if it is so then, before i had the 750W power supply i had a chieftec 650W supply that would stop when both cards were running. i did again some math and :
a 650W power supply with 80% efficiency gives me 520W of power available for my computer. it seemed it wasnt enough as it was shutting down the computer every time both cards started work
the 750W power supply does not have this problem!
it goes beyond my understanding what goes on with the power consumption, i'm just glad it's less than i anticipated
thanx for your help
You're mistaken about the efficiency. A 650W with 80% efficiency gives you up to 650W of power, but to do so it will use up 813W measured at the wall. A 750W with 90% efficiency gives you up to 750W of power, but to do so it will use 833W. So you get 100w more power but pay bills for 20W more with a more efficient PSU.
I'm leaning towards the possibility that you're estimating your power consumption wrongly.