Electric Transformer Efficiency: 4 Answers For Beginners

What is transformer efficiency?

The proportion of a transformer’s output power to the input power is referred to as the efficiency of a transformer.

The impact of losses on transformers can be measured using the term transformer efficiency.

Copper losses, hysteresis losses, iron losses and flux losses cause the output power delivered to the load lower than the input power of the transformer.

The following formula can be used to determine the efficiency of a transformer:

Efficiency of transformer η = (Output Power/Input Power) X 100

The power transformer’s efficiency typically ranges between 97 and 99 percent.

The input power is always higher than the output power due to the losses.

Are Transformers more efficient when cooled?

The cooler the transformer the higher its efficiency. The efficiency of a transformer changes with the load.

Transformers produce heat, the cooling methods of transformers keeps the temperature rise withing allowable limits, and increase the efficiency of the transformer.

There are various cooling methods used to prolong the life span of the transformer, each method increases the output the transformer can provide. Let’s take an overview of them, if you want more information about transformer cooling methods, read my article here.

ONAN Method

ONAN, “Oil Natural Air Natural”, is the simplest method of cooling transformer. No air fans or oil pumps are used.

The output power of the transformer is equal to or less than 100% when using ONAN cooling.

ONAF Method

ONAF, “Oil Natural Air Forced”, is a technique used to help disperse the heat off the surface of radiators and is even more efficient.

In this process, it is utilized to force air to speed up the cooling process through the aid of fans.

The output power is increased to 133%, Yeah, That’s the fact! you can overload the ONAF cooled transformer up to 133%

OFAF Method

OFAF, “Oil Forced Air Forced”, is a technique used to cool transformers more quickly than ONAF method. In the OFAF method, oil is forced to move faster.

The output power of the transformer is increased up to 167%.

How does transformer efficiency vary with load?

The efficiency of a transformer depends on the load; it may vary according to the variations in load.

The efficiency rises with a rise in output power up to a specific value and, after a particular value of output power, efficiency decreases.

With no loads, the Transformer draws a small amount of energy, primarily due to the current of magnetization.

In a transformer designed to maximize efficiency, the energy input with no load is usually 1 to 2 percent of rated power because, the power loss at no load occurs regardless of the secondary load’s power output.

However, the efficiency is, in fact, zero. It is the result of the output power divided by the input power, which is zero.

The transformer that loads are variable (like distribution transformer) is designed to maximize efficiency at around 75% of the load.

If it’s continuously operating at or near full load (like power transformers), the design is to achieve maximum efficiency, or at least close to the maximum load.

When the workload on the transformer rises, the efficiency of the transformer increases. When the load is around 75%, it will be the most efficient.

When the output rises above 75 percent, the voltage drop within the windings of the transformer becomes more extensive, and the efficiency decreases.

A good transformer efficiency can operate between 90 and 95 percent when fully loaded.

What is all day efficiency of transformer?

The efficiency discussed so far is the transformer’s ordinary, commercial, or power efficiency. But the distribution transformer does not give an accurate idea about the transformer’s performance because the distribution transformer’s load fluctuates throughout the day.

This transformer is energized for twenty-four hours, but it delivers a very light load for a significant portion of the day.

The efficiency of such transformers (like distributor transformers) cannot be evaluated through power efficiency. However, it can be assessed through a specific type of transformer efficiency referred to in energy efficiency and all-day efficacy.

The efficiency of the whole day is calculated by calculating the amount of energy used during 24 hours.

The efficiency for the whole day of a transformer can be defined as the ratio of energy output (in units of kWh) to energy input for 24 hours.

To determine the efficiency throughout the day of the transformer, it is necessary to know about load cycle of the power transformer is used.


A 20 KVA transformer operating on domestic power, which could be considered to be of a unity power factor.

It has a full-load efficiency of 95.3 percent, with the loss of copper equal twice the iron loss. Calculate the efficiency of its all-day by following the day-to-day cycle:

  • No-load during 10 hours.
  • Half-load for 8 hours.
  • The total load for 6 hours.


Full output of load = 20 x 1 = 20 KW

Full load input = output/efficiency = (20/95.3) 100 x 20.986 KW

Total losses = Pi + Pcu = Input – Output = 20.986 – 20 = 0.986 KW

Then You have to calculate Pcu = 2P I (given)

So, the equation P 1 plus 2P I = 0.986 KW

Or Iron losses (Pi) = 0.3287 KW

Copper losses at full load (P cu) = 2 times 0.3287 = 0.6574 Kilowatts

kWh of output over 24 hours is = {(1/2) x 20 x 8} + (1 x 20 x 6) = 200 KWH

Iron losses over 24 hours = 0.3287 × 24 hours = 7.89 kW

Copper loss during 24 hours=Cu losses of 8 hours for half load + Cu losses for 6 hours when at full load

= {(1/2)2 x 0.6574 x 8} + (0.6574 x 6)

= 5.259 KWH

Input over the 24 hours (kWh) output over 24 hours + copper and iron losses for 24 hours

= 200 + 7.89 + 5.259 = 213.149 KWH

All-day performance of the transformer = (kWH output over 24 hours ÷kWH input during 24 hours) *100

All-day performance of the transformer = (200/213.149) = 93.83%

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