Dual fuel
A dual-fuel generator runs on either gasoline or propane, switched by a selector and a regulator. Propane stores indefinitely and stays available when storms close gas stations, at the cost of roughly 10% less output than the gasoline rating.
Propane’s lower energy density per unit of fuel-air mixture is why the derating exists — a generator rated 7,500 W on gasoline typically delivers about 6,750 W on propane. Size against the propane figure if propane is your emergency plan.
Propane also burns cleaner (no carburetor gumming from stale fuel), which matters for a machine that may sit unused for a year between outages. The standard 20 lb grill cylinder holds about 4.6 gallons of propane and runs a mid-size generator for roughly half a day at moderate load.
| Output on propane | Roughly 8–12% below the gasoline rating, on both the running and the surge figure |
|---|---|
| Propane energy | ≈91,500 BTU/gal; a full 20 lb cylinder holds 4.6–4.7 gal (≈420,000–430,000 BTU) |
| Vapour draw limit | A 20 lb cylinder supplies roughly 60,000 BTU/hr in mild weather, far less when cold or nearly empty |
| Gasoline shelf life | 30–90 days untreated for ethanol blends, 12–24 months with stabiliser; propane does not degrade at all |
| Cylinder connection | QCC1 / Type 1 valve with a flow-limiting device — open the valve slowly or it restricts flow |
| Typical runtime | About 5 hours at half load from one 20 lb cylinder on a 7,000–9,000 W unit; 8+ at quarter load |
Why propane costs you output
An engine breathes a fixed volume per revolution. Propane arrives at the intake already a gas, so it occupies part of that volume and displaces air; gasoline arrives as a liquid and vaporises in the port and cylinder, taking up far less of the incoming charge. Less air per cycle means less fuel can be burned per cycle, and the result is roughly 8–12% less peak output. Combine that with jetting and timing that must be a safe compromise across both fuels, and you have the whole explanation. It is not a knock limitation — propane’s octane rating is above 100, so detonation is not what stops you.
The practical rule is to size against the propane figure whenever propane is the actual plan. A generator advertised at 7,500 running watts on gasoline is a 6,700–6,900 W machine on propane, and its starting rating derates by about the same fraction. That is often precisely the margin between starting a half-horsepower well pump and listening to it stall.
On the other side of the ledger, propane is the better fuel for a machine that is stored rather than used. There is no varnish in the carburettor, no phase-separated ethanol in the bowl, less oil dilution, and less carbon. The most common reason a stored generator will not start in an outage is a gummed carburettor after a season of stale gasoline, and running on propane removes that failure mode entirely. A dual-fuel unit stored with the fuel bowl run dry and a full cylinder beside it is about as reliable as a portable gets.
Cylinder logistics: vapour draw, cold weather, and runtime
Propane leaves a cylinder as vapour, and vaporising liquid propane takes heat drawn from the remaining liquid and the tank wall. Draw rate is therefore limited by tank surface area, liquid level, and ambient temperature, not by the regulator. A 20 lb cylinder supports roughly 60,000 BTU/hr in mild weather with a decent fill; below freezing, or when the cylinder is down to its last quarter, that figure falls sharply. The symptom is a generator that runs fine for an hour and then starts surging and dropping voltage as the regulator starves, and frost forming on the cylinder is the visible tell.
The fix is more tank, not a different regulator. A 100 lb cylinder holds 23.6 gallons, supports well over 100,000 BTU/hr, and gives roughly five times the runtime; a fixed 250 or 500 gallon tank ends the question permanently. Two 20 lb cylinders manifolded through a changeover regulator both raise the available draw and let you swap one while the other keeps running — the only way to refuel a propane generator without shutting it down, and a genuine advantage over gasoline in a long outage.
Runtime arithmetic is simple division: usable gallons ÷ published gallons per hour at your load fraction. A 7,000–9,000 W dual-fuel at half load burns roughly 0.8–1.0 gal/hr, so a single 20 lb cylinder is about a five-hour evening, not an overnight. Plan on at least three cylinders for a multi-day outage, and understand that exchange cages within twenty miles of a storm track are empty inside a day.
Storage, cylinder expiry, and the conversion-kit question
Storage is a safety matter with clear rules. Cylinders live outdoors, upright, in ventilated shade — never in a garage, basement, crawl space, or vehicle passenger compartment, because propane is heavier than air and pools in low spaces rather than dispersing. Cylinders also expire: US cylinders must be requalified twelve years after the manufacture date stamped on the collar and roughly every five years after that, and a dealer will refuse an out-of-date cylinder even though an exchange cage may quietly hand you one.
Aftermarket conversion kits that add propane to a gasoline-only generator do work, and they are also a modification of a certified engine’s fuel system. The Clean Air Act prohibits tampering with emission controls, and a fuel conversion needs its own EPA certification — plus a CARB Executive Order in California — to be legal on a certified engine. Most inexpensive kits have neither, void the engine warranty, and disclose both facts in the fine print. A factory dual-fuel generator is certified as sold, and that is the clean way to the same capability.
Switching fuels without breaking anything
The fuel selector must be set before starting, not while running, and most manuals specify a starting procedure for each fuel — often priming on gasoline in deep cold, because propane vaporises poorly at low temperature and cold-starting on it can be genuinely difficult. Read the sequence once in daylight rather than at 2 a.m. in a storm.
The cylinder valve deserves attention. Modern QCC1 (Type 1) valves contain a flow-limiting device that clamps the flow to a trickle if it detects what looks like a ruptured hose. Opening the valve quickly against a generator’s demand regulator can trip it, and the symptom is an engine that starts and immediately starves. The remedy is to close the valve, disconnect, wait a few seconds, reconnect, and open the valve slowly with the generator off.
Frequently asked questions
How long will a generator run on a 20 lb propane tank?
Divide the cylinder’s 4.6 gallons by the generator’s consumption at your load. A 3,500–4,000 W unit at half load burns roughly 0.4–0.6 gal/hr, giving eight to eleven hours. A 7,000–9,000 W unit at half load burns roughly 0.8–1.0 gal/hr, giving about five. An exchange-cage cylinder filled to 15 lb holds only about 3.5 gallons, so knock a quarter off those figures.
Is it better to run a generator on gasoline or propane?
Gasoline gives about 10% more output and usually costs a little less per unit of energy at retail. Propane stores indefinitely, does not gum a carburettor, stays available when gas stations lose power, burns cleaner, and can be swapped mid-outage using a changeover regulator. The common answer for a home backup generator is propane as the default and gasoline held in reserve for the day you need the extra output.
Does running on propane damage a generator?
No — it is generally easier on the engine, with less carbon, less oil dilution, and no fuel-system varnish. The one real caveat applies to high-hour industrial use: gaseous fuels provide less valve-seat lubrication than gasoline, and engines that accumulate thousands of hours on propane can see faster exhaust valve recession. A home standby or portable generator will never accumulate the hours for that to matter.
Can I convert my gasoline generator to run on propane?
Physically yes, legally it depends on the kit. Modifying a certified engine’s fuel system is regulated tampering unless the conversion kit holds its own EPA certification for that engine family, plus a CARB Executive Order in California. Most cheap kits have neither and void the engine warranty as well. If dual fuel is what you want, a factory dual-fuel generator is certified as built and avoids the whole question.
Why does my propane generator lose power after about an hour?
Almost certainly vapour draw, not the generator. A 20 lb cylinder can only vaporise so fast, and as the liquid cools and the level drops that rate falls. The engine starves, the governor hunts, and voltage sags. Frost on the cylinder confirms it. Move to a 100 lb cylinder, manifold two 20 lb cylinders through a changeover regulator, or reduce the load.