How to choose a right kind of solar inverter?

You might have got an idea of the working of the inverter after reading the article "What is the functioning of the solar inverter?". But you cannot purchase inverter without having the answer to the following questions:

• What should be the rating of the inverter?
• Which type of inverter I should purchase?

A proper match between your energy requirement and the inverter rating along with right type can provide solution to your power needs.

(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:

• 2 Fans
• 2 Tube lights
• 2 CFL 
• 1 Television

There is a difference between a Starting power and the Running power. A 25 watt CFL may take around 32 watts(Starting power) for first few seconds and after that it comes down or stabilizes to 25 watts(Running power).Therefore, an additional of  (32 watts – 25 watts) = 7 watts is required to run the 25 watts CFL.
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:

• The total running power, Wrp = (160 watts + 120 watts + 50 watts + 150 watts) = 480 watts
• The highest additional starting power, Wsp = 200 watts
• The highest power for sizing the inverter (Your total power need) = the total running power, Wrp + The highest additional starting power, Wsp = 480 watts + 200 watts* = 680 watts

(*I am assuming that you start one appliance at a time that is you switch on the electrical appliances one by one not all in one time. Otherwise the sizing of the inverter will increase and hence its cost)
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:

• Watt = PF X VA

Where PF is the power factor, whose value is between 0 to 1.
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:

• Watts = VA x efficiency
• 680 watts = VA x 0.8
• VA = 680 watts/0.8 = 850 watts

So, we will look for inverter in the market having rating 850 VA or more.

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,

• one is Modified sine wave inverter
• and the other is Pure sine wave inverter.

Pure sine wave inverter:

• The output voltage of this type of inverter is sine wave just like a wave form which we get from grid supply or mains. The voltage rises and falls smoothly and it instantly changes the polarity when it crosses the zero volts. The phase angle also changes smoothly.

Modified sine wave inverter:

• The output voltage in the modified sine wave is not of the same form as that of grid supply, its voltage changes abruptly and it remains at zero volts for some time before changing its polarity.
  

(4) Pure sine wave inverter vs. Modified sine wave inverter

Advantages of pure sine wave inverter:

• The electrical devices run smoothly with less noise and disturbance.
• The heat produced is less.
• Motors run faster with less noise.
• The operating life of the appliances increases and less maintenance is required.

The following devices will not run properly through modified sine wave inverter:

• Motor
• Microwave oven
• Toaster
• Laser printer
• Coffee maker
• Washing machine

The prolonged operations of the above mentioned appliances on Modified sine wave inverter may affect the life of these electrical devices.

• The efficiency of Modified sine wave inverter is around 70% whereas Pure sine wave inverter has efficiency more than 90%, but Modified sine wave inverter is cheaper than the Pure sine wave inverter.

If you are not using above mentioned devices then you can go for Modified sine wave inverter to trade off between price and the performance otherwise it is advisable to go for Pure sine wave inverter for better performance and the safety of the electrical appliances.
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.

Conclusion

I am concluding the topic in the following points:

• Calculate the total running load, which is to be run through inverter, of the electrical appliances in watts.
• Find the highest additional starting power required to run the device.
• Add the total running load and the highest additional starting power to get your total power need.
• Calculate the VA rating of the inverter by dividing your total power need by the efficiency of the inverter.
• Choose between Modified sine wave inverter and the Pure sine wave inverter (preferred) based on types of devices to be run through inverter.
• Do not switch ON all the electrical appliances in one go, rather switch ON one by one by keeping a gap of at least few seconds between each device.

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