Can someone tell me the formula to use to come up with the right size and number of batteries when installing an inverter

Since I don't know the Load(s), try this:
UPS Selector Sizing Applications (http://www.apc.com/tools/ups_selector/)
They are very helpful.

You can start here:

http://www6.zetatalk.com/docs/Batteries/Banks/Sizing_an_Inverter_Battery_Bank_FAQ_2004.pdf

lets say u have a load drawing x amount of current and u want an inverter and battreys which can able to run your equipment,say for 20 hrs when main power is off. What's the fomula to come up with the right size of battries and inverter and also the formula to see how long it will take to fully charge the battries when starting to charge them