Your calculations about PSU power requirements is wrong.
When companies list the power requirements for graphics cards they assume a fully loaded system, E.g. a CPU (125w+), a number of optical drives, HDDs, fans, USB devices, etc. They also add a large cusion to allow for cheap PSUs that don't run to specs. Each 6 pin PCI-E power connector delivers 75 watts, each 8 pin delivers 150 watts. In this article, an R9 270 has a draw of less than 150 watts. A closer calculation of your system PSU requirements would be 150 x 2 (2 R9 270s) + 125 (CPU) = 425 Watts, this doesn't take into account other components though and you would have to add their draw as well. Also you want to run your PSU at less than 85% load, otherwise you can reduce its lifespan.
For your system I would suggest a minimum 650 watt PSU, or greater, this would allow 100 watts to other components such as HDDs and your MoBo, USB devices etc. + 15% to keep the load within 85%. 750+ would allow more headroom though. I run an R9 290 on a 650w Corsair TX, I can run the GPU and CPU at full load and not have any issues.
@Nuclear_Bob wrote:Your calculations about PSU power requirements is wrong.
When companies list the power requirements for graphics cards they assume a fully loaded system, E.g. a CPU (125w+), a number of optical drives, HDDs, fans, USB devices, etc. They also add a large cusion to allow for cheap PSUs that don't run to specs. Each 6 pin PCI-E power connector delivers 75 watts, each 8 pin delivers 150 watts. In this article, an R9 270 has a draw of less than 150 watts. A closer calculation of your system PSU requirements would be 150 x 2 (2 R9 270s) + 125 (CPU) = 425 Watts, this doesn't take into account other components though and you would have to add their draw as well. Also you want to run your PSU at less than 85% load, otherwise you can reduce its lifespan.
For your system I would suggest a minimum 650 watt PSU, or greater, this would allow 100 watts to other components such as HDDs and your MoBo, USB devices etc. + 15% to keep the load within 85%. 750+ would allow more headroom though. I run an R9 290 on a 650w Corsair TX, I can run the GPU and CPU at full load and not have any issues.
You forgot the additional 75 watts for PCI Express. A R9 270 with two 6 pin connectors needs 2 x 75 watts + 75 watts = 225 watts
My own R9 290 with 8 and 6 pin connector needs 75 + 150 + 75 = 300 watts max. I use it with a 480 watts power supply without problems.
