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 solar energy requirement and the inverter rating along with right type can provide solution to your power needs.

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Starting Power & Running Power

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Your inverter should be able to provide 2 types of power to the electrical appliances: one is Starting Power and another is Running Power.

• Starting Power > Running Power

For example, when a ceiling fan with power rating of 80 watts is switched ON will withdraw around 130 watts for first few seconds and after that it will settle down to 80 watts.

We can say, 130 watts is the Starting Power and 80 watts is the Running or Continuous Power of the fan.

When I switch ON the fan, my Inverter should take enough D.C. power from the battery and convert it into 130 watts A.C. power and after few seconds it should settle down to 80 watts.

In short, the Inverter should have 2 types of rating:

• Peak Rating: The maximum power that an Inverter can handle for short period of time (up to few seconds or minutes). Generally, up to 15 seconds of Peak rating is enough to cover most of the electrical appliances.
• Continuous Rating: It is the power that Inverter needs to provide on steady basis. It is usually much lower than the Peak Rating of the Inverter.

Let us understand more about these terms with another example.

Suppose, I want to run the following electrical appliances on solar power:

• 2 Fans
• 2 CFL (Compact Fluorescent Lamp)
• 1 Television

Let us evaluate the Starting and Running power of these electrical appliances.

• A 80 watts fan actually needs 130 watts of Starting power for few seconds before settling to 80 watts.

• 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.

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• 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 Starting Power = (260 watts + 64 watts + 350 watts)
  

         = 674 watts

• The Running Power = (160 watts + 50 watts + 150 watts) 
  

         = 360 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)

Therefore, I should look for an Inverter with Peak Power Rating of 674 watts and Continuous Power Rating of 360 watts.

​The rating of the inverter generally comes in VA (Volt-Ampere).

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Watt vs. VA

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In the market, the rating of Inverter is generally specified in VA (Volt-Ampere) not in Watts.

But we calculated our Inverter Rating in Watts.

Dose this mean that our calculations are good for nothing?

Cheer up Guys!

We have a relation between VA and Watts

It is true that both are the units of power but the former (VA) is the Apparent Power and the latter (Watts) is the Real Power.

The relation between the two is as follows:

• Watt = PF X VA

Where PF is the power factor (efficiency), whose value lies between 0 to 1.

Assuming, you buy an Inverter having Power Factor of 0.9 (90% Efficient).

We sized the Peak Rating of Inverter as 674 Watts and the Continuous rating is 360 watts.

We can easily calculate its VA rating using this formula:
Watts = Power Factor x VA
Or
VA = Watts/Power Factor

Calculating Peak Power Rating in VA:

• VA (Peak) = 674 watts/0.9 = 749

Calculating Continuous Power Rating in VA:

• VA (Continuous) = 360/0.9 = 400 

Finally, I should look for Inverter with Peak Power Rating of 749 VA or above and Continuous Power Rating of 400 Watts or above.

Our electrical appliances are designed to run on pure sine wave that we get from the grid.

In other words, 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 these 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.

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The types of Inverter

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There are basically two types of inverters which are available in the market,

• One is Pure Sine Wave Inverter
• Another is Modified 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.

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Pure sine wave inverter vs. Modified sine wave inverter

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 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.
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Now, we have to decide whether to Modified sine wave inverter or 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 Pure sine wave inverter.

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Conclusion

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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 Starting Power of each appliance and add them all to get the Peak rating of the inverter.​
• 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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