100AH battery: (7k ~ again, shop around for a better deal, go with tubular)
100AH battery contains 1.2kwh of storage (i.e 1.2 units of power).
Theoretically, it can supply a maximum of 1200 watts for an entire hour. However, real world use would severely limit this on top of converting DC to AC and you cannot use more than 80% of a lead acid battery, so let's do some maths.
100AH (only 80% capacity usage) - 20% efficiency loss / C20 discharge rating of a battery.
Batterybhai.com ~ I see prices of 8.5k for 100AH new from Tata Green / Exite Insta etc. If you happen to have an old 100AH lying around, you can exchange it and it'll be around 6.5k. Again, shop around for more options.
I got 200AH red charge Luminous from the same site for 9.9k a few months ago so I'm sure 100AH is possible.
I would still advise to go for a bigger setup as there is literally 0 upgrade paths if you choose to do it this way.
You're wanting 5 to 6 hours MINIMUM backup for 70W continuous power delivery. SMF batteries cost more for 100AH compared with tubular. You cannot go with less capacity (i.e like 65AH) as you'll have very less backup.
Tubular also lasts long, but I guess as long as you get the capacity you'd need, I guess it depends on how long you want the battery to last?
A local dealer quoted 9500 INR for Inverter + Battery
Microtek 400 VA + SF Sonic 65 AH lead acid battery
He told me that a single ceiling fan will run for 7-8 hrs on this combo.
I'm still looking for better battery brand and price