Propane vs Diesel Generators: Pros and Cons [9 Points]

Do you decide to buy a portable generator? However, even before having an idea of the size, or the brand, you need to know the type of fuel to use. Propane or diesel generators are quite different. Making the wrong choice is something that you want to avoid. Here propane and diesel generators are meticulously compared.

The choice between a portable propane generator vs a diesel generator depends on how the following 9 factors are relevant for its application:

1. Weight: propane ones have an external tank
2. Fuel Price: diesel sometimes cheaper
3. Fuel Availability: diesel easier to find
4. Fuel Storage: propane is the best
5. Noise: diesel occasionally better
6. Emission: propane way lower
7. Safety: diesel slightly better
8. Maintenace: propane easier
9. Start: cold start problem for diesel

Let’s dive into every single advantage and disadvantage. Remember, not all of them might be important for you as they might not be a problem for you or you just do not care for that specific aspect.

1 – Weight and Size

If space is a serious concern diesel generators are probably a better choice. Indeed, such generators come with an embedded tank that is embedded with fuel.

On the other hand, propane generators are not equipped with an embedded tank (as this will be quite dangerous). What you need to do for this type of generator is to connect an external propane tank. Hence, on top of the generator and the cables supplying your devices you will also have an extra cable with the propane tank. Something that you do not want especially if the area where the generator is located can, by accident, be accessed by not authorized people (if you work on a field) or very small kids (if in your garden).

However, when checking this generator out, I was wondering if their design affects their weight or size.

Do propane generators weigh more than diesel generators? The fuel type does not affect the weight of the generator, especially for portable ones. For instance, a 7000W rated power generator (excluding the tank filled with fuel) will still weigh around 200+lb independently from using propane or diesel.

For those of you looking around, you might also notice that nowadays the so-called “dual fuel” generators are becoming very common (for instance like the Pulsar on Amazon). These can run both on propane and gasoline. This is another direct evidence the engine is not that different for the two types of fuel.

Moreover, there were a few hardcore Youtubers that also attempted to convert a diesel (or gasoline) engine to propane (or natural gas). Despite being this possible, it is not easy and definitely something I would not recommend (considering also how expensive generators are nowadays in case you break yours).

The fuel was not found to affect the size of a portable generator. I did not notice any significant difference (again, excluding the propane tank) between the size of a diesel and propane generator. They both occupy a volume of a cube of around 23 inches for an average 5000W rated power for instance.

What about those of you camping? In these, I would avoid propane generators. The tanks will add a significant load. Go for a small size diesel generator.

2 – Fuel Price

As discussed by the USA EIA, the USA produces about 1.8 billion barrels of diesel compared to a consumption of 1.5 billion. The situation is even better for propane. Indeed, the USA is exporting more and more propane due to its growth in house production. So, both fuels are readily available in the USA.

However, what about the price.

Is propane price more stable than diesel? Propane and diesel price follows very similar price fluctuations. As discussed in this interesting comparison, diesel and propane, which are produced following similar processes, experience the same price fluctuations. The price of diesel is constantly rising from 1 to 3 dollars per gallon (for residential use) in the last 15 years with just a drop a few years ago. This is clearly illustrated by the EIA study.

Is propane more expensive than diesel? The price per gallon is comparable to diesel and changes throughout the year. However, this does not mean that propane generators involve the same cost to run as diesel generators.

Propane is around a third the price of diesel per amount of volume. The retail price of diesel is around $2.3 per gallon against the 2 to $2.8 of propane (considering domestic application). Hence, we can assume that propane

Are then propane generators cheaper to run than diesel generators?

Propane generators are more expensive to run than diesel generators when a fair assessment in which both the retail price and fuel energy density are considered.

Indeed, the price to run a generator does not depend only on the price of fuel per unit of volume but mainly on the amount of electricity that can be produced from that given volume of fuel.

Suppose that you buy 1 gallon of fuel (let’s call fuel A) that costs half the price of another fuel (let’s call fuel B). This does not mean that a generator using fuel A is cheaper to run. What about if fuel A (the half-price) produces a tenth of the energy produced by fuel B. Then the energy produced with fuel A is going to be way more expensive, even if the fuel is cheaper per volume.

Checking some more technical data, we know that 1 gallon of diesel can ideally produce at most 33kWh of energy. However, 1 gallon of propane can produce only 27kWh of energy. Hence, doing the math, the energy produced by one gallon of propane will cost 8.8 pennies per kWh (obtained as $2.4 divided 27kWh). However, for diesel, the cost will be 7.3 pennies per kWh (obtained as $2.4 divided 33kWh).

Of course, be careful. The price of each kWh of electricity you produce with your generator is going to be way higher than the 7c I calculated here for both fuel types. This is because, to make the technical analysis easy, I skipped the efficiency of the generator. Of those 33kWh of energy that 1 gallon of diesel has “inside”, the generator can only extract 20% of it. It sounds little, but yes, 80% of the energy of your fuel is lost.

So, a more realistic price of what you are going to pay for your kWh of electricity is 5 times higher (around 44c per kWh for propane and 36c per kWh for diesel). Hence, we can estimate that diesel generators are 22% cheaper to run.

3 – Fuel Availability

This is something you need to keep in mind when choosing a generator. You do not want to lose yourself in technical details in choosing a generator to be then unable to use it when needed because of the lack of fuel.

First do not confuse propane with natural gas. They are quite different types of fuels. This is important because propane, differently from natural gas, is not distributed through pipe networks.

However, this does not mean that it cannot arrive directly at your home. In many countries such as the USA and the UK, gas companies deliver at your footstep tanks of compressed liquid propane (LCP) ready to be connected to your generator. Once the tank is empty, they can also come to collect it next time when they bring a new one. Moreover, some petrol stations do offer the possibility to refill your empty propane tank. Next time you are queuing fueling your car, just ask.

The same cannot be said with diesel. I am not aware of any company delivering diesel at home. So, in such a case, you need to get out of the house and head towards the nearest diesel station. The good thing though is that you can have diesel with no delays and all the time (gas stations are open 24/7) compared to the few hours (or days) needed to wait for a propane delivery or finding a station able to fuel your propane tank (in working hours).

Hence, if fuel availability is one of your concerns, diesel generators are better given the ease it is to fund such a fuel almost everywhere, anytime.

However, if you are an RV user, then propane is not an issue. Indeed, many of your devices (such as refrigerators) run already on propane and its less availability (or extra effort to get it compared to diesel) is not a disadvantage anyway.

4- Fuel Storage – Shelf Life

Propane fuel in properly sealed tanks lasts way longer than diesel fuel. If having a safety backup of fuel in your garage to run in moments of emergencies, with no delays, then propane generators are very likely the best choice.

To understand why let’s see the three main categories of portable generator users:

1. Those people living off-grid for choice, pleasure (traveling with their RV for instance);
2. Those that need to work on-site;
3. Those that need to supply their main home appliances in case of an emergency;

If you belong to the third category, you will very likely be interested in how long you can store your fuel to be able to use when needed. In case of extreme weather conditions, the last thing you want to do is to get out of your house to the petrol station (that might be even not available, flooded, or plenty of other people just doing the same).

Without any special maintenance procedure, a simple tank of diesel will last around 1 year without undergoing major degradation. In a warm environment, it can last as little as 6 months.

Moisture and fungi growth (yes living forms can grow on diesel) are the most common problems. You can recognize old diesel easily not for its color change (as this depends also on the additive present) but for its odor and presence of fungi.

What about a tank of propane? How long does it last?

A tank of pressurized propane gas can last easily 20-30 years. This is around 20 times the shelf life of diesel. The main limitation to propane lifespan is actually the tank rather than the fuel itself.

Hence, if you are looking for a generator to use mainly in case of emergency, I would recommend propane generators. A propane tank will surely serve you well years from now when you need it the most.

5 – Noise

Propane generators are as noisy as diesel generators. The type of fuel is not one of the main factors that dictate how loud a generator is.

Here a catch though.

Many sustain that portable diesel generators are quieter. Those people are not lying. Their diesel generators are quieter just because they are very likely inverter generators. These generators are a special type of diesel (and even propane) generators. They are equipped with an extra special device (the inverter) that, essentially, allows them to run at the optimum speed (not the maximum) in case of a variable load.

Hence, what makes them quiet is the inverter, not the fuel. Propane inverter generators (a bit less common than diesel ones in my experience) are as quiet as the inverter diesel counterpart.

Another point to demonstrate this aspect is the conversion kits. Conversion kits are just a bunch of cables and plugs that you need to connect to an existing generator to change the fuel they are using. A Honda 2200i can be easily converted from (gasoline, yes not diesel, but the concept is the same) to propane! What changes is just the fuel that “explodes” within the cylinder. However, the engine does not change at all, the explosion of the gas (that causes the noise in the first place) happens in the same way and the cylinder (that contributes to the noise) moves as before. If you do not believe, check yourself the before and after fuel conversion in the video below (it is for gasoline, however, the same concept applies to diesel).

Hence, if noise is a major concern focus on buying an inverter generator. This can be either propane, gasoline, or diesel. The choice will hence depend on the other factors discussed in this article.

Of course, all the above might not apply to large backup generators. This discussion is focused on portable generators (those on wheels that you can carry around). Large backup units installed permanently on your garden might have more complex (and expensive) engines for which a difference in noise can be found.

Indeed, a few friends installer claims that large backup diesel generators units are noisier than their propane counterpart.

6 – Emission

Propane generators emit way less carbon dioxide, NO2 gases compared to diesel generators. Propane is the best option if, for any reason, they cannot be placed far enough to avoid their exhausted fumes to reach your house.

However, as an engineer, I was curious to know how environmentally friendly propane generators are. Do they pollute half, ten times less than diesel?

A study reported by eia found that propane emits 139 pounds (around 63kg) of CO2 for each million Btu, compared to the 161 pounds (73kg) of CO2 for the same 1 million Btu of diesel. Without getting into much technical detail a Btu (British thermal unit) is the energy produced by the fuel. This means that given the same energy (and so electricity) diesel produces 14% more CO2 than propane. It is a lot and important of course to reduce our carbon footprint, but it is not half as someone claimed.

However, fuel does not only produce CO2, but also others (despite in less amount) more dangerous chemicals. I am talking about Nitrogen Oxide gases for instance. These are under indicated as NOx or N2Ox family of gases if you have some basics of chemistry. These gases are quite bad for the environment by reacting with the air we breathe, producing ozone (O3) and acid rain [PDF].

Finding trustworthy quantitative information on the variation of the NO2 emission by switching from diesel to propane is quite difficult due to different type of engine, manufacturer and the fact such info is often omitted by the label “CARB compliant” (this means that they emit less of what is, by law, allowed).

However, this study [PDF] shows how by replacing diesel with propane with an engine goes down from 160 ppm to as low as 40 ppm. Hence, I would not take this with absolute truth (as the engine was probably from a car) but rather as an indication.

Hence, we can say that propane can, on average, be around 4 times less NOx pollutant than diesel.

In addition to the above, diesel also produces ashes. This is a very well known phenomenon that affects all the filters of diesel-based engines as discussed here. However, as the University of Wisconsin mentioned [PDF], this is not the case with propane that, once burned, produces virtually no ash.

Needless to say that they both produce carbon monoxide (CO). This is extremely dangerous because it is an odorless bioproduct so you can get gas poisoned without even realizing it. Despite propane producing less CO per gallon (compared to gasoline), it does produce enough to be hazardous. This is the reason why portable generators, independently from the fuel they use, should never (ever) be run indoors or outdoor close to windows or doors.

7 – Safety Handling

Both propane and diesel can cause poisoning and explosion if handled improperly. The main risk from handling such fuels comes from how they are spilled or released out the can (tank) that contains it. However, given the high damage that can arise from propane spillage, mainly due to its gas nature, such fuel can be more dangerous to handle.

The danger of diesel is that it can be easily spilled by accident in the environment. It is just a liquid stored in (hopefully) a sturdy plastic can. However, even if some droplets end in the case of your generator, on your clothes, this is not causing for major concern. Be sure to clean those droplets with a towel and wash your clothes (separately from others). Also, diesel does not ignite as quickly as gasoline or propane.

No need to say that if you drop the whole can of diesel on the floor, or somewhere, this can be a serious hazard also for the gas that diesel can release when it evaporates. You should avoid at all costs that anything hot or able to produce sparks pass nearby. Avoid what the guys below did (or did not)

Propane, on the other hand, does not spill out of the tank so easily. The tank is designed to avoid spillage as it can be extremely dangerous. However, when this happens, the risks of serious damages are way higher than in the case of diesel.

First, you cannot see a gas moving in the air while you can clearly see diesel on a surface. However, you can smell commercial propane. Indeed, as natural propane nature is odorless, a smell of rotten egg is added to make it detectable.

Second, gas is a terrible enemy in general. It tends to fill all the available space. One of the most common accidents is actually in transporting a propane tank in your car. The security valve, if you lay the propane tank horizontally, can by accident get slightly loose.

This might fill your car with gas. This is the worse place ever when this can happen as sparks and high temperatures of your car engine are a recipe for disaster. In such a case open all the doors, leave the car, and call the firefighters. To avoid problems, always keep the propane tank outdoors, and always upright (you can use some hard plastic support to help you with that).

Another hazard in propane handling is the line transporting the gas to the generator. If that line is broken (or the valve is not in perfect conditions) the consequences can be serious.

8 – Maintenance

Propane generators, given the stability of the fuel (does degrade dozens of times slower than diesel) and its cleanliness, are easier to maintain over time. Indeed, as a very rough rule of thumb, the dirtier the fuel, the more particulate it creates. Such particulates start accumulating within the engine (cylinder, exhaust stack) causing a blockage. This will cause a decrease in efficiency first and malfunction if the problem is left untreated.

Diesel portable generators need more attention and maintenance over time compared to propane generators. Among the most common problems that affect diesel generators, but not propane, are wet stacking, rubber, corrosion, clogged filters, and reduced efficiency.

Wet Stacking

Wet stacking is often caused by diesel generators run too often at low load. In this condition, the fuel does not burn properly causing a higher percentage of carbon to accumulate within the cylinder.

After a while, you will also see such particulate as a black and oily liquid in the exhaust stack (here the name “wet stack”). Such liquid is created by those dry carbon particles absorbing the moisture of the surrounding air exhaust gases.

Clogged Filters

Fungus growth, engine cloaking, and water separation are all common problems that might affect the engine if diesel is left for longer than a month in the generator tank.

In a diesel generator (as the tank is embedded) it is tempting to leave the fuel inside the tank once you have finished using it. This is something that you must avoid.

Indeed, diesel tends to develop the so-called sludge. This has the aspect of an algae slimy substance that tends to create clumps, adhere to the wall of the tank and the water. These slimy clumps tend to drop at the bottom and can easily clog filters when the vehicle is fueled.

So diesel should always be removed from the tank of a diesel generator. If the generator is going to be used within a month or less, the fuel can be left within the generator tank but using diesel stabilizers is recommended.

For security reasons also you need to take out the gas from a propane generator (by letting the generator run without a gas connection so to exhaust the gas left over in the engine). However, this is not related to damages to the engine, just for the security of future operations.

Corrosion

Diesel can corrode rubber components in the generator if not removed. The carburetor for instance (a small component that mixes air and fuel) has rubber and brass elements that the diesel can consume over time. You need to clean the carburetor (follow manufacturer guidance on the matter) before leaving aside a diesel generator for a long time.

Here we need to be clear. I am not stating that propane generators do not need any intervention. Portable propane generators share many common parts with their diesel counterparts. Hence, many of the problems that affect diesel do also affect propane and they do need regular maintenance (such as air filters, oil replacement, etc…)

However, the frequency of such intervention, given the same amount of working hours, is less for propane. Moreover, propane does not have all those additional maintenance requirements of a diesel generator due to the fast degradation of such fuel.

9 – Start

Diesel generators start having difficulty starting at temperatures below 30F (-1C). This is called the “cold start” problem. Hence, in a cold environment, propane (or gas-based generators) is a better option than diesel generators.

First, the evaporation process in diesel engines is heavily affected by the fuel temperature. For liquid fuel vaporization (the process in which your diesel needs to turn in vapor/small droplets) gets harder and harder as the temperature decreases. This is not a problem with propane as, once out of the tank, is in gas form.

The second factor that explains the cold start problem is the higher (than propane) temperature needed for diesel to start the combustion. Indeed, in cold weather the whole engine gets cold. Its cylinder, piston, and frame are cold. Hence, this makes it even more difficult for the diesel engine of your portable generator to reach the needed temperature (thousands of kelvin) through compression (of the air in the cylinder). Indeed, at first, that energy generated through compression gets lost in the cylinder, pistons rather than in the fuel/air mixture.

To note that the above problem might not apply to all diesel engines. Indeed, car diesel engines, especially modern ones, do not suffer (at least as badly as in the past) of such problems. However, a small portable generator of a few hundred dollars does not have the same quality as a car engine. So, do not expect a similar performance.

If you can manage to warm up the generator (place in a warmer environment, do never try to turn it on indoor). You can also purchase starting fluids and apply them to the carburetor of your portable diesel generator.

Takeaways

Hence, if you manage to go through all this discussion on propane vs diesel generator you should have made up your mind (or hopefully have a better idea) on what is best for your situation.

1. Propane is cleaner compared to diesel generators;
   
2. Portable propane and diesel generators have very similar noise levels. The use or not of an inverter is what makes much small portable diesel generators quieter than open-frame (no inverter) propane generators. It is not the fuel itself;
   
3. Propane is less energy-dense than diesel, so you need more space in case you want to store the fuel;
   
4. Propane can be stored for a decade or more in a cool environment. However, diesel after as little as a few months start deteriorating;
   
5. Diesel generators might undergo more maintenance than propane generators due to the high degradation of diesel fuel;
   
6. Propane generators might be more suitable for RV as propane is already in use for other appliances within the RV. Moreover, an RV generator is going to be placed closer to those living around the RV.

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