Knowing how to calculate energy efficiency of an appliance will help you in deciding whether to invest in a particular appliance or not. Energy efficiency of an appliance actually gives you the value of the useful output generated by an appliance that is put into use. The energy efficiency of an appliance is usually calculated in the form of percentage.

- To calculate the energy efficiency of an appliance you need to know only two values i.e. the energy supplied to the appliance and the amount of energy of the appliance that is put into use.
- There is no appliance which is perfectly efficient i.e. an appliance that gives 100% efficiency.
- Therefore, an appliance which gives 80% to 90% efficiency is considered as perfectly efficient, so if your appliance lies in this range then it is perfectly efficient.

The formula for calculating energy efficiency of an appliance is given as,

** % energy efficiency = (Useful energy produced × 100) / Total energy used**

- Before calculating the energy efficiency of the appliance we need to know the values of useful energy produced by the appliance and the total energy supplied to the appliance.
- To calculate both these values you need to calculate the power used and power produced.

Formula for calculating power is as follows.

** P = V × I**

Where,

- P stands for power
- V stands for voltage
- I stands for current drawn

**Example:**

Here is a simple example that you can try at your home using your mobile and the charger. You can actually calculate the energy efficiency of the mobile phone yourself using the method discussed below.

- On the bottom of the charger it is generally given that it uses 21 mAmps current and voltage of 230 volts. These values will help you in calculating the amount of power it draws from the mains.

Power = V × I

Power = 230 × 21

Therefore,

Power = 483

- As we used the value of power in mAmps we need to divide the value obtained by 1000 to convert the current value in Amp and get the value of power in Watts

Therefore,

Power = 4.83 Watts

Power drawn by the charger from the mains is 4.83 Watts.

- Now, we want to calculate the total amount of energy used by the appliance. To do this we need to use the values of output voltage and current. These values are also given on the charger of the cell phone.
- Suppose the values for outputs of the charger are given as 355 mAmps and 3.7 Volts. Then using the same formula as above, we can calculate the output power of the charger as follows.

Power = V × I

Power = 355 × 3.7

Power = 131

- To convert mAmps into Amps and obtain the value of power in Watts, divide it by 1000.

Therefore,

Power = 1.31 Watts

Total energy of the charger that is put into use is 1.31 Watts.

- Looking at the above values you will get an idea that for every 4.83 joules of energy drawn by the charger only 1.31 joules of energy is put into use while the remaining 3.52 joules is wasted.
- From the above calculation you must have got an idea about the efficiency of the charger.
- Now, let us estimate the actual value of the efficiency of the charger using the formula for calculating efficiency of the charger so that we will be able to interpret the efficiency of the battery more precisely.

% energy efficiency = (Useful energy produced × 100) / Total energy used

Therefore,

% energy efficiency = (1.31 × 100) / 4.83

% energy efficiency = 27%

- This states that energy efficiency of the battery is 27%.
- Looking at the above figure you may feel that your battery is not at all efficient, but there are other factors which need to be considered while calculating the energy efficiency of any appliance.
- In case of a battery i.e. a transformer there is generally no loss of energy because a large amount of energy drawn by the battery is used in converting the electrical energy into heat energy as the mobile also gets heated while it is getting charged.
- Some amount of energy is also converted into chemical energy and the remaining amount is wasted.
- Using the above formula and the steps given above you can calculate energy efficiency of any appliance knowing the power supplied to the appliance and the amount of power put into use.

Calculating energy efficiency of the appliance is useful to minimize the usage of electricity and reduce your electricity bills by smart shopping.

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