Here is the rough estimate of battery capacity you will need:
Add all kwh usage of all applicances you intend to use. Divide by 12. You will get number of amperhours you will need. Each battery has its amperhours, e.g. 200 amperhours. Battery discharging equally would theoretically give you 100%. Battery discharged at let's say double the equal discharge mode would give you about 80%.
But, I was surprised how little I use up ON INVERTER.
4 bulbs, big TV, small TV, DVD, 2 x cable box, cellphone charges, computer, 12ft fridge, etc.
it gets to use about 3 amps, which is 360 watt (0.36 kwh consumption) (not charging the inverter).
So my use 360 watt / 12 = 30 amps.
If your battery has 225 ampshours, and you have 4 batteries, it is 1000 amps hours
your consumption would last you, technically, 33.3 hours.
1000 ampshours discharged equally over 20 hours, it's 50 ampshours every hour, so calculated above your 22ampshours every hour would be better than recommended discharge, you would not put a strain on batteries using higher discharge than recommended 20-hr equal discharge.
Scenario 2:
Now, suppose you have a load of 200 amps on your inverter, with the same battery capacity of 1000 ampshours.
In theory, that whould provide you 5 hours of service (1000 ampshours / 10)
but the bigger the battery discharge rate, the more strain you put on battery and the fewer power it gives out.
Recommended discharge is 20 hours, but here you discharges it in only 5 hours. 5 Hour theoretical discharge would give you only 80% of capacity, so you would only have 800 ampshours to use out of 1000. Discharging at 200 amps would give you, in reality only 4 hours of service.
Now, here you can learn more
http://www.majorpower.com/inverters/battery_sizing_faq.pdf