How to Calculate the power consumption and How to Rate an Inverter and battery


Calculating the power consumption, inverter rating and battery rating:

Wattage rating of typical home appliances:

Appliances  Power
Fan (Ceiling) 50-75 Watts
Fan (Table) 25-50 Watts
CFL 18 Watts 18 Watts
Computer (Desktop) 80- 150 Watts
Laptop 20- 75 Watts
LCD TV (32”) 150 Watts
Refrigerator 160 – 250 Watts
Tube Light (4 Feet) 40 Watts

First calculating the power consumption of your home:

You want 3 Fans, 4 Tube lights, 2 CFL & 1 Television to operate at the time of power failure. Below is the power consumed by these items as per the above table:

1 Fan – 70 Watts

1 tube light – 40 watts

1 CFL – 18 watts

1 Television – 150 watts

Therefore your total power requirement is (3*70 +4*40 + 2*18 + 150) = 556 watts

Now calculating the VA rating of inverter as per the Power Requirement:

It stands for the Volt ampere rating. It is the voltage and current supplied by the inverter to the equipments. If an inverter operates with 100% efficiency, then the power requirement of the electrical items and power supplied by inverter is same. But we all know that 100% or ideal conditions don’t exist in real.  Most inverters have the efficiency range from 60 % to 80%. This efficiency is also called power factor of an inverter and is simply the ratio of power required by the appliances to power supplied by an inverter. Power factor of most inverters ranges from 0.6 to 0.8.

Hence Power supplied (or VA rating of inverter) = Power requirement (power consumed by equipments in watts) / Power factor (efficiency).

Here average value of power factor or efficiency is considered as 0.7

Power of inverter (VA) = 556/0.7 = 794 VA

In the market 950 VA inverters are available. So an inverter with 950 VA will be the right choice for your home.

Now calculating the battery rating based on the inverter Capacity:

Battery is the backbone of an inverter system. The performance and life of an inverter largely depend upon the battery quality. The next big question is “how much back up will an inverter provide?” or for “how many hours it can run all of your equipments?”. This is what is called the battery capacity. It is the battery capacity that decides the backup hours. It is expressed in Ah (Ampere Hours).

In the market batteries with capacity of 100 Ah, 150 Ah, and 180 Ah etc are readily available. So how to decide which one you need? To find this out let’s do a reverse calculation. Consider that you need a battery that provides back up for 3 hours.

Battery capacity = Power requirement (in watts) * Back up hours (in hrs) / Battery Voltage (in volts)

Battery Capacity = (556 * 3) / 12 = 139 Ah

** Value of Battery voltage is taken 12V

Therefore a battery with a capacity of 150 Ah will work for you.

So if you want to run 3 fans, 4 tube lights, 2 CFL and 1 TV for 3 hours during power failure you would need 950VA inverter and 150 Ah battery.

By understanding this simple calculation you have not only saved yourself from the misleading information shared by inverter dealers but also helped yourself in taking correct decision.

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