How to calculate the size of Inverter for my home/office

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It is very common to face this question from your friend or your relatives to suggest a good inverter for their home or office. Being an Electrical Engr we can’t just suggest anything surfing the net and tell them go buy Lumious 2000 VA inverter. I faced a similar situation while selecting inverter for our RCS AutoCad coaching center recently.  Nobody wants to shell out extra money, guess we need 1250 VA and we are going to purchase 2000 VA, I won’t.

I went through some books and internet blogs, I found some solutions. Wanted to share with you –> Inverter Size PDF


To start we must know

  1. Total Electrical Load (in Watt) of the premises (How to calculate load Wattage? Load = No of electrical equipments X Wattage X Diversity Factor)
  2. Total Electrical Load (in VA) (How to calculate load VA? VA = Load in watts / Power Factor


Now we can calculate the size of the inverter

Before start the calculation we need to know these details about the inverter and battery bank we have decided to buy.


Inverter / Battery Detail:          (Assumed)

Additional Further Load Expansion (Af)=20%

Efficiency of Inverter (Ie) = 80%

Required Battery Backup (Bb) = 2 Hours.

Battery Bank Voltage = 24V DC

Loose Connection/Wire Loss Factor (LF) = 20%

Battery Efficiency (n) = 90%

Battery Aging Factor (Ag) =20%

Depth of Discharge (DOD) =50%

Battery Operating Temp =46ºC



 Size of inverter

  • Size of Inverter=Total Load (VA) x (1+Af) / Ie VA

Above equation gives us the size of the Inverter circuit we require.

Now we will calculate size of battery bank,

 Size of Battery Bank

  • Total Load of Battery Bank (BL) = (Total Electrical Load in watts x Backup Capacity) / Battery Bank Voltage  Amp Hr
  • Size of Battery Bank=[ (BL Load) x (1+LF) x (1+Ag) x Tp] / [n x DOD] Amp Hr
  • Battery Bank Voltage= 24 V


Now we have Amp Hr and voltage required for the battery bank, we can easily select the suitable battery or batteries connected in parallel, series or parallel-series. In series we get voltages added up, in parallel we get the Amp hr added up. With some permutations and combinations we get the required voltage and Amp Hr with lowest costing battery combinations.



(How to calculate Diesel Generator for your office/ Home ? Click Here)




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