It really depends on how you configure your cards. I could configure my cards (R9 290) to use as low as 250w and still maintain a decent KH/s (750+) though right now I'm at 300w per card.
Best thing I can tell you to do is to get something like this:
http://www.thegreenhead.com/imgs/kill-a-watt-ez-1.jpg - a device that measures power consumption from wall, than you would easily know how much wattage each single card of yours consumes and will be able to calculate how much power you'll need if you want to add x amount of cards in the future.
I can't stress enough how big of an impact configuration is on the wattage consumption. I've seen people pull as high as 400w from the wall with a single R9 290 (making it a total of 2000w for 5 in your case) or as low as 250w (a total of 1250w in your case again).