What is a Truck Auxiliary Power Unit (APU)?

An APU unit or an Auxiliary Power Unit is a game changer for the trucking industry. It is a small, efficient motor that is capable of producing the power needed to keep the air/heat running, interior appliance power, and anything else you require all without the need to idle your truck’s engine. In the past when long haul drivers had to stay overnight they had a couple of choices. They could either idle their engine to keep the cab warm or cool, they could find a hotel room, or they could try to sleep through the winter’s cold or the summer’s heat without any air conditioning or furnace.

As you can imagine most truckers opted for idling their engine to provide them with some form of comfort while they slept. I would have done the same as I cannot sleep in a hot environment. We have all become used to the creature comforts of air conditioning and heating. The problem with this is years ago different states began introducing anti-idling laws. The introduction of these laws brought the innovation of APUs into the trucking industry.

I am sure we are all familiar with anti-idling laws. These laws are designed to reduce air pollution. Unfortunately here in the United States each state can have a different anti-idling law then the next. You could be in compliance in Kansas City, Missouri and then cross the state line in a few miles and be fined in Kansas City, Kansas. As I write this article there are thirty states with varying anti-idling laws. It is difficult to keep up.

In short the laws are designed to prevent idling of engines on specific types of vehicles. In nearly every case heavy duty trucks fall into these categories. So, now the trucker is left with the choice of idling his engine during the night to stay warm/cool and risk being caught and fined OR he can look at investing into an APU. When I say invest into an APU I mean invest. APUs are not cheap, but they are also one of those things that once you have it you will never want to go back.

Auxiliary Power Units

There are various types of APUs brands and manufacturers out there today. Typically though an APU comes with four main components: The APU engine itself, the air conditioner, the furnace, and the interior control panel. You will also have various electrical harnesses, ductwork, and other tubing to connect everything. First, lets look at the engine. In essence an APU is just like a portable generator that you would bring with you if you were on a camping trip. The engine section of the APU is what generates the power through the alternator. The engine creates the energy which feeds into the alternator which rotates to provide power. This power is then distributed to your furnace, air conditioner, and other appliances that you wish to power. It is also worth mentioning that when the APU is running it will also charge your truck’s batteries as well to ensure they are ready to start back up.

The APU is typically mounted either just below your sleeper/cab next to the fuel tank or in some cases it is mounted right behind your cab wall next to the fifth wheel. The air conditioner’s condenser can be mounted in a variety of spaces but most often is found either on the side of the cab or on the ceiling of the cab. The furnace, evaporator, and control panel are mounted inside the cab. The control panel will function and look similar to a typical air conditioner/heater controls found in your cab.

APU Pros & Cons

The biggest pro when it comes to APUs is being able to generate cold or hot air whenever you need without having to idle your engine. This puts you in compliance with various state laws and also allows you to be comfortable in your cab while sleeping or while just having some time off. This alone is worth it to many truckers. Besides the obvious benefit there are a few others to look at though.

The APU is typically tied right into your truck’s diesel fuel tanks. However when the APU is running it will only use twenty to thirty percent of the fuel that your truck’s engine does while idling or running. That means an instant fuel savings when running your APU. Along with the fuel savings you will also save your engine from additional wear and tear. Idling is never good for your engine and by reducing the hours that you idle you also reduce the maintenance and servicing that your engine needs. This holds especially true nowadays with trucks made from year 2010 and up. As most of you know these trucks all come with a diesel particulate filter, or a DPF. Idling your truck not only increases the wear and tear on your engine but also on your DPF… and maintenance or replacement on a DPF is NOT cheap… that I can assure you.

I mentioned this earlier but the APU will actively charge your truck’s batteries as well. So, if you wish to power other electronics or appliances in your truck then you could also run a power inverter. A power inverter converts your trucks stored battery direct current (DC) power over to alternating current (AC) power. AC electricity is what all of our standard home appliances use such as microwaves, refrigerators, etc. Now some APUs come with built in power inverters and some do not. But, if your batteries are being charged by the APU then you do not have to worry about draining your batteries while running your power inverter.

There really is only one major downside when it comes to APU folks and that is price. Adding an auxiliary power unit to your truck is NOT cheap. These can range from five-thousand dollars all the way up to twelve-thousand dollars or more. This cost is assuming the purchase of the unit as well as the installation charge. It is because of this cost hurdle alone that many fleet owners do not put APUs in their trucks. This creates an overall bad experience for their company drivers and may cause them to end up leaving to another fleet that provides APUs for their drivers. In fact many drivers may reject a job offer if the truck they’ll be driving does not have a built-in APU.

There are ways to reduce the cost numbers that I stated above. Of course if you are an owner operator and you have the money you may spend twelve thousand and purchase the top of the line Thermo King TriPac APU. But, if you just want to purchase a middle of the road APU you can as well for a much cheaper price. If you really want an APU but just can’t swing the expense there are refurbished APUs out there that you can purchase from various APU houses. One such example of this is . Lastly, if you still can’t swing the cost then you can always look into what’s known as a ‘Red Neck’ APU. I will get into what those are in our next section.

Types of APUs

When APUs first started they were nearly all diesel or gasoline built engines. In fact they were very similar to portable generators that we use today for power emergencies or for camping. Over the years as technology has advanced different types of APUs have been introduced. In this section we are going to take a look at each type and which one would be best for your needs.


Even today the most common APU you are going to run into is diesel or combustion based. These include some of the big brand names in the APU market such as Thermo King, Carrier, Rigmaster, Cummins, Centramatic, and many others. A diesel APU comes with a specific set of advantages and disadvantages. The first big selling point is that a diesel engine can run indefinitely or until your fuel tank is empty. They can also typically power much more equipment then a standard electric APU.

The downside of diesel is that they can be loud. Now, not as loud as an idling truck engine, but still loud. In most cases though this isn’t a problem as a truck stop isn’t the quietest of places in the first place. The other downside of diesel is that it will require more maintenance then their electric counterparts. This is an engine that you are running and depending on the brand you will require maintenance between five-hundred to two-thousand hours of use. This can include changing the oil, belts, and other moving parts.

It is recommended that diesel APUs be used by long haul truck drivers. Say you have hit your hours for the day and need a place to stop for the night. Your diesel APU will be able to provide you power for the entire night without issue. In some cases if you have a twenty-four hour layover until your next pick-up/drop-off your diesel APU can come to rescue as well. Remember, that this APU uses only about twenty to twenty-five percent fuel then your standard truck engine does.


The concept of the APU is to give the truck driver an alternative means to power their vehicle other then idling their engine. Remember, that all of this was needed due to environmental concerns. The States saw their anti-idling laws as cleaning up the environment. Going with an electric APU is by far the most environmentally friendly of the APUs. There is no noise and no fuel consumed when using these battery powered APUs.

The big downside of these APUs though is that because they are battery powered they have a limited run time and can also only power so much before being maxed out. When you do run out of juice you have to wait hours before the system can be recharged again. The average electric APU can provide you with around ten to twelve hours of air conditioning time and a bit longer for heating (This excludes extreme conditions like Minnesota in January or Phoenix in August). You will also impact your run time if you decide to power other appliances such as a microwave, coffee maker, or even a television.

When the APU is drained and needs to be recharged it will typically take anywhere between six to twelve hours before the system is fully charged and ready to go again. If you couple this recharge time along with the limited run time and limited power supply it is recommended that electric APUs are used by small cab drivers on shorter trips. A long haul driver with a full sleeper would not have the best experience with an electric APU.

For an example scenario where this would be needed, say you are on site waiting to drop-off or pick-up but there is a one to two hour wait before you can even get tot he dock. This is a great opportunity to use the electric APU as you will only be using it for a few hours at a time. Then, when you start your truck back up the batteries will begin to recharge. Just remember that for a few hours these work great but if you plan on spending the night and running the APU for twelve hours then you are most likely going to need a diesel system. Also consider what climate you will be driving in mostly. If it is going to be an extreme climate like say Arizona or Maine then you should probably go with a diesel APU.


These systems did not get as popular as many folks would have hoped. The concept behind them is that they function as an electric APU until the batteries get too low on their charge and then the APUs diesel engine kicks on. The engine charges the batteries back to one-hundred percent and then the engine shuts off again. This process continues. The idea behind this is to not be running the diesel APU engine constantly and instead on an as needed basis.

This allows for fuel savings and a more environmentally friendly system. The downside here is that I could not find much literature or articles on the topic of hybrid APUs. The most recent article I found on this was back in 2006 and as you can imagine it was quite dated. If you know anymore about these types of models please reach out to me and I can update this article with more information.


Alright folks so we have covered the various ‘normal’ auxiliary power units out there. But now we can get into the fun stuff. As we mentioned earlier the cost to purchase an APU is quite high ranging from five-thousand dollars all the way up to twelve-thousand or more. Many folks do not have this type of money lying around and just are not able to afford these APUs, but they also want to be able to sleep in comfort during cold and hot nights. So, what do they do?

Well, these innovators come up with their own APU system. Remember how I said earlier that the APU is typically just a portable generator? Well in these innovative setups truckers will purchase their own portable generator and typically insert in an enclosure behind the cab close to the fifth wheel. I’ve seen some folks build custom enclosures for the generator to sit in. Others will have the generator boxed away until they need it and then set it up outside their cab with no enclosure.

No matter how you do it, you now have a portable generator running that can provide you with power. The next thing to do here is to route the extension cords from the generator into your cab. Now, inside your cab is the next step. Let’s say you are up in the north part of the country during winter and you need some heat. All you need now is to purchase a space heater and setup a place in your cab for it to sit. Then plug in the heater to your generator’s extension cord and bam you now have heat!

Air conditioning is a bit trickier as you need a way to expel the heat out of your vehicle. There are a few options here. You can purchase a portable air conditioner and set it up inside your cab with the exhaust port going out your passenger window. Just be sure to get this air tight or else all of the heat is going to come right back into your vehicle. This will take some playing around with it to get it just right.

You could also look at purchasing a window air conditioner and trying to mount it to your truck’s passenger window, but I would not recommend this approach as the unit could fall and break and it could also end up causing damage to your door or window. Not to mention having to mount and unmount this thing each and every time you wish to have air conditioning. It could end up being a pain.

The last option for air conditioning is installing an actual mini-split system. Mini-splits are much more expensive then a portable or window unit but they also provide you with a good source of air conditioning. These systems have two sections to be installed: One on the outside and one on the inside. They are then connected by copper refrigerant tubing. As you can expect this takes quite a bit of work to install and will have to stay on you truck permanently or until the air conditioner fails. For the sake of those on a budget I would suggest you get the portable air conditioner and play around with the exhaust tube and your window.

Now depending on the generator you have running you may have room to run other things in your truck along with the air conditioner or heater. Perhaps you have a refrigerator or microwave that you wish to run. It can all be done by using that same generator. I have seen some people do these redneck generator builds for as low as eight-hundred dollars. While you can get a heck of a lot of savings by doing this I do need to state that it is not the RIGHT way to do it.

I also need to put a disclaimer to protect myself here. We here at ToughAssTools do not recommend this approach as there are risks involved. If you decide to do this approach we are not liable for any damages to you or your vehicle that occur. It is always best to go with a true APU system that comes from the various manufacturers out there.


As you can tell by the length of this article I can go on and on about auxiliary power units until I am red in the face… and I will but I will save that for other articles. To close out this article though folks an APU is basically a built-in generator for your truck that provides you with an air conditioner, a furnace/heater, and power to anything else you wish to run. These APUs are meant to run while your truck is parked either while waiting to unload or while at an overnight stop. They can range in price from four or five-thousand dollars all the way up to twelve-thousand or more.

Having an APU is a comfort item for your drivers or for your vehicle if you are an owner operator. They are not necessary but man they can make a hell of a difference. I’m out of rural Kansas and I can tell you the summers get cruel here. I cannot imagine trying to sleep in my bed without my air conditioner keeping the house cool. I am sure without the AC I would eventually fall asleep but it would be a pain and it would not be comfortable sleep.

The same can be said about APUs. Sure, you can get by without them… but do you really want to? And, if you are in the boat where you truly just cannot afford it then maybe you look at that redneck approach we mentioned earlier? If you do decide to go that route be sure to be safe with it!

Thanks for reading folks,

Alec Johnson

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