Auxiliary Power Units (APUs): Diesel vs. Electric

Auxiliary power units (APUs) are small engines or generators installed in vehicles, typically large trucks or marine vessels, to provide power for functions other than propulsion.

While diesel APUs have been used for decades, electric APUs are a newer technology that offers some advantages. This article will compare diesel and electric APUs’ operating principles, efficiency, emissions, noise, maintenance, and costs.

How Diesel APUs Work

How Diesel APUs Work

Diesel APUs are essentially small diesel engines that can provide power independent of the main engine. In a semi-truck, the diesel APU allows the truck’s cab to be heated, cooled and electrified when the main engine is off. This avoids having to idle the large diesel engine just to run basic systems.

The diesel APU has its own fuel supply and alternator to generate electricity. Diesel APUs typically provide 3-15 kW of electrical power, heating, and air conditioning. They allow trucks to avoid idling their main engines at rest stops, reducing fuel costs and emissions.

Also read: Maximize Fuel Efficiency With Apus In Trucking

How Electric APUs Work  

How Electric APUs Work

Electric APUs also provide power, heating, and air conditioning without running the vehicle’s main engine. But instead of having their own engine, they use a battery bank that the vehicle’s main engine charges when running.

The batteries can power the APU’s generator when the main engine is off. Electric APUs also have a connection that allows the battery bank to be charged by plugging into an electrical outlet.

Also read: Are Truck Auxiliary Power Units Apus Worth It?

Efficiency Comparison of Diesel and Electric APUs

  1. Fuel efficiency

Electric APU systems are generally more efficient than diesel units. Whereas diesel APUs burn fuel to generate power directly, electric APUs store energy from the main engine in batteries and use it much more efficiently. Electricity can be delivered on demand with fewer energy losses. The higher efficiency means electric APUs use less fuel overall.

  1. Emissions Reduction with Electric APUs

Reducing air pollution is a major benefit of electric APU adoption. Diesel APUs produce emissions, including particulate matter (PM), nitrogen oxides (NOx), hydrocarbons (HC), and carbon monoxide (CO).

Electric APUs produce no direct emissions. This allows vehicles equipped with them to avoid idling their larger engines, which cuts down on emissions significantly, especially in urban areas. Many cities are encouraging electric APU use for this reason.

  1. Noise Reduction with Electric APUs  

Excessive noise pollution is another downside of diesel APU use. The small diesel engines run at high RPMs and produce 60-75 decibels noise levels. Electric APUs have a very quiet operation, producing only about 45 decibels.

This significantly reduces noise impacts from idling vehicles and improves the quality of life for those nearby. The quiet operation also provides a better rest environment for drivers trying to sleep. 

  1. Maintenance Differences

Routine maintenance is critical for both diesel and electric APUs. Diesel units require oil changes, air, fuel and oil filter changes, inspections for leaks or damage, and replacement of belts and hoses.

Electric APUs need far less routine maintenance besides checking fluid levels and ensuring tight connections. The battery banks do degrade over time and need replacement, but lifespans typically exceed 5 years. Overall, electric APUs have lower maintenance requirements.

  1. Cost Comparison

Upfront purchase costs for electric APUsare typically higher than comparable diesel units by $3,000 or more, depending on specifications. However, the savings from reduced fuel consumption generally provide a return on investment in less than 2 years.

Considering the value of lower emissions and noise, many fleets find the electric APU’s benefits outweigh the higher initial investment. Costs for both systems can be offset by tax credits up to $3,500 for electric APUs and $1,000 for diesel units under the Inflation Reduction Act.


Which type of APU is right for me?

The best type of APU for you will depend on your individual needs and budget. A diesel APU is a good choice if you are looking for a reliable and affordable APU. However, if you are looking for a quiet and emission-free APU or need an APU that can be powered by shore power, then an electric APU is a better choice.

What are the benefits of using an APU?

There are some benefits to using an APU, including:
Fuel savings: APUs can help you save fuel by eliminating the need to idle your truck’s engine.
Improved driver comfort: APUs can provide air conditioning and heating while the truck is parked, improving driver comfort and productivity.
Reduced emissions: Electric APUs are emission-free, which can help to reduce your environmental impact.
Increased resale value: Trucks with APUs are typically worth more when you sell them.

How long can an APU run?

The runtime of an APU will depend on the type of APU, the size of the battery, and the amount of power you use. Diesel APUs can run for over 10 hours, while electric APUs typically run 6-8 hours.

How much does an APU cost?

The cost of an APU will vary depending on the type of APU, the size of the battery, and the features you choose. Diesel APUs typically cost around $5,000-$10,000, while electric APUs typically cost around $10,000-$15,000.

Where can I buy an APU?

APUs can be purchased from various sources, including truck dealerships, online retailers, and APU manufacturers.

How do I install an APU?

A qualified technician should install APUs. The installation process will vary depending on the APU type and your truck’s make and model.

What are some common APU problems?

Common APU problems include:
Dirty fuel filters
Clogged air filters
Low oil levels
Overheated engines
Dead batteries

What should I do if my APU breaks down?

If your APU breaks down, you should contact a qualified technician to repair it. Do not try to repair the APU yourself, as this could be dangerous.


Electric APUs for auxiliary power in vehicles have compelling fuel-efficiency advantages, reduced emissions and noise, lower maintenance, and faster ROI than diesel units. As electric technology continues improving and costs decrease, the adoption of electric APUs is expected to accelerate. Their ability to cut fuel use and pollution without the drawbacks of diesel operation will make electric APUs a key strategy for sustainable transportation. Companies making the switch are already seeing the benefits.

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Henry Hunter

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