Electrical Efficiency Calculator – Input, Output & Power Loss

η = P_out / P_in — Efficiency, Loss & Heat

P_in + P_out P_in + η P_out + η V + I (in/out)

P_in Input Power

P_out Output Power

Efficiency Analysis

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Efficiency (η)
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Input Power

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Output Power

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Power Loss

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P_in − P_out = heat

Loss %

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Efficiency: η = P_out / P_in

Every power conversion wastes some energy as heat. Efficiency is the ratio of useful output to total input. The difference is power loss — dissipated as heat in the converter, motor, transformer, or regulator.

P_in — Total power drawn from the source (mains, battery, solar panel).
η (eta) — Efficiency ratio. 85% means 85% of input becomes useful output, 15% becomes heat.
P_out — Useful power delivered to the load (motor shaft, LED light, DC rail).
Loss — Wasted power = P_in − P_out. Always dissipated as heat. Sizes the heatsink.

Electrical Efficiency Calculator

Efficiency (η) is the ratio of useful output power to total input power, expressed as a percentage. A device that consumes 100 W and delivers 85 W of useful output is 85% efficient — the remaining 15 W is lost as heat. Every electrical component has losses: resistors dissipate heat, voltage regulators drop voltage, motors lose energy to friction and winding resistance, power supplies lose energy in transformers and switching circuits. This calculator answers the final question in the electrical chain: of all the power going in, how much actually does useful work?

Where This Calculator Fits

Ohm’s Law → gives you voltage and current
Power Calculator → gives you watts
Electrical Energy Calculator → gives you kWh and cost
Power Triangle Calculator → breaks AC power into real and reactive
Efficiency Calculator (this tool) → of all that power going in, how much is useful output?

How It Works

A triangle solver for η = (P_out / P_in) × 100%. Enter any two of the three variables — the calculator solves for the missing one. It also shows P_loss = P_in − P_out (the heat your device generates) and the loss percentage (100% − η).

η = (P_out / P_in) × 100% — find efficiency
P_in = P_out / (η / 100) — find input power
P_out = P_in × (η / 100) — find output power

P_loss = P_in − P_out — power lost as heat (always shown)

The Three Inputs

P_in (Input Power) — total power consumed from the supply, in mW, W, or kW. What you measure at the input terminals. Leave empty to answer: “how much input power do I need to deliver this output at this efficiency?”

P_out (Output Power) — useful power delivered to the load. For a motor: mechanical shaft power. For a power supply: DC output power. Leave empty to answer: “how much useful output does this input produce?”

η (Efficiency) — percentage from 0% to 100%. Leave empty to answer: “what is the actual efficiency from measured input and output?”

The power loss figure is critical for thermal design. It tells you exactly how many watts of heat your device generates — which determines whether you need a heatsink, a fan, or a larger enclosure.

Linear Voltage Regulator

12 V input at 500 mA (6 W in), 5 V output at 500 mA (2.5 W out).

η = 2.5 / 6.0 × 100 = 41.7%
P_loss = 6.0 − 2.5 = 3.5 W of heat
Loss percentage = 58.3%

41.7% efficiency. Over half the input power is wasted as heat. The 3.5 W of loss is why linear regulators need heatsinks — and why switching regulators replaced them for high-current applications. A linear regulator’s efficiency is fundamentally limited to η = V_out / V_in (5/12 = 41.7%). No design improvement can change this; it is inherent to the topology.

Switching Power Supply

230 W input, 200 W output. Typical desktop computer PSU.

η = 200 / 230 × 100 = 87.0%
P_loss = 230 − 200 = 30 W of heat
Loss percentage = 13%

87% efficiency. 30 W of heat extracted by the PSU’s internal fan. An 80 PLUS Gold PSU achieves 87–90% at typical loads; Platinum hits 90–92%; Titanium reaches 94% at 50% load. The difference between 87% and 94% at 200 W output: 30 W vs 12.8 W of heat — less noise, longer component life, and lower electricity cost.

DC Motor

500 W electrical input, 420 W mechanical output.

η = 420 / 500 × 100 = 84.0%
P_loss = 500 − 420 = 80 W of heat
Loss percentage = 16%

80 W of losses split across winding resistance (I²R copper losses), iron core losses (eddy currents and hysteresis), bearing friction, and windage. This heat must be removed by the motor’s cooling system. For the thermal resistance calculation that determines heatsink sizing, see the Thermal Resistance Calculator.

Reverse Solve Examples

“I Need 100 W Output at 90% Efficiency”

P_in = 100 / 0.90 = 111.1 W
P_loss = 111.1 − 100 = 11.1 W

You need 111.1 W from the supply to deliver 100 W to the load. Size your power supply, wiring, and circuit breaker for the input power (111.1 W), not the output.

“A 2 kW Motor at 88% Efficiency”

P_in = 2000 / 0.88 = 2273 W
P_loss = 273 W
At 230 V: I = 2273 / 230 = 9.9 A

The supply must deliver 2273 W and 9.9 A. The 273 W of heat determines the motor’s cooling requirements.

“500 W Input at 92% Efficiency”

P_out = 500 × 0.92 = 460 W
P_loss = 500 − 460 = 40 W

Cascaded Efficiency

Multiple conversion stages in series multiply their efficiencies:

η_total = η₁ × η₂ × η₃ × …

Mains → PSU (90%) → DC-DC (92%) → LED driver (88%):
η_total = 0.90 × 0.92 × 0.88 = 72.9%

Three individually “good” stages cascade to 73% — over a quarter of the input is wasted. Every stage matters. Eliminating one conversion stage saves more energy than improving any single stage by a few percent. For the LED driver side of this chain, see the LED Driver Calculator.

Efficiency by Device Type

Device	Typical η	Loss at 1 kW Output
Large power transformer	97–99%	10–31 W
Solar inverter	95–98%	20–53 W
Switching PSU	85–95%	53–176 W
LED driver	85–93%	75–176 W
Induction motor (large)	90–96%	42–111 W
Induction motor (small)	75–88%	136–333 W
Linear regulator	V_out/V_in	Depends on ratio

Heat and Thermal Design

P_loss = heat. Every watt of loss must be removed or the device overheats:

<1 W — PCB copper and package thermal pad sufficient.
1–10 W — small heatsink or thermal vias.
10–50 W — dedicated heatsink, natural convection.
50–200 W — heatsink with forced-air cooling (fan).
>200 W — liquid cooling or large fin-stack with high-CFM fans.

Frequently Asked Questions

What is a “good” efficiency?

Depends on the device. Switching PSU: above 90% is good. Motors: above 90% for large (>5 kW), above 80% for small (<1 kW). Linear regulators: η = V_out/V_in, fixed by the voltage ratio — no design improvement can change it.

Can efficiency exceed 100%?

No. Conservation of energy. If your calculation shows >100%, the output measurement includes energy from another source (e.g. a heat pump’s COP includes ambient heat, but the compressor’s electrical efficiency is still <100%).

Why is my linear regulator so inefficient?

η = V_out/V_in. A 3.3 V regulator on 12 V: η = 3.3/12 = 27.5%. The other 72.5% is heat. This is inherent to the topology. Switch to a buck converter for 85–95% efficiency regardless of voltage ratio.

Does efficiency change with load?

Yes. Most converters peak at 50–80% of rated load. At light loads, fixed losses (quiescent current, switching losses, core losses) dominate. At full load, I²R losses increase. Check the data sheet’s efficiency curve for the full picture.

How do I measure efficiency?

Measure P_in (V_in × I_in) and P_out (V_out × I_out) simultaneously. η = P_out/P_in. For AC, use a power analyser that measures true power — a simple V × I reading gives apparent power, which understates efficiency.

How does this relate to the Power Triangle Calculator?

The Power Triangle resolves real, reactive, and apparent power — it deals with power factor. This calculator deals with conversion efficiency — how much input power becomes useful output. A motor has both: a power factor (determines current drawn) and an efficiency (determines mechanical output). Use both together for complete motor analysis.

Last updated: March 2026

Efficiency: η = Pout / Pin