You can go to Don Rowe and use their calculater to figure various ways.
This is not working for me now with Firefox but I opened a (yuk) IE window and it worked fine.
http://donrowe.com/inverters/inverter_faq.html#how_long
They have a pretty good FAQ also if you read through a bunch of it. They have a little chart that shows about how much power various things use ect..
Basically enter 1 your battery banks voltage, 2 the amp hours of the battery bank, 3 the watts you plan to use. Hit calculate and box 4 shows your hours of runtime.
So play with it a bit and you can figure various things out.
12V with 1000 amp hours battery bank at a 500 watt load gives how many hours?
Enter your Voltage 12 or 24, geuss at amp hours, enter your average hourly load, click calculate and get how many hours you can run at that load on that many amps. If you don't like the answer enter higher amount for amp hours till you get the answer you like for the hours you can run.
To figure your average hourly load, Mine is 510KW per month average over 6 months time.
So I divide 510,000 by 4.3weeks (actaully 4.3 weeks in a month) Almost 119Kw a week.
Divide 119Kw by 7 days = almost 17kw a day, divide that by 24hours and I get about 706watts per hour average. Of course you could probably just divide your average monthly usage by the hours in a month, but I like to know the other figures as well myself like weekly and daily.
So I go to donwrowes.com calculator and enter my 12v system, geuss amp hours (1000amps), enter my 706 watts I want to run and I find out how many hours I can run it on a battery bank of that size. That's 14.18 hours for me at 1000 amps. Go back and change to 2500 amps and I can run 35.46 hours. Divide the hours of course by 24 to find out how many days that would be. I find that at my present usage 1000 amps is less than one days backup, and 2500 amps is only 1.5 days backup.
I find by playing with the numbers a bit at my 705watts per hour I need 12,500 amp hours to last 7.38 days on battery power
Luckily at some point durring that 7.38 days I will have some power comming in most likely so I don't really need that many storage amps. But that's how you can easily figure out what you need for battery storage, decide how long you want to last on batteries only and figure your average useage out, then calculate various amps.
My average may be less now, I changed all lights to CFL that run more than a couple minutes at a time. But then I also bought several power tools that will increase usage alot like my lathe/milling machine. So now I need a new average soon myself.