I bought a display port cable connect to my dell 30 inch lcd and I turn of built in display. It works great. This way I don't need to use my intel i7 4770k desktop to save some energy if I am not requiring heavy computing resource and increase lifetime of my desktop and save some energy bill. Anything can save electricity is better.
The used laptop cost me around nzd 300 and my brand new desktop cost me around nzd2000 two years ago.