Calculate transformer efficiency, output power, or total power losses by selecting a solve mode and entering its two relevant values in W, kW, or MW. Use real power at the same operating point; kVA is not real power.
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Transformer Efficiency Formula
The calculator uses output power and total power losses to find transformer efficiency. It can also rearrange the same relationship to solve for output power or losses when efficiency is known.
- Efficiency = transformer efficiency, expressed as a percentage
- Pout = transformer output power, in watts, kilowatts, or megawatts
- Ploss = total transformer power losses, in watts, kilowatts, or megawatts
The efficiency calculation compares the useful output power to the total input power. Since input power equals output power plus losses, higher losses reduce the efficiency percentage.
Select Efficiency for output power and total losses, Output power for efficiency and losses, or Total losses for output power and efficiency. Power must be nonnegative and efficiency between 0% and 100%. Zero/100% inverse cases can be inconsistent or underdetermined. A positive output with zero losses is the ideal 100% limit.
Typical Transformer Efficiency Ranges
Actual efficiency depends on loading, design, temperature and material. The following categories do not define a universal numerical efficiency range. Use tested manufacturer data at the intended load; DOE and ENERGY STAR guidance distinguishes load and no-load losses and load-specific performance.
| Transformer Type or Size | Efficiency Basis | Common Notes |
|---|---|---|
| Small low-power transformer | Use measured output and total losses | Losses are more noticeable relative to output power. |
| Distribution transformer | Use model-specific tested data at a stated load | Compare performance at the intended loading; rated capacity alone does not determine efficiency. |
| Large power transformer | Use manufacturer test data | Large units are designed for very low percentage losses. |
Power Unit Conversions
The calculator converts all power values to watts internally before solving. Use consistent physical meaning even if the displayed units differ.
| Unit | Equivalent in Watts | Example |
|---|---|---|
| W | 1 W | 500 W = 500 W |
| kW | 1,000 W | 5 kW = 5,000 W |
| MW | 1,000,000 W | 2 MW = 2,000,000 W |
Example Calculations
Example 1: Find transformer efficiency
Suppose a transformer delivers 50 kW of output power and has 1.5 kW of total losses.
The transformer efficiency is 97.09%.
Example 2: Find total power losses
Suppose a transformer has an output power of 100 kW and an efficiency of 98%.
The total power losses are about 2.04 kW.
FAQ
What counts as total power losses in a transformer?
Total losses include the power that does not reach the load as useful output. The main parts are core losses, also called iron losses, and winding losses, also called copper losses. Stray losses and dielectric losses may also be included in more detailed transformer testing.
Why is transformer efficiency less than 100%?
A real transformer always has losses. Some energy is lost as heat in the windings, some is lost in the magnetic core, and small additional losses can occur from leakage flux and insulation effects. Because of these losses, output power is always less than input power.
Is efficiency the same at every load?
No. Transformer efficiency changes with load. Core losses are present whenever the transformer is energized, while copper losses increase as load current increases. Efficiency is usually highest near the load level where fixed losses and variable losses are well balanced.
