Generally speaking any new monitor you purchase today will use far less energy than something that is 10+ years old. The technology has just gotten so much better. Here is a link to energy star rated monitors.
https://www.energystar.gov/productfinder/product/certified-displays/results
Notice the most efficient monitor uses a mere 3.64 watts when it's on and 0.05 watts in sleep mode which is mind blowing compared to the way they used to be.
