I'm not sure if you'll really save yourself much electricity with what you're planning, if any.

GPUs shouldn't really be pulling much power when they aren't being used, but as another poster mentioned you can test this with a kill-o-watt or other similar tool. If you care about electricity usage and don't have one it's a great lil thing to have around.

You can test by doing the following:

-run PC at idle without GPU inserted into mobo and then test again with GPU plugged in.

If you want to check if switching back and forth between output ports (igpu vs card) you can try and switch your monitor cable while the PC is running, but there's a good chance that the GPU will be active even if you don't have a display plugged in. You can test this by:

-boot PC with GPU inserted and monitor plugged into GPU, then swap the cable to igpu and see if that makes a difference. I highly doubt it will make a noticeable difference, but if it seems worth it to you I think the easiest way to switch is a KVM switch or other device made to swap a single display amongst different devices.

But at that point you're saving pennies at best and it'll all go towards your new kvm switch unless you want to be plugging and unplugging a cable all the time... And I'm no expert in how various components draw power in a PC when they're not being used (probably firmware/OS dependent) but I still think your GPUs will draw power even if not actually connected to a display

Just keep your stuff plugged into your GPU my guy. If you want to drop power use and noise then tweak the power/fan curves, underclock it, make sure you've got good airflow, etc