(1) The starting power & the running power
Let us suppose I want to run the following electrical appliances through solar power system. These are as follows:
In case of the television, the starting power requirement is more than double its rating. A 150 watt television takes around 350 watts of power in starting and then after few seconds of running, it stabilizes to 150 watts. So, we need to consider this before choosing an inverter for our solar power system.
Let us consider the following table:
So, I want an inverter which can provide 680 watts to these electrical appliances. The rating of the inverter generally comes in VA (Volt-Ampere).
(2) Watt vs. VA
First of all, there is the need to understand the difference between Watt and VA.
It is true that both are the units of power but the former (watt) is the real power and the latter (VA) is the apparent power.
The relation between the two is as follows:
It is necessary to know the power factor or the efficiency of the inverter. Let us suppose you buy an inverter having rating 800 VA with 80% efficiency. The inverter will supply 800 VA X 0.8 = 640 watts to the electrical equipments.
In our case, we want inverter to supply 680 watts to the electrical appliances/devices. If we assume 80% as an average efficiency figure, we can calculate the VA rating of the inverter by using the following formula:
All the electrical appliances in India are designed to run on pure sine wave with 50 Hz frequency. The performance of these appliances is dependent on the shape of the input wave form. A change in shape of input waveform can affect the performance of the electrical appliances. Let us suppose you run your television on the inverter and you find some noise or the flicker on the television screen, this is due the fact that the output from your inverter is not the pure sine waveform, it is distorted. The prolong operation or running of the electrical appliances on the distorted waveform can considerably affect the life of these devices.
(3) The types of Inverter
There are basically two types of inverters which are available in the market,
(4) Pure sine wave inverter vs. Modified sine wave inverter
Advantages of pure sine wave inverter:
By now, we have calculated the desired rating of the inverter based on our requirement as mentioned in the above example.
Now, we have to decide whether to purchase 850 VA Modified sine wave inverter or 850 VA Pure sine wave inverter.
It is true that initial cost with Modified sine wave inverter is less, but if we look at the reduction in the performance, increase in the maintenance cost of the devices and negative impact on their operational life, it is concluded that one should go for 850 VA rated Pure sine wave inverter.
I am concluding the topic in the following points: