A desktop computer is made to use around 400 watts of electricity per hour when you're really using a lot of power from the computer.
That would be for editing videos, gaming etc.
But during average use such as browsing websites, viewing Youtube videos etc your desktop computers uses on average of 100 watts to 150 watts of electricity per hour.
That is not a huge amount and a space heater would use more electricity per hour than a desktop computer would.
I leave my desktop computer on 24/7 and do a lot of uploading and downloading of large files overnight when I'm asleep or when I'm away from the computer.
When I started leaving my desktop computer on all the time I only noticed my electricity bill went up by $15.00 per month.
That's not a huge amount and is cheap enough for the convenience of having my computer ready all the time so I don't have to wait for the computer to boot up all the time.
Also it's better for the computers lifespan to leave it booted up and turned on all the time instead of turning it off and on all the time.