What an electric APU does and why trucks need one
An auxiliary power unit (APU) is a small generator mounted on a semi truck that runs the cab's heating, air conditioning, and electrical systems without running the main diesel engine. When a driver parks for a break or overnight, the APU powers the sleeper cab instead of idling the engine for eight or ten hours straight. This cuts fuel waste, reduces noise, and lowers emissions — which is why many states now restrict or ban engine idling during driver rest periods.
Traditional APUs run on diesel fuel, the same fuel as the truck's main engine. Electric APUs, by contrast, draw power from a battery pack mounted on the truck and recharge when the main engine runs. Some electric APUs also plug into shore power at truck stops or rest areas, similar to how an RV connects to a campground pedestal. The battery stores enough charge to heat or cool the cab for a full ten-hour rest period without the engine running.
Trucking companies install APUs because idling restrictions carry fines — often $300 to $1,000 per violation depending on the state — and because fuel savings add up quickly. A truck that idles eight hours a night burns roughly 1 gallon of diesel per hour, or 8 gallons per night. Over a year, that is thousands of dollars in wasted fuel. An electric APU reduces that cost significantly, though the upfront purchase and installation price is substantial.
Key Takeaways
- Electric APUs run on rechargeable batteries rather than diesel fuel, powering the cab's climate control and electrical systems during driver rest periods.
- Most electric APUs recharge when the truck's main engine runs and can also plug into shore power at truck stops to extend runtime.
- Idling restrictions in many states make APUs necessary to avoid fines, and fuel savings typically recover the installation cost within three to five years.
- Battery capacity, climate conditions, and power draw determine how long an electric APU can run on a single charge before the engine must start.
- Maintenance costs for electric APUs are lower than diesel APUs because they have fewer moving parts and no fuel injection system to service.
How the battery and charging system work
An electric APU system includes a large lithium or lead-acid battery pack, a charging circuit that draws power from the truck's alternator when the engine runs, and an inverter that converts DC battery power into AC power for the cab's air conditioning and other appliances. When the driver shuts down the main engine, the battery supplies power to the APU compressor and heating elements. The battery typically holds enough charge for 8 to 12 hours of moderate climate control, depending on outside temperature and how much heating or cooling the driver needs.
Recharging happens automatically when the main engine is running. The truck's alternator charges the APU battery the same way it charges the truck's starting battery, though the APU battery is usually larger and has its own dedicated charging circuit to prevent the starting battery from being drained. Some systems allow the driver to monitor battery charge level on a dashboard display, showing how many hours of runtime remain before the battery is depleted.
At truck stops and rest areas equipped with shore power pedestals, drivers can plug in a cable to charge the APU battery directly from the facility's electrical grid. This is especially useful in cold climates where heating demand is high and battery charge depletes quickly. Shore power charging is faster than engine-based charging and allows the battery to reach full capacity without running the main engine.
Fuel savings and operating costs over time
A truck that idles the engine for eight hours per night uses approximately 1 gallon of diesel per hour, totaling 8 gallons per night or roughly 2,920 gallons per year (assuming 365 nights on the road). At current diesel prices, which vary by region and season, that represents a significant annual expense. An electric APU reduces that consumption to near zero because the battery supplies all the power needed for climate control during rest periods.
The actual savings depend on local diesel prices, how often the truck idles, and how much the battery needs to recharge. A truck that runs its engine for 10 hours per day to recharge the APU battery will use more fuel than one that only runs the engine for 8 hours. However, most trucking companies find that the fuel saved during rest periods outweighs the extra fuel burned during charging, resulting in net savings of $2,000 to $4,000 per truck per year.
Installation and purchase costs for an electric APU system typically range from $8,000 to $15,000, depending on battery capacity and whether the truck already has the electrical infrastructure to support it. At $3,000 per year in fuel savings, the system pays for itself in three to five years. After that, the savings go directly to the company's bottom line. Maintenance costs are also lower than diesel APUs because electric systems have fewer moving parts and no fuel injectors or combustion chambers to service.
State idling restrictions that make APUs necessary
Many states have enacted laws that prohibit trucks from idling the engine for more than a set time period, usually 5 to 15 minutes, except in specific circumstances. California, New York, Massachusetts, and several others enforce these rules strictly. Fines for violating idling restrictions range from $300 to $1,000 per violation, and enforcement happens through roadside inspections, weigh stations, and complaints from the public. A trucking company that operates in multiple states must comply with the strictest rules in any state where its trucks operate.
Exceptions to idling restrictions typically include engine warm-up time in cold weather, running the engine to power auxiliary equipment like refrigerated trailers, and brief periods needed to operate safety systems. However, these exceptions do not cover the eight to ten hours a driver spends resting in the cab. An APU allows the driver to maintain cab temperature without triggering an idling violation, making it a practical necessity in states with strict enforcement.
Some states offer tax credits or rebates for trucking companies that install APUs or other idle-reduction technology. The amount and availability of these incentives vary by state and change year to year, so companies should check with their state's environmental or transportation department for current programs.
Battery capacity and runtime in different climates
Battery capacity is measured in kilowatt-hours (kWh), and a typical electric APU system carries 10 to 20 kWh of usable capacity. In mild weather, 15 kWh can power a cab's climate control for 10 to 12 hours. In extreme cold or heat, the same battery might only last 6 to 8 hours because the heating or cooling system works harder and draws more power.
Cold weather is the biggest drain on battery capacity. Heating a cab in subzero temperatures requires continuous power, and the battery loses efficiency in cold. A truck parked overnight in Minnesota in January will see much faster battery depletion than the same truck parked in Texas in spring. Drivers in cold climates often rely on shore power charging to extend runtime, or they run the engine periodically to top up the battery during long rest periods.
Heat also drains the battery, though less severely than cold. Air conditioning is less power-intensive than heating, so a truck in hot weather can usually run longer on battery alone. However, if the cab is parked in direct sun and the interior temperature rises before the driver starts the AC, the system has to work harder to cool down, consuming more battery power.
Comparison of electric APUs to diesel APUs and engine idling
| Method | Fuel Type | Annual Fuel Cost | Noise Level | Maintenance |
|---|---|---|---|---|
| Engine Idling | Diesel | $2,500–$3,500 | High | Engine wear |
| Diesel APU | Diesel | $800–$1,200 | Moderate | Fuel system, spark plugs |
| Electric APU | Battery (recharged by engine) | $500–$800 | Very low | Battery monitoring, inverter |
Engine idling is the cheapest option upfront because it requires no additional equipment, but it is the most expensive to operate and violates idling restrictions in many states. Diesel APUs are a middle ground: they cost less to install than electric APUs and can run indefinitely as long as fuel is available, but they still produce noise and emissions and require more maintenance.
Electric APUs have the lowest operating cost and the smallest environmental footprint, but they depend on battery charge and may not be practical in extreme climates without shore power access. For trucking companies operating in states with strict idling rules and regular access to shore power, electric APUs usually make the most financial sense. For companies that operate primarily in states without idling restrictions or in remote areas without shore power, a diesel APU or engine idling may still be the practical choice.
Installation requirements and truck compatibility
Installing an electric APU requires the truck to have adequate electrical infrastructure: a large alternator to charge the battery during engine operation, sufficient space for the battery pack (usually mounted under the cab or in a frame rail), and wiring to connect the battery to the inverter and the cab's climate control systems. Newer trucks often come pre-wired for APU installation, making the process simpler. Older trucks may need significant electrical upgrades, which can add $2,000 to $5,000 to the total installation cost.
The truck's sleeper cab must also have compatible heating and cooling systems. Most modern sleeper cabs use electric heating elements and AC compressors that work well with an APU inverter. Older cabs with engine-driven AC systems or fuel-fired heaters cannot easily be converted to electric APU operation and may require replacement or significant modification.
Installation typically takes one to three days at a truck service facility. During installation, the service technician will mount the battery, run wiring through the truck frame, connect the inverter, and test the system to may support it charges and powers the cab correctly. After installation, the driver should receive training on how to monitor battery charge, when to plug in at shore power, and what to do if the battery depletes during a rest period.
Frequently Asked Questions
Can an electric APU run the refrigerated trailer as well as the cab?
No. Electric APUs are designed to power the sleeper cab only. Refrigerated trailers require their own dedicated power source, usually a diesel-powered trailer APU or a plug-in connection to shore power at the truck stop. Some newer trailers can use battery power, but this is separate from the truck's cab APU system.
What happens if the battery runs out during a rest period?
The driver can start the main engine to recharge the battery and resume cab climate control, or they can plug into shore power if available. If neither option is available, the cab will gradually lose heating or cooling until the driver restarts the engine. Most systems alert the driver when battery charge drops below a certain threshold, giving them time to act before the battery is completely depleted.
Do electric APUs work in very cold climates?
Electric APUs work in cold climates, but battery capacity decreases in freezing temperatures and heating demand increases, so runtime is shorter. Trucks operating in extreme cold should have larger battery capacity and should use shore power charging whenever possible. Some companies in cold regions use diesel APUs instead because they can run indefinitely without battery limitations.
How long does it take to charge an electric APU battery at a truck stop?
Shore power charging typically takes four to eight hours to fully charge a depleted battery, depending on the charger's power output and the battery's capacity. Engine-based charging is slower and depends on how long the engine runs. Most drivers charge overnight at truck stops, allowing the battery to reach full capacity by morning.
Can I install an electric APU on an older truck?
Yes, but older trucks may need electrical upgrades to support the system, which increases installation cost. The truck must have a large enough alternator and adequate space for the battery pack. A truck service facility can assess your specific truck and provide a cost estimate for installation.