Power question (adding a 5th card)

[snootch]

Okay, i've read like 13 posts regarding power, and was contemplating expanding my rig. The parts are in the mail for my rig (should be getting them in a day or two)
I have a kill-a-watt, and could probably figure this out myself in a week, but i wanted to ask here so I could be proactive and order parts to enable a 5th card. (if possible)
So, heres the question: Running this rig:

Mobo: ASROCK 970 Extreme4
CPU: AMD Sempron 145 (single core)
RAM: 2 x 2GB DDR3
PSU: Seasonic Platinum 1000w
4 x MSI R7950 Twin Frozr

I assumed the following: Mobo, CPU, RAM & one 7959 = 300 watts. Each additional 7950 = 200 watts. So system laid out above should equal 900 watts is the cards are slightly undervolted.
Do you think I am correct (or incorrect) in assuming I will (or won't) have the power for a fifth card? (1100 watts?) Would I need to upgrade to a platinum 1200W?

BTW, three cards are running unpowered risers, and one x1 slot is using a powered riser. i assume adding a 5th would require a powered riser.

[1715051807]

1715051807

[1715051807]

1715051807

[BBQKorv]

Get a kill-a-watt or some other power meter, divide the reading by 1.1 since you're talking about platinum series PSU (usually 90% efficiency).

Example:
950W from the wall is ~860W system power consumption and ~90W for the PSU to waste during voltage conversion.

[snootch]

Interesting, So I might be able to swing 5 cards with my existing power supply? BTW, I will be mining LTC. How will this influence power draw versus BTC?

[madkind]

I'd add a secondary PSU for better efficiency, and run 2 cards+ mobo on the first, 3 cards on the second.