{
    "name": "Power Transformer Efficiency Calculator",
    "url": "https://calculators.geekmidst.com/calculators/power-transformer-efficiency-calculator",
    "markdown": "https://calculators.geekmidst.com/calculators/power-transformer-efficiency-calculator.md",
    "category": "Energy & environment",
    "description": "Calculate a power transformer’s efficiency at any load and power factor from its kVA rating, no-load core loss and full-load copper loss.",
    "summary": "A transformer loses a fixed core loss whenever it is energised and a copper loss that grows with the square of load. Efficiency is output ÷ (output + both losses).",
    "also_called": [
        "Transformer loss calculator"
    ],
    "applies_to": "Worldwide",
    "last_updated": "2026-10-29",
    "computation": "Results are computed in the visitor’s browser on the HTML page. This API describes the calculator; it does not compute results.",
    "inputs": [
        {
            "name": "rating",
            "label": "Rating",
            "type": "number",
            "default": "100",
            "unit": "kVA"
        },
        {
            "name": "load",
            "label": "Load",
            "type": "number",
            "default": "100",
            "unit": "% of rating"
        },
        {
            "name": "pf",
            "label": "Load power factor",
            "type": "number",
            "default": "0.8"
        },
        {
            "name": "core_loss",
            "label": "No-load (core) loss",
            "type": "number",
            "default": "1000",
            "unit": "W"
        },
        {
            "name": "cu_loss",
            "label": "Full-load copper (load) loss",
            "type": "number",
            "default": "1500",
            "unit": "W"
        }
    ],
    "outputs": [
        {
            "key": "efficiency",
            "label": "Efficiency",
            "format": "percent"
        },
        {
            "key": "output_kw",
            "label": "Output power",
            "format": "number"
        },
        {
            "key": "total_loss_w",
            "label": "Total losses",
            "format": "number"
        },
        {
            "key": "cu_loss_w",
            "label": "Copper loss at this load",
            "format": "number"
        },
        {
            "key": "max_eff_load",
            "label": "Load for maximum efficiency",
            "format": "percent"
        }
    ],
    "formula": {
        "text": "Output = kVA × x × pf;  Copper loss = Pcu,FL × x²;  η = Output ÷ (Output + Pcore + Pcu,FL × x²) × 100;  Maximum efficiency at x = √(Pcore ÷ Pcu,FL)",
        "variables": {
            "x": "load as a fraction of rating",
            "pf": "load power factor",
            "Pcore": "no-load (iron) loss",
            "Pcu,FL": "copper loss at rated load"
        }
    },
    "worked_example": {
        "inputs": {
            "rating": "100",
            "load": "100",
            "pf": "0.8",
            "core_loss": "1000",
            "cu_loss": "1500"
        },
        "steps": [
            "Output: 100 kVA × 1 × 0.8 = 80 kW.",
            "Losses: 1,000 W core + 1,500 W copper = 2,500 W.",
            "Efficiency: 80 ÷ (80 + 2.5) = 96.97%. Maximum efficiency at √(1000 ÷ 1500) = 81.6% load."
        ],
        "result": "At full load and 0.8 power factor the transformer is 96.97% efficient; its efficiency peaks at about 82% load, where copper loss equals core loss."
    },
    "interpretation": "Because core loss never stops, a lightly loaded transformer can be surprisingly inefficient over a year. For energy over time, weight the losses by how many hours the transformer spends at each load (all-day efficiency).",
    "assumptions": [
        "Copper loss scales with the square of current; core loss is constant at rated voltage.",
        "Stray and dielectric losses are included in the two test figures."
    ],
    "limitations": [
        "Temperature correction of copper loss and harmonic loads are not modelled."
    ],
    "rules": null,
    "faqs": [
        {
            "question": "Why does power factor change efficiency?",
            "answer": "Losses depend on current (kVA), but output is measured in kW. A lower power factor means less useful output for the same losses."
        },
        {
            "question": "When is a transformer most efficient?",
            "answer": "When the variable copper loss equals the fixed core loss, at a load of √(core ÷ full-load copper loss)."
        }
    ],
    "sources": [],
    "related": [
        {
            "name": "Cable I²R Loss Calculator",
            "url": "https://calculators.geekmidst.com/calculators/cable-i2r-loss-calculator"
        },
        {
            "name": "Inverter Efficiency Calculator",
            "url": "https://calculators.geekmidst.com/calculators/inverter-efficiency-calculator"
        },
        {
            "name": "Percentage Calculator",
            "url": "https://calculators.geekmidst.com/calculators/percentage-calculator"
        }
    ],
    "disclaimer": null,
    "publisher": "GM¹X CalcVerse"
}