice during storms, that is the strongest practical argument for propane or dual-fuel over gasoline-only, even before weighing the running cost. It is also why propane and dual-fuel units feature so often in generator maintenance guidance aimed at people who want a machine that starts on the first pull after a year in the shed.
The trade-off: about 10% less power on propane
Propane’s energy density is lower than gasoline’s – about 91,500 BTU per gallon of liquid propane against roughly 114,000-120,000 BTU per gallon of gasoline, a gap of roughly 20-25% by volume. In practice the loss shows up smaller than that at the generator’s output shaft, because propane also burns slightly more completely and engine compression can be tuned around it. That is why most dual-fuel portable generators are rated for about 10% less running wattage on propane than on gasoline – a unit rated 7,500 running watts on gasoline is typically listed around 6,750 watts on propane. Size your load planning off the propane number if that is the fuel you intend to run day to day, and check your specific model’s spec sheet rather than assuming a flat 10% across every machine.
Propane vs a battery: the honest cost per kWh
Retail propane runs roughly $3-4 a gallon, or about $18-25 to refill a 20 lb tank depending on region and whether you exchange or refill. Combined with the 1.3 kWh-per-pound figure worked out above, that puts fuel cost alone at roughly $0.75-$1.00 per kWh of electricity actually delivered – before counting the generator’s purchase price, oil changes, or the wear of running an engine under load. A LiFePO4 power station has no fuel cost once you own it: charging it from a wall outlet at typical US residential rates runs closer to $0.02-0.04 per kWh stored, and the battery itself is rated for thousands of charge cycles, so its purchase price amortizes to a small fraction of a cent per kWh over its life. A unit like BLUETTI AC200L Portable Power Station stores 2,048 Wh for $899, and EcoFlow DELTA 3 Max Series Portable Power Station (2048Wh) holds 2,048 Wh for $799 – both roughly matched to what a 20 lb propane tank delivers to the wall over a several-hour outage, at zero ongoing fuel cost after the purchase.
That does not make propane the wrong choice. A generator refuels in minutes from a tank you can stockpile at a hardware store, and it does not care how long the outage runs – a battery is capped by its stored watt-hours and needs sun, a wall outlet, or a car alternator to top back up, which is exactly the case made in how a solar generator actually works. For an outage measured in hours, a battery is usually the cheaper and quieter answer, particularly for sensitive loads at the CPAP or TV and router end of the spectrum rather than heavy compressor loads. For an outage measured in days, propane’s energy density per dollar and its refuel-in-minutes convenience win, provided the tank stays warm enough to keep vaporizing. The full run-down of what each dollar buys across both fuel types and battery capacities is laid out in the power station price index, and the chemistry difference behind that battery math is covered in LiFePO4 vs lithium-ion.
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