The efficiency of a machine is the percentage of energy you put in that actually comes out as useful work. You calculate it with a simple formula: efficiency equals work output divided by work input, multiplied by 100%. A machine that receives 100 joules of energy and delivers 75 joules of useful work has an efficiency of 75%. No real machine ever reaches 100%, because some energy is always lost to friction, heat, or sound along the way.
How the Formula Works
Work, in physics, is what happens when a force moves something through a distance. When you use a machine, you put energy in (work input), and the machine delivers energy out (work output). The ratio between those two numbers is the machine’s efficiency.
For example, if you push a box up a ramp by applying 200 joules of effort, but only 160 joules go toward actually raising the box, the efficiency is (160 / 200) × 100% = 80%. The missing 40 joules were eaten up by friction between the box and the ramp surface. That lost energy doesn’t vanish. It turns into heat, warming the surfaces slightly. The total energy is conserved, but a portion of it becomes unusable for the task you’re trying to accomplish.
Why No Machine Reaches 100%
The second law of thermodynamics is the fundamental reason. It states that in any cyclical process, some energy will always transfer to the surrounding environment as heat rather than converting entirely into work. The only way to hit 100% efficiency would be if zero energy escaped as heat, and that simply doesn’t happen in the real world.
Friction between moving parts is the biggest culprit in mechanical systems, accounting for up to 33% of total energy losses in engines. In fact, friction consumes roughly one-third of the world’s total energy resources across all mechanical applications. Beyond friction, energy also escapes as vibration, sound, and heat radiated into the air. Even the drag of air moving around spinning components inside a motor (sometimes called windage loss) chips away at efficiency, typically contributing 8 to 12% of a motor’s total losses.
Simple Machines
The six classic simple machines (lever, pulley, wheel and axle, inclined plane, wedge, and screw) are useful for understanding efficiency because their losses come almost entirely from friction. In a frictionless world, every simple machine would be 100% efficient: you’d get out exactly the work you put in. In practice, adding more moving parts increases friction. A single fixed pulley might operate above 90% efficiency, while a compound pulley system with multiple wheels and ropes loses more energy at each contact point and drops lower. Levers tend to be among the most efficient simple machines because they have only one pivot point generating friction.
Internal Combustion Engines
Car engines are a striking example of how much energy machines waste. A modern gasoline engine converts only about 30 to 36% of the fuel’s chemical energy into useful motion at the wheels. Diesel engines do better, reaching 42 to 47% thermal efficiency because they compress fuel more tightly before igniting it.
Where does the rest go? Roughly 30 to 37% escapes as heat through the exhaust and cooling system. Friction inside the engine accounts for another 33%, and air resistance takes about 5%. By the time energy reaches the driving wheels, only about 12% of the original fuel energy is doing the job of moving the car forward. That’s why engines get hot, radiators exist, and exhaust pipes blow warm air.
Electric Motors and Electric Vehicles
Electric motors are far more efficient than combustion engines. Industrial motors rated at the international IE3 (premium) standard run at roughly 91 to 96% efficiency depending on their size, with larger motors performing better. IE4 (super premium) motors push even higher, reaching 96 to 97% efficiency at larger power ratings. The losses that remain come mostly from electrical resistance in the windings, friction in the bearings, and heat generated by magnetic fields in the motor’s core.
This efficiency gap explains why electric vehicles use energy so much more effectively than gasoline cars. In real-world driving tests, EVs showed a relative efficiency advantage of about 68% over combustion vehicles in mixed driving, rising to 77% in urban stop-and-go conditions. City driving favors EVs even more because they can recapture braking energy through regenerative braking, while a gasoline car converts all that kinetic energy into wasted brake heat.
Solar Panels
Solar cells convert sunlight into electricity, and their efficiency tells you what fraction of the sun’s energy hitting the panel actually becomes usable power. Standard commercial silicon panels today operate at roughly 20 to 22% efficiency. The current lab record for a silicon solar cell, set by LONGi in early 2025, is 27.8%. The gap between lab records and rooftop panels exists because manufacturing at scale involves cost tradeoffs, and real-world conditions like dust, heat, and imperfect sunlight angles reduce output further.
The rest of the sunlight is either reflected off the panel surface, absorbed as heat, or consists of wavelengths that silicon simply can’t convert into electricity. Multi-layer cells that stack different materials can capture a broader range of wavelengths and push efficiencies above 40% in laboratory settings, though these remain expensive and are mainly used in spacecraft and concentrated solar installations.
Human Muscle Efficiency
Your own body is a machine, and it follows the same rules. Human skeletal muscle converts chemical energy from food into mechanical work at about 25% efficiency. The other 75% becomes heat, which is why you warm up during exercise. This 25% ceiling is set by the biochemistry of how your cells produce and use their energy molecule (ATP) to power muscle fibers. It holds true across many animals, not just humans.
That number helps explain why you burn far more calories during a workout than the mechanical energy your muscles actually produce. If you do 100 calories worth of physical work on a bike, your body burned roughly 400 calories total to make it happen. The “wasted” energy isn’t entirely pointless, of course. It’s what keeps your body temperature at 37°C (98.6°F).
How to Compare Machines
When evaluating any machine’s efficiency, the key question is: efficiency of what conversion? A gas turbine’s thermal efficiency measures heat-to-motion conversion. A solar panel’s efficiency measures light-to-electricity conversion. An electric motor’s efficiency measures electricity-to-rotation conversion. These numbers aren’t directly comparable because they describe different energy transformations, each with its own physical limits.
- Gasoline engines: 30 to 36%
- Diesel engines: 42 to 47%
- Electric motors (industrial): 91 to 97%
- Solar panels (commercial): 20 to 22%
- Human muscle: about 25%
A machine with low efficiency isn’t necessarily a bad choice if the energy source is cheap or abundant. Solar panels convert less than a quarter of sunlight into electricity, but sunlight is free and available worldwide, which makes them economically competitive despite the modest percentage. Conversely, a gasoline engine’s 30-something percent efficiency matters a great deal when fuel is expensive and the waste heat contributes to environmental warming.