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APUs: Money-saver or pricey indulgence?

I mentioned some time ago that I felt an auxiliary power unit (APU) would be a wise investment that would hopefully save some money by reducing idling.

My problem is that I could not find any definitive data to help my decision.

I’m sure it makes financial sense, but a five-figure purchase really needs a bit of evidence to support it, and I cannot find anything anywhere from a trusted source.

Internet trucking forums would seem to be a good place to ask questions, but that didn’t help me at all. I received opinions, but I wanted cold hard facts, yet I got none.

So I decided it was about time that somebody did a definitive test and
got some numbers down for comparison. Does an APU make financial sense or is it just a luxury?

To do this I needed a whole year of running without one. Comparing
January figures alone would surely skew the results in favour of an APU and the same applies to August.

But what about the milder months? There is no need for heating or cooling all year-round, even in Canada, so is the APU just an extravagance for half the year?

And will the savings from the times when it is more cost-effective than idling outweigh the times when it just sits there hanging on the frame rails serving no purpose whatsoever?

My first anniversary of truck ownership in Canada coincides with this issue of the magazine and I’m finally getting an APU fitted.

I now have a full year of data without one, which I can compare to the results I get over the next year.

I am getting a full ECM download, which will tell me idle times, the number of diesel particulate filter (DPF) re-gens that have occurred while idling overnight and the fuel used while idling. I have idled when I’ve needed heating or cooling and I have idled when temperatures dipped below -15 C (to ensure my truck would start in the morning) and I have idled when I’ve used my microwave.

For the next year I will do the same, but instead of using the 13-litre engine, I will use the little one on the frame rails. My new cut-off point for idling will be determined by an experiment in the yard. I was thinking -25 C, but I’m sure that -30 C will be okay.

However I don’t want to take that chance out on the road, so doing it in the safety of the yard where I can plug in the block heater and warm it up a bit if it doesn’t work at lower temperatures is the safest method.

I know that using an APU will cut down on diesel particulate filter DPF re-gens as my truck does one every night it idles in winter.

In summer it’s different, but with colder temperatures or winter blend fuels it needs to re-gen after idling all night.

This is a real pain as it usually occurs just as I’m getting out of bed in the morning. I then have to let it do its thing for up to 40 minutes.

This increases the amount of fuel burned from idling significantly and the fact that a re-gen is required at all is confirmation that idling is not good for my engine and all the stuff that’s attached to it to save the world.

As this experiment is going to be conducted over a full year, the results are not going to be available until the end of the test. However I’m planning to get a monthly download of the ECM data to compare things on a like-for-like basis.

I’m pretty much convinced that the APU makes absolute sense during the winter months, the re-gens alone confirm that. The milder months and the summer will be the deciding factor in terms of whether there’s an overall economic advantage.

Of course there is more to it than just money – convenience is another bonus. At present, when I get back to the yard and park the truck, I have to take out everything that will freeze or spoil.

It may not seem like a big deal, but emptying the fridge and cupboards is an inconvenience.

I also get into a cold or hot cab when I return to work. It’s not the end of the world, but with an APU I can leave everything where it is and let the APU take care of maintaining a comfortable temperature inside the cab while I’m at home.

There are downsides, of course. The APU itself has a running cost and needs servicing.

It also takes up space on the frame and adds weight. I want mine mounted just in front of the drive axle on
the passenger side, but this will require the fuel tank to be cut down.

It will also mean the mounting brackets for my side skirts and my fuel tank straps will need modification.

I have determined that by moving things around a little I won’t lose
any fuel capacity and I will have the extra weight of the APU going on my drives, not my steer axle, which is already close to the legal limit.

The whole process is quite complicated – certainly a lot more than it appears on the surface – and hopefully my efforts will be of use to anyone thinking about fitting an APU.


A fourth generation trucker and trucking journalist, Mark Lee uses his 25 years of transcontinental trucking in Europe, Asia, North Africa and now North America to provide an alternative view of life on the road.

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1 Comment » for APUs: Money-saver or pricey indulgence?
  1. Jeff McCord says:

    Full Disclosure: I sell Espar Fuel Operated Heaters.
    Hello Mark;
    Love your methodology. Think about using a Fuel Operated Heater to provide engine-off heat? Standalone or as part of an APU system they are very attractive. The most popular APU’s use them as the heat component because they provide heat more efficiently than even the smaller diesel engine, are quieter, AND they minimize the hours the smaller diesel runs and by doing so adds years to the life of it. Then when you compare results, download the heaters ECM and compare them to the difference of the propulsion engines hours run, then you have the basis of how much it cost for engine-off heating and A/C. If you really want to know how much A/C cost, price the installation of the standalone heater and do the math. APU’s, Fuel Operated Heaters, Engine Idle Management systems are tools that when used to do what each component does best minimizes the cost of operating a truck. Know your business. Know your Cost. Make money!

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