Energy Efficiency Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/8/2026
Energy efficiency is calculated with η = (useful energy ÷ energy supplied) × 100. For 70 kWh of useful energy obtained from 100 kWh supplied, the efficiency is 70%.
Explanation
Efficiency (or energy efficiency) expresses the proportion of energy supplied to a system that is actually converted into useful energy, with the rest lost, most often as dissipated heat (friction, electrical resistance, thermal losses). It's a key indicator in engineering for comparing systems: a typical combustion engine reaches an efficiency of 25 to 40%, an electric motor often exceeds 90%, and a condensing boiler can approach 100% by recovering the latent heat from water vapor in the flue gases. A 100% efficiency would correspond to a perfect conversion with no losses at all, a theoretical case no real system achieves in practice due to unavoidable physical losses (second law of thermodynamics). This calculator gives an overall efficiency for the whole system; for a multi-stage conversion chain (for example, generating, then transmitting, then using electricity), the overall efficiency is the product of each stage's efficiency, not their simple average.
Example: a system delivering 70 kWh of useful energy from 100 kWh supplied
Inputs
Useful energy: 70 kWh. Energy supplied: 100 kWh.
Calculation
Efficiency = (70 ÷ 100) × 100 = 70%.
Result
This system has an energy efficiency of 70%, meaning 30% of the supplied energy is lost.
Frequently asked questions
Can efficiency exceed 100%?
Not in the strict sense of converting a single form of energy, where 100% represents the theoretical limit with no losses at all. Some systems do show a 'coefficient of performance' above 100%, however (heat pumps, for example): they don't create energy, but move heat already present in the environment, which isn't comparable to a classic conversion efficiency.
How do you calculate the overall efficiency of a multi-stage chain?
By multiplying the efficiencies of each stage together (expressed as a proportion, not a percentage): a two-stage chain with 90% and 80% efficiency each gives an overall efficiency of 0.90 × 0.80 = 72%, not the average of the two (85%). Losses accumulate at each stage.
Why does no real system reach 100% efficiency?
Because the second law of thermodynamics requires that any energy conversion be accompanied by entropy production, generally as heat dissipated into the environment (mechanical friction, electrical resistance, thermal losses). Some systems come very close (modern electrical transformers, for example, often above 98%), but none reach it exactly.