Diesel APU’s

Diesel APUs can provide cooling, heating, and electrical power to the sleeper cab while the truck’s engine is off. Some diesel APUs on the market today provide heating, bunk cooling and AC power for hotel loads, while others provide bunk cooling and AC power only and then also use a fuel-operated air heater for cab heat. All diesel APUs also charge truck batteries and can provide electrical power for a block heater if the engine is so equipped.

They generally burn between 0.1 and 0.5 gallons of fuel per hour depending on their design, the ambient temperature, the sleeper’s insulation, and the resulting HVAC load, along with the AC power being generated.



Provide cab cooling, heating, AC power for hotel loads, and battery charging

Can operate as long as the truck has fuel. This provides nearly an unlimited amount of time for cooling, heating, and AC power without the need to restart the truck engine. Battery HVAC systems are limited in the hours of cooling they can provide before truck engine restart and battery recharging is required. Also, their hotel load AC power capabilities usually are less than that of their diesel-powered counterparts

High Btu capacity allows operation in extreme temperatures. Colder weather reduces battery capacity thus negatively affecting hotel load AC power capabilities of battery HVAC systems, while diesel APUs can avoid those issues


Expensive to purchase, install and maintain compared to other idle-reduction technologies. Compared to battery HVACs, for example, diesel APUs require periodic oil and filter changes and have more parts and components to be maintained. A typical service interval for a diesel APU is every 6 to 12 months. With a battery HVAC system, batteries need to be changed approximately every 2 years unless and engine start/stop system is employed to keep the batteries from being discharged too often. It is likely that while diesel APUs need maintenance on a more frequent basis, the cost of that maintenance will be equal to or less than the cost of replacing batteries on battery HVAC systems

Due to their mounting location, diesel APUs are exposed to road debris, salt and spray, and need adequate frame space to be installed. Additional work is required at installation for an open-loop system

Need diesel fuel to operate and generate emissions and noise, while reducing fuel savings less than other idle-reduction technologies

Urban municipalities have zoning restrictions prohibiting the use of diesel APU. In California, for example, a fleet may need to add an expensive DPF to make a diesel APU compliant with that state’s specific emissions regulations. With the upcoming Greenhouse Gas Phase 2 regulations, the emissions regulations on diesel APUs are likely going to get tighter

Diesel APUs weigh between 450 and 550 lbs. when installed.  In addition, you have to add another 20 pounds for fluids. However the Energy Policy Act of 2005 allowed for a national 400-pound exemption for the additional weight of idle-reduction technologies on heavy-duty vehicles. In 2012 the Moving Ahead for Progress in the 21st Century Act increased the weight allowance to 550 lbs. However, each state can adopt the exemption at its own discretion and not all states have done so. The Office of Energy Efficiency & Renewable Energy has a map showing states that recognize the APU weight exemption

Fleet Survey Results

  • If a fleet does not use APUs, their idle percent tends to be in the 30% to 50% range
  • If a fleet uses APUs, idle percent is in the single digits to 20%
  • Fleets seem to be split on diesel APUs vs. battery HVAC systems with some fleets using both (but not on a single vehicle)
  • Use of auto start-stop systems is increasing
  • Almost everyone uses diesel fired heaters
  • Most fleets are looking for positive ROI, but most of them also think there is significant value in driver retention and hiring

Decision-Making Tools

Our team produced top-line tools to assist fleets in making their idle-reduction technology choice:

  • The confidence matrix summarizes the study findings and indicates NACFE’s confidence in the various idle-reduction technology solutions.
  • The Payback Calculator allows fleets to estimate the payback in months for various idle-technology solutions. Fleets input data into the form and the calculator uses the data along with the information gathered by the study team to quantify and benefits and consequences likely to be experienced by the fleet in terms of upfront costs and year-over-year costs.

In addition the report contains challenges and changes charts for each of the anchor technologies. The charts can be found in section 9 of the report.



As a result of research and interviews the study team reached the following conclusions concerning idle reduction in the sleeper cab market:

  1. Reducing idle is good for the environment, saves fuel and improves a fleet’s “green image.
  2. A 10% reduction in annualized idle percentage is worth about 1% in fuel economy that results in a $500 to $700 annual savings assuming 100,000 miles a year and diesel at $3 a gallon.
  3. Driver hiring, comfort and retention are mentioned in almost every conversation about idle-reduction technologies. Some have said that offering a good solution is a requirement for hiring and retaining drivers.
  4. There is no “one size fits all” solution to idle reduction.
  5. Drivers are still a very important part (If not the most important part) of successful management of idle times.
  6. Depending on “shore power” (outside electrical connection) to be available is not a reasonable idle-reduction solution for most fleets as the number of electrified parking spaces is extremely small compared to the number of trucks that need to be parked at any given time.
  7. Most suppliers of idle-reduction technologies mention that their solution helps minimize maintenance costs, particularly in the vehicle’s aftertreatment system. However, we have been unable to uncover any data that actually substantiates those claims.
  8. The answer to the question of the resale value of trucks equipped with idle-reduction technology is unclear.
  9. Maintenance of the vehicle’s battery connections and electrical systems is critical to getting the best performance out of idle-reduction technologies.