According to a study by the American Trucking Association (ATA), the US trucking industry consumed over 44.8 billion gallons of fuel in 2020 alone. A significant percentage of the consumption is from truck idling, a common practice in the trucking industry in which drivers leave their engines running while the vehicle remains stationary. 

Drivers draw power from their trucks during rest periods for heating and cooling or to run different appliances in their cabs. By doing so, trucks consume hundreds of gallons of fuel which adds to the overall truck driver expenses

With fuel efficiency being a popular topic among carriers and owner-operators, many truck manufacturers have collaborated with fuel cell manufacturers to provide auxiliary power units (APUs) to the industry to serve as an alternative power solution for truckers to reduce idling. 

In this article, we’ll discuss auxiliary power units in detail and explain everything you need to know about them, from their types to cost, benefits, return on investment (ROI), and purchasing considerations. 

What Is an Auxiliary Power Unit (APU) In Trucking?

An auxiliary power unit (APU) acts as a power generator for semi-trucks and other freight vehicles. An APU device is installed in freight trucks and is capable of providing energy for heating, cooling, and other essential appliances and devices, such as laptops, phones, TVs, electric stoves, and more.  

thermo king apu on a truck

The core purpose of this device is to provide an energy-efficient alternative to truckers instead of relying on their engines to provide electrical power. In turn, this saves them money on fuel, lowers maintenance costs, and idling violation penalties. 

APUs come in different shapes and sizes. They typically look like sound-proof generators or enclosed engines attached to the side of a semi-truck and can weigh hundreds of pounds. This adds to the gross weight of the vehicle. 

APUs are not only more cost-effective but also serve as an eco-friendly alternative to truck idling. In other words, they can help save hundreds of gallons of diesel during layovers or waiting times and reduce your overall carbon footprint when hauling cargo. 

Types of APUs

Today, you can find two types of auxiliary power units in the market, with diesel combustion APUs being the pioneering technology used to reduce idling. Below, we’ll explore the two types of APUs in detail along with their benefits and drawbacks for truckers and carriers.

Diesel Powered

Diesel-powered APUs are the most popular type in the market. They enable truckers to power several appliances on top of heating and cooling systems without the need to keep the engine running.

In other words, you can run microwaves, televisions, power tools, and other appliances or machinery. The only downside is that diesel APUs are maintenance-heavy requiring regular oil changes and part replacements (filters, plugs, etc.).

From a cost perspective, the most significant benefit diesel APUs offer truckers and carriers are savings of approximately 75% in fuel consumption compared to idling (they consume about 0.1 to 0.5 gallons of diesel per hour).

Moreover, auxiliary power units can supply continuous power supply as long as you have diesel in the fuel tank. 

Electric Powered

An electric-powered APU works similarly to a battery-powered one and is installed on trucks to provide power for heating, cooling, and other appliances. However, unlike diesel APU, it doesn’t offer an endless supply of power. Instead, it delivers power until the batteries are fully depleted.

Most modern electric APUs can last for about 10-12 hours on a full charge and can be plugged into the truck engine or external power outlet for charging. They’re more affordable than diesel APUs and don’t require as much upkeep as their counterparts.

However, depending on your routine and energy requirements, they might not be the best option due to their storage capacity limits. They’re also prone to power disruptions and may require drivers to idle their trucks to charge them in certain cases. 

How Does an APU Work?

As mentioned earlier, an auxiliary power unit is installed at the side of a semi-truck and looks like a small enclosed generator. The device serves as a secondary power source and runs either on diesel or battery-powered, depending on the type.

how apus work on a truck

APUs are equipped with a capacitor that stores and releases sufficient power for heating, cooling, and appliances. They have varying functions, depending on their type, brand, model, and the truck they’re installed in.

Some units turn on immediately, while others can be turned on whenever needed. Electric APUs have an automatic shut-off sensor that switches the unit off when the voltage is low due to drained batteries. 

Similarly, a diesel APU is connected to a truck’s fuel tank that supplies diesel to a powerful 2-cylinder engine which produces AC and DC power. 

Cost Breakdown of an APU

A study by the North American Council for Freight Efficiency shows that diesel APUs consume between 0.1 to 0.5 gallons per hour. In contrast, when freight trucks idle, they consume around 0.6 to 1.1 gallons of fuel.

Auxiliary power units for trucks can cost around $7,000 to $12,000, depending on the type, quality, brand, and other factors, according to the American Transportation Research Institute. These numbers are also commensurate with our findings and when speaking to industry professionals.

There are several APU manufacturers are in the market, including Go Green APU, Thermo King, Carrier, and RigMaster Power. All of these brands offer different APU types, designs, and features.

For instance, Ringmaster’s newest unit costs about $9,500 without installation. Due to the high upfront costs of these devices, understanding the return on investment on APUs is essential for truckers.

In light of this, we have created a return on investment calculation for truckers, in which we explain the financial impact and yield of a typical diesel APU that costs $10,000.

ROI Simulation for Truck APUs

The table below is a simulation of how much fuel a truck saves by switching to an auxiliary power unit. It shows how much time it takes for the device to pay for itself:

Average Cost of an Auxiliary Power Unit (APU)$10,000
Average Annualized Idling Time1,800 hours per year
Fuel Consumption During Idling0.8 gallons of fuel per hour (Reported by the U.S. Department of Energy)
Fuel Consumption of APU0.1 – 0.5 gallons of fuel per hour
Fuel Saved Using APU0.3 – 0.7 gallons of fuel per hour
Average Fuel Cost in the U.S.$4 – $5 per gallon
Average Annualized Fuel Savings Using APU$2,160 – $6,300 per year
Return on Investments Timeline1.6 – 4.6 years

The first thing you’ll notice from the table is that a diesel-powered APU saves around 0.3-0.7 gallons of fuel per hour. In the long run, this amounts to around $2,160-$6,300 in annual savings.

With these numbers, a trucker can get a 100% return on investment anywhere between 1.6 – 4.6 years (depending on usage and fuel prices). However, these savings do not consider the cost of APU maintenance.

The good news is that by reducing idling, you can reduce the wear and tear on your engine caused by incomplete combustion cycles and keep your truck operational for much longer. 

A case study for electric APUs would show similar trends. However, there will be several differences in the approach. For starters, electric APUs are relatively cheaper and do not require any fuel.

As a result, you can achieve ROI much faster, usually between 8 months to a few years. However, there are certain drawbacks to using an electric APU compared to a diesel alternative.

You would have to account for several costs when using and maintaining electric APUs, such as the trade-off hauling time for charging your APU using a power outlet or the fuel used to charge the batteries while traveling or in idle mode. 

Benefits of Installing an APU on Your Truck

Installing an auxiliary power unit on your truck offers a host of lucrative benefits, including the following:

Better Fuel Efficiency

Diesel-powered APUs can help improve fuel efficiency during transit. You can save up to 90% of the cost of powering heaters, air conditioning units, TV, and other appliances when idling by burning through less fuel per hour.

Similarly, electric APUs consume slightly more fuel when charging but can store power to facilitate heating, cooling, and other energy needs without the consumption of additional fuel. 

Idling Regulatory Compliance

Nearly 50% of US states have strict idling regulations set by the Environmental Protection Agency’s transportation and air quality division. The core purpose of these regulations is to minimize carbon monoxide, nitrogen oxides, and particulate matter pollution.

Idling restrictions are also imposed to ensure energy security by minimizing the demand for fuel. By installing APUs in their vehicles, truckers can significantly reduce idling and rely on an alternative power source to meet their energy requirements. 

Reduced Truck Maintenance

One of the biggest downsides of truck idling is that it wears down your engine faster, due to incomplete fuel combustion cycles. This can damage several essential parts over time, including the cylinders and exhaust systems.

As a result, many truckers incur thousands of dollars in repair and maintenance costs. By opting for an APU, you can minimize or prevent the damage caused by idling. As such, parts would have a longer life span and require less frequent replacements with slower wear and tear, thus, reducing overall trucking expenses. 

Environmental Friendliness

According to the US Department of Energy, rest-period idling alone produces nearly 11 million tons of carbon dioxide, over 55,000 tons of nitrogen oxide, and hundreds of tons of PM2.5 solid particles or droplets annually in the United States alone.

By using APUs instead of your truck’s engine to power your HVAC and other devices, you can play a significant role in improving air quality and reducing the rate of environmental degradation. 

Considerations for Installing an APU

As you can see, auxiliary power units make a valuable addition to any truck. They provide an alternative energy source to truckers looking to power different appliances.

They also reduce the overall trucking expenditure by improving fuel efficiency and prolonging maintenance needs. However, there are certain things you need to consider before purchasing and installing one for your truck:

  • Cost of Equipment – APUs can be expensive. Even entry-level models in the market can cost over $7,000. So, before you purchase one, make sure you have enough working capital to afford and maintain one while waiting for its return on investment. 
  • Additional Gross Vehicle Weight – Auxiliary power units, especially eco-friendly ones with enhanced power storage, can easily weigh hundreds of pounds. From a cargo-hauling perspective, an APU’s weight can add to the gross vehicle weight forcing truckers to haul less to avoid exceeding the maximum limit. However, according to the Energy Policy Act of 2005, trucks with APUs can exceed the maximum limit by 400 pounds in their efforts to reduce fuel use and carbon/nitrogen emissions. Furthermore, a non-mandatory amendment by the US Government Publishing Office has increased the limit to 500 pounds. Truckers are still encouraged to double-check other states’ idling laws and regulations to ensure these exemptions are valid on their transport routes. 
  • Upkeep of Auxiliary Power Unit – APUs require more than a one-time upfront investment. Truckers must also ensure they have enough capital to meet their high maintenance costs. For instance, electric APUs require battery replacements every few years. Depending on the type, brand, and size, a typical battery can cost anywhere between $350 and $1,000. Furthermore, APU repair and maintenance experts charge more than regular technicians to ensure this sophisticated piece of machinery operates at optimum levels. 
  • Selection of Auxiliary Power Unit – Finally, you should choose an auxiliary power unit according to the environmental conditions and regulations of the states where you plan to use it. US states have different policies related to installing APUs on your truck. Some states don’t permit diesel-powered APUs. Similarly, states with extreme heat or cold conditions might require more powerful (non-electric) APUs to keep cabins cool or warm. 

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Andrew Lin

Co-Founder & Writer
at freightcourse

About the Author

Andrew is a multi-business owner with over 12 years of experience in the fields of logistics, trucking, manufacturing, operations, training, and education.

Being the co-founder of freightcourse has given him the ability to pursue his desire to educate others on manufacturing and supply chain topics.