By Richard Asmus
Updated March 24, 2022

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Transformers are fundamental components in electrical systems, stepping voltage up or down while maintaining power balance. Although they can alter voltage and current, the product of voltage and current—power—remains unchanged apart from inevitable losses. These losses, expressed as heat, are crucial to understand for sizing, efficiency, and safety.

To quantify transformer losses, you need the actual voltage and current on both the primary (input) and secondary (output) sides. Power in watts is calculated as P = V × I. Because a transformer cannot increase total power, the secondary power will always be less than the primary power by the amount of loss.

Step 1: Compute Primary Power

Multiply the primary voltage (V₁) by the primary current (I₁) and record the result: P₁ = V₁ × I₁.

Step 2: Compute Secondary Power

Multiply the secondary voltage (V₂) by the secondary current (I₂) and record the result: P₂ = V₂ × I₂.

Step 3: Determine Loss

Subtract the secondary power from the primary power: Loss = P₁ – P₂. The difference represents the power dissipated, typically as heat.

Example: For a primary of 440 V at 3 A, P₁ = 440 × 3 = 1320 W. With a secondary of 220 V at 5.7 A, P₂ = 220 × 5.7 ≈ 1254 W. The loss is 1320 W – 1254 W = 66 W, indicating a 66‑W loss.

TL;DR (Too Long; Didn’t Read)

Transformer efficiency is calculated by dividing output power by input power: Efficiency = P₂ ÷ P₁. Using the example above, efficiency = 1254 W ÷ 1320 W ≈ 0.95, or 95 %.