Transformer Efficiency Calculator
Transformer efficiency tells you what fraction of input power is delivered to the load. The formula is eff = Pout / (Pout + losses), or equivalently eff = Pout / Pin. Losses come in two forms: core losses (eddy current and hysteresis losses in the magnetic core, roughly constant with applied voltage) and copper losses (resistive heating in the windings, proportional to load current squared). Enter the output power and the two loss components to calculate overall efficiency and input power requirements.
Transformer efficiency formula
eff = Pout / (Pout + Pcore + Pcopper)
Pin = Pout + Pcore + Pcopper
Efficiency is expressed as a percentage. Example: Pout = 10,000 W, Pcore = 50 W, Pcopper = 100 W gives Pin = 10,150 W and eff = 10,000 / 10,150 = 98.52%.
Transformer efficiency considerations
- DOE efficiency standards (10 CFR Part 431) set minimum efficiency requirements for distribution transformers sold in the US, typically 98 to 99.5% at rated load.
- Core losses are load-independent; a transformer energized all day with no load still dissipates core loss energy continuously.
- Copper losses at full load can be measured by the short-circuit test; core losses by the open-circuit test at rated voltage.
- For variable loads, efficiency over the load cycle matters more than peak efficiency; transformers are designed for peak efficiency at typical operating load, not full load.
Frequently asked questions
What is transformer efficiency?
Transformer efficiency is the ratio of output power to input power: eff = Pout / Pin = Pout / (Pout + losses). Losses include core losses (eddy currents and hysteresis, constant with voltage) and copper losses (winding resistance, proportional to current squared).
What are the two main types of transformer losses?
Core losses (also called iron losses or no-load losses) occur due to eddy currents and magnetic hysteresis in the core. They are roughly constant regardless of load. Copper losses (load losses) occur in the winding resistance and increase as the square of the load current.
What is a typical efficiency for power transformers?
Large distribution transformers achieve efficiencies of 98 to 99.5% or higher. Small signal transformers may be 85 to 95% efficient. Switching power supply transformers at high frequency can range from 90 to 98% depending on design.
At what load is transformer efficiency maximum?
Efficiency is maximum when copper losses equal core losses. Since copper losses increase with load squared and core losses are constant, maximum efficiency typically occurs at 50 to 70% of full load for utility distribution transformers.
How are transformer turns ratio and voltage related?
Turns ratio N1/N2 = V1/V2. A step-down transformer has more primary turns than secondary turns. Ideal transformer power is conserved: V1*I1 = V2*I2. Real transformers have slightly less output power due to losses.
Official sources
- U.S. DOE: DOE Distribution Transformer Efficiency Standards.
- IEEE Standard C57.12.00: IEEE Standard for General Requirements for Liquid-Immersed Distribution Transformers.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.