Brief introduction:
I'm programming a game and, till now, I've runned it on my desktop. This morning I decided to try it on my laptop (a i3-4005u with a gt920m) to see cpu/gpu usage and, with my surprise, it couldn't even reach 60 fps... then I discovered that it didn't use the nvidia gpu but the intel's integrated one. I looked a bit on Google and found the problem: <https://stackoverflow.com/questions/10535950/forcing-nvidia-gpu-programmatically-in-optimus-laptops>.

The usage of the GPU (the Nvidia one now) was still to high for such the game I'm making (kind of 2D platform). Trying it on my desktop (gtx 780) I discovered that it uses even 50% of its power (MSI Afterburner).

A screenshot:

[![game screenshot](https://i.stack.imgur.com/8wK1X.png)](https://i.stack.imgur.com/8wK1X.png) 

To generate the light map I use a render-to-texture and here is the problem: rendering the lights uses all the power (profiled with visual studio). The only difference between rendering the lights and the background is the active RenderTargetView and the only different between the default back buffer and the light map RTV is the format so I changed it:

```
textureDesc.Format = DXGI_FORMAT_R8G8B8A8_UNORM; //it was DXGI_FORMAT_R32G32B32A32_FLOAT

```

After that the gpu usage dropped to 3-4% (and the game looks the same).
Now I'm wondering why things changed. Can you help me?

Still now rendering the lights is the most expensive things and I can't understand why (the shader is simpler than the sprites' one. I can post the code if you want, but is a basic "load data from texture and output")

[![enter image description here](https://i.stack.imgur.com/QJFIj.png)](https://i.stack.imgur.com/QJFIj.png)

The first draw call is for lights, then there is the background's one.
I really can't understand what's going on.

P.S
I also found that using a borderless window instead of the fullscreen mode by default adds a 10% of usage on the gpu, while fullscreen mode uses all the cpu power (100% of usage). Can you help me again understanding why?