I believe some motherboards offer a feature that can come in handy. The name varies from vendor to vendor. They will activate the video ports on the motherboard and enable a monitor to be run using the iGPU.
Not sure regarding switching to iGPU when on the Windows desktop, but about the last three generations of GPUs have their own low - power modes. My GPUs sometimes clock all the way down to 20Mhz - though usually 'idle' at 80Mhz.