75
%
250
unit
25
%
0.75
25
unit
75
%
250
unit
25
%
0.75
25
unit
Efficiency measures how effectively a system converts input energy (or power) into useful output. No real machine is 100% efficient — some energy is always lost to friction, heat, sound, or other dissipative processes. The efficiency formula $$\eta = \frac{\text{useful output}}{\text{total input}} \times 100\%$$ applies universally to engines, motors, power plants, biological systems, and any energy conversion process.
This Efficiency Calculator accepts either energy values (joules) or power values (watts) and computes the efficiency percentage, total losses, and loss fraction. Understanding efficiency is critical for engineering design (selecting motors and transmissions), energy policy (comparing power generation methods), sustainability (reducing waste), and physics education (applying the first and second laws of thermodynamics). The calculator also provides the output-to-input ratio, which for heat pumps is called the Coefficient of Performance (COP) and can exceed 1.
Efficiency is defined as:
$$\eta = \frac{E_{useful}}{E_{total}} \times 100\%$$
or equivalently for power: $$\eta = \frac{P_{out}}{P_{in}} \times 100\%$$
Energy lost equals $$E_{lost} = E_{total} - E_{useful}$$, and the loss fraction is $$\frac{E_{lost}}{E_{total}} \times 100\%$$. Efficiency plus loss fraction always equals 100%. The output/input ratio is the decimal form of efficiency. For conventional machines, this ratio is between 0 and 1. For heat pumps and refrigerators, the COP can exceed 1 because they move heat rather than generate it.
An efficiency of 100% would mean zero energy loss (impossible for real machines per the second law of thermodynamics). Typical efficiencies: electric motors 85–95%, car engines 20–35%, LED bulbs 40–50%, solar panels 15–25%, coal power plants 33–40%. Higher efficiency means less waste, lower operating costs, and reduced environmental impact.
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Results
A motor consuming 1000 W of electrical power and producing 850 W of mechanical power has 85% efficiency with 150 W lost as heat.
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Results
A car engine converting 30 kJ of fuel energy into 8.4 kJ of mechanical work has 28% efficiency — 72% is lost as heat.
The second law of thermodynamics states that in any energy conversion, some energy degrades to thermal energy (heat) that cannot be fully recovered. Friction, air resistance, electrical resistance, and sound all represent unavoidable losses in real systems.
For standard machines, no. However, heat pumps and refrigerators have a Coefficient of Performance (COP) that can exceed 1 because they move thermal energy rather than converting it. A heat pump with COP = 3 delivers 3 kW of heating using only 1 kW of electrical input by extracting 2 kW from the environment.
The Carnot efficiency $$\eta_C = 1 - \frac{T_{cold}}{T_{hot}}$$ (temperatures in Kelvin) is the theoretical maximum efficiency for any heat engine operating between two temperatures. No real engine can exceed this limit. It increases with larger temperature differences.
Reduce friction (better lubrication, bearings), minimize heat losses (insulation), use regenerative systems (recapture waste energy), optimize operating conditions, and select high-quality components. Regular maintenance also prevents efficiency degradation over time.
Efficiency is a quantitative ratio of output to input energy. Effectiveness is a qualitative measure of whether the system achieves its intended purpose. A system can be highly efficient but ineffective if it produces the wrong type of output.
Yes. For electrical circuits, efficiency = (output power / input power) × 100%. For a power supply rated at 80 PLUS, it converts at least 80% of AC input power to DC output power, with up to 20% lost as heat.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
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