Auxiliary power is any energy source on a vehicle, building, or device that handles everything except the primary function. On an airplane, the main engines provide thrust; auxiliary power runs the lights, air conditioning, and cabin electronics. On a ship, the main engine pushes through the water; auxiliary power keeps navigation equipment, pumps, and galley appliances running. The concept is the same everywhere it appears: a secondary power system that supports the systems the primary engine or power source wasn’t designed to handle.
How Aircraft Use Auxiliary Power
Commercial jets carry a dedicated auxiliary power unit, commonly called an APU, usually housed in the tail section. This small gas turbine engine can be started using nothing but the aircraft’s onboard batteries, and once it’s running, it provides two critical outputs: electrical power for cockpit instruments, cabin lighting, and avionics, and compressed air (called bleed air) that feeds the air conditioning packs and spins the main engines during startup.
Without an APU, an airplane sitting at the gate would need external ground equipment to supply electricity, cooled air, and high-pressure air for engine start. The APU makes the aircraft self-sufficient. It also serves as a backup in flight. If an engine flames out at altitude, the APU can supply bleed air to help relight it. During takeoff from hot, high-altitude airports where every bit of engine performance matters, crews sometimes run the APU to handle air conditioning so the main engines can devote full power to thrust.
A typical narrow-body jet APU produces around 45 kilowatts of electrical power. Honeywell’s RE220 unit, one of the more capable models in its class, delivers 60 kilovolt-amperes of output from ground level up to 45,000 feet, enough to handle heavier electrical loads than many competing designs.
Auxiliary Engines on Ships
On commercial vessels, the main engine has one job: propulsion. A separate set of auxiliary engines generates electricity for everything else onboard. Navigation systems, radar, communication equipment, lighting, water pumps, refrigeration, and cooking equipment all run on power from these auxiliary diesel generators. Without them, a ship could move through the water but couldn’t safely navigate, feed the crew, or maintain cargo conditions.
The distinction gets interesting on diesel-electric vessels, where the engines generate electricity that then drives electric propulsion motors. Even though these engines technically contribute to movement, they’re still classified as auxiliary because their direct output is electrical power rather than mechanical thrust turning a propeller shaft. Formally, an auxiliary engine is any diesel engine on a vessel that provides power for functions other than propulsion or emergencies.
Backup Power for Hospitals and Buildings
Hospitals rely on auxiliary power systems to keep patients alive when the grid goes down. Engine-driven generators, typically running on diesel fuel, are the standard choice for facilities with heavy electrical loads supporting life-support equipment, surgical lighting, HVAC, and other critical systems. These generators can run indefinitely as long as fuel is available, making them the backbone of hospital emergency power.
The transition has to be fast. Industry standards require backup generators to reach full output within 10 seconds of a power interruption. To cover that brief gap, hospitals use uninterruptible power supplies (battery-based systems) that kick in instantly and hold the load until the generators spin up. This layered approach means ventilators and monitors never lose power, even for a moment.
Military Vehicles and Silent Watch
Modern tanks and armored vehicles carry far more electronics than their predecessors: targeting systems, communication arrays, climate control, fire suppression, and chemical detection equipment. Running the main engine just to power these systems burns enormous amounts of fuel and generates heat and noise that make the vehicle easy to detect.
Auxiliary power units solve this by supplying electrical, hydraulic, and pneumatic power while the main engine stays off. This capability, known as “silent watch,” lets a vehicle sit in position with all its electronics active while producing minimal acoustic and thermal signatures. The result is lower fuel consumption, longer mission endurance, and better survivability. A unit like Nero Industries’ A20K powers everything from weapon systems and communication equipment to onboard climate control, all without firing up the primary engine.
Electric Vehicles and the 12-Volt Problem
Electric cars have a large high-voltage battery pack (typically 400 or 800 volts) that drives the motor. But dozens of smaller systems, including headlights, windshield wipers, infotainment screens, door locks, and the computer that manages the car itself, still run on a conventional 12-volt system. An auxiliary power module bridges this gap, stepping the high voltage down to 12 volts and keeping a small lead-acid or lithium 12-volt battery charged. If this module fails, the car can have a full main battery and still be unable to start, because the computers that control everything run on that low-voltage side.
Computers and Standby Power
Even a desktop computer has a form of auxiliary power. When your PC is “off” but still plugged in, its power supply continues to deliver a small amount of electricity on a rail called +5V standby. This trickle of power keeps the motherboard alive enough to respond when you press the power button or when a scheduled wake event occurs. It also powers USB ports that charge your phone overnight while the computer sleeps. The standby rail stays active whenever AC power is connected, regardless of whether the system is running, sleeping, or shut down.
Spacecraft and Deep-Space Power
In deep space, where sunlight is too weak for solar panels to be practical, spacecraft rely on radioisotope power systems. These devices convert heat from the natural decay of a radioactive element into electricity, providing both thermal energy to keep instruments warm and electrical power to run them. NASA has used radioisotope thermoelectric generators on missions to the outer planets, where a solar-powered design would need impractically large panel arrays. Closer to the Sun, solar arrays paired with batteries handle auxiliary power needs, but beyond roughly Jupiter’s orbit, radioisotope systems become the only viable option for missions lasting years or decades.
The underlying principle across all of these applications is the same. Primary power handles the main task, whether that’s thrust, propulsion, or high-voltage drive. Auxiliary power handles everything else that makes the system functional, livable, or safe.