Diesel vs. Propane Generators

When we moved into our off-grid home, the previous owners bequeathed us a 10-kilowatt (kW) diesel generator to back up an existing solar panel system. The system was woefully undersized, so the generator came on pretty much every day. It sounded like an 18-wheeler with no muffler, spewing thick black smoke through an exhaust pipe in the roof of its enclosure.

We gave that behemoth away (no one would buy it) and replaced it with a 22-kW Generac propane generator. That decision has been a mixed bag. The propane model runs cleaner with a noticeably loud hum instead of a much more annoying chug-chug-chug. But we use a lot more propane, and our energy bill has skyrocketed.

During the stormy winter of 2023, the generator ran constantly, and we needed an emergency propane delivery to keep it going. We were lucky the propane company fit us into its schedule.

Propane generators and modern diesel ones, which are quieter than the outdated workhorse we gave away, come with pros and cons. If you’re looking for a small generator for a camping trip or a week at Burning Man, you’re probably better off with a diesel model.

What Is Diesel?

Diesel is a liquid fuel like gasoline. Both are derived from crude oil, but diesel is less refined than gasoline, which makes it denser, less volatile and less combustible.

It’s so dense spark plugs won’t ignite it, so diesel engines don’t have plugs. Instead, compression in the piston chambers ignites the fuel. That means a diesel engine must have a more robust design than a gas engine.

That design increases its lifespan. In a car, a gas engine can be expected to last for 200,000 or 300,000 miles, but a diesel engine can go for 1,000,000 to 1,500,000 miles. The lack of spark plugs simplifies maintenance. Plus, diesel is safer to store than gasoline because it isn’t as flammable.

People long assumed diesel engines were better for the environment than gas engines. But a 2017 study conducted by the International Council of Clean Transportation in Europe suggests otherwise.

The study compared the emissions from a Volkswagen Golf diesel engine to those of a Volkswagen Golf gas engine, and found the diesel engine emitted 13% more carbon dioxide than the gas one. The U.S. Department of Energy reports propane engines emit 13% fewer greenhouse gases than gas engines. So putting the numbers together, diesel engines are significantly dirtier than propane engines.

What Is Propane?

Also known as liquid petroleum (LP) gas, propane is listed as a clean fuel in the Energy Policy Act of 1992. It’s derived from a combination of crude oil and natural gas and is even more refined than gasoline.

Propane is a gas at atmospheric pressure, so it must be stored under compression to maintain a liquid state. Because it’s so highly refined, it contains less usable energy than diesel or gasoline. That means you need more of it to keep your generator running for a specific amount of time.

What Are the Differences Between Diesel and Propane Generators?

When comparing diesel to propane generators, keep the following points in mind:

• Noise: Because they generate more torque per piston stroke, diesel generators may be louder, but the difference isn’t that much. For comparison, the sound level of the 3,200-watt Cummins Onan Diesel Generator is 72 decibels at 10 feet, while the 3,600-watt Cummins Onan Propane Generator runs at 68 decibels.
• Fuel consumption: According to Santa Energy, an 8-kW diesel generator uses about one-half gallon of fuel per hour. A 6-kW propane generator burns from one-and-a-half to two gallons per hour, or three to four times as much. The price of diesel fuel has skyrocketed in recent years (it’s currently about $6/gallon in California) while propane has remained relatively stable (about $2.50/gallon in California), which helps equalize operating costs.
• Fuel storage and delivery: A diesel generator comes with an onboard fuel tank, and diesel fuel is easy to store in approved containers. A propane generator has no onboard tank and must be connected by hose to an external one. If a propane tank holds several hundred gallons, the supply can last months depending on use. But a five-gallon bottle can easily run out in a day.
• Emissions and odors: Propane generators burn cleanly and produce no odors. Diesel generators produce thick, black smoke and noticeable odors. Most require the addition of diesel exhaust fluid (DEF) to control emissions, which reduces smoke and odors.
• Maintenance: Propane generators come with spark plugs and combustion system parts you must maintain and replace. Diesel generators don’t have plugs, and the combustion system is simpler. Moreover, diesel fuel lubricates the engine’s moving parts, so there’s less maintenance overall.

When Is a Propane Generator the Best Choice?

Many homeowners in the mountain community where I live use propane for heating and have a 500-gallon tank on site. For people like them, a propane standby generator makes a lot of sense. They don’t have to haul extra fuel onto the property. And because standby generators only work occasionally, they never worry about the tank running empty.

A diesel generator used in conjunction with DEF to control emissions makes more sense for people who need a full-time power source. Diesel fuel is easier to transport and store, and diesel engines last longer.

If you’re driving your RV to Burning Man or a campground, consider taking a diesel generator with you and save your onboard propane for cooking, heating and showering. A solar generator is an even better idea.