How do i go about powering 15m of LED strips
This is my room:

There's another 4m RGB LED strip on the opposite wall. Both LED strips and the custom LED lamps in the middle of the ceiling are connected to an ATX PSU ( this one, which is overkill by the way, but silent which is important )
My particular RGB LED strips power is about 10W per meter. So about 80W of RGB LEDs in my room. They operate at 12V so there is 6,66 Amps flowing through them. My ATX power supply is rated at 45.8 Amps in the +12V rail, that's 550W, much more than needed for my setup. The LED lamps in the middle of the ceiling are 150 Watts of high power Cree LEDs, also operating at 12V and from the same power supply. ATX power supplies tend to be powerful and cheap for projects like the one you're attempting.
You should power your LED strips only on one end, but you're better off dividing strips into separate segments. The copper traces that carry current in LED strips tend to be thin and offer relatively high resistance. When the RGB strips are full-on there is no noticeable effects in the lighting throught my 4 meter strips, yet when they are controlled to light really dimly, the eye gets quite sensitive to slight lighting differences and LED strips &quot;tail&quot; gets noticeably dimmer than the &quot;head&quot; closely connected to the PSU. I suggest you divide your installation into 4 different equally lengthed segments, all connected to the same PSU with &quot;big diameter&quot; wires. You can search online for nominal wire resistance per meter of a given diameter and calculate the difference in power that your &quot;close to the PSU&quot; LED strips will have compared to the &quot;far away from the PSU&quot; ones.
Let me make one extra comment you didn't ask for. If this RGB system is the only source of light you will have in your room, bare two things in mind:
1- RGB LEDs are inefficient compared to white LEDs, you won't get the same luminosity from 10W of RGB LEDs than from 10W of white LEDs.
2- Combining red, green and blue does give you white light, but a very bad quality one. There's a thing called Color Rendering Index. Things illuminated with &quot;white&quot; light coming from RGB sources will seem strange. In contrast, the white LEDs in the middle of my ceiling have a very high CRI, which makes the room feel like if it was being illuminated by natural sunlight.