How to determine what inverter I need.

With the worsening power situation in our cities, Inverters are becoming an essential part of our lives. In this post I will just summarize how to determine what capacity inverter to buy.

Step 1: Determine your usage. I.E. list out what all appliances you would want to run on the inverter.
For our calculation let us say that we are going to run 2 CFLs, 1 fridge, 2 fans and one laptop+1DSL modem on inverter.
In my house the power rating for these appliances are
CFL 25W = 2*25 = 50Watts
Fridge 300W
Laptop 90W
Modem 25W
Fans 40W = 2*40 80W

Total usage = 545Watts or 545VA

Add 25% to it as conversion loss , 545+136=681 VA is the bare minimum rating of the inverter that you would need.
In the market, generally 800 VA is what is available. So that is the one that you should be getting.

Step 2: Determine how long you would need your backup system to run.
Assuming that you are going to run at full load, how long would you want your backup to run.

For my usage, I decided that I would want my backup to run for at least 4 hrs.

The ampere hour attribute of the inverter battery determines how long it will last.

So 800 VA *4 hrs = 12 V * Ah is the formula. Here your battery is 12V battery. Ah is the ampere hour.
This gives 266 Ah.

So an inverter with a rating of 800 VA with a battery pack of 250 Ah is what I need to run my backup for roughly 4 hrs at full load.

Running on a minimal load, however will help me achieve more mileage out of my inverter.

A key factor to keep in mind when buying an inverter is to check the rating of the battery because this is where the big money is hidden and this is where the sellers will try to dupe you.

Generally sellers will talk about "optimal" load(which will be 60% of the maximum load) when informing you about the duration of backup.

Hope this helps.