You may not realize this, but when battery saver says your screen is using battery... that doesn't mean the screen hardware itself. That's just battery life that being used while the phone is not in standby, and which cannot be attributed to a particular app. In WP8.1 the battery app was better at tracking which apps used power, so it didn't show as much for screen on. In Android, screen on, or display on, or whatever is almost always the highest battery drain, since Android doesn't do a good job of tracking which app is using power. It's usually Display On using the most, then Standby, then other random stuff. Even though I knew that most of the power was used on Clash of Clans and Instagram.
In fact not really, cause in W10m the 80% power consumption by the screen is just opposed to the cellular, wifi and process.
So you have a 80% display, 8% process, 5% cellular and 7% wifi (this always sum to 100%).
And then messaging at 50%, a game at 25%, etc (for example)
That just means 40% of the battery have been used to display the messaging app and 4% to process it, 20% to display the game, 2% to process it, etc.
It's an average of course, a game will probably ask for more process power.
That's why display+wifi+process+cellular =100%
AND the sum of the battery use of all the app listed also do 100%. The ration in the display, cellular, process and data doesn't add to the app's one like if it was a leftoverof some app not recognised. It's just two way to study the battery use. Hardware wise and software wise.
In fact, 100% used in display would means the only way the phone drains the battery is by turning the screen on, with no process, no data in background, just like a lamp. It's in fact the way the phone would last the longest.
But the more you use it, the more process and data stuff use energy, and in proportion the display use less. But that just means the phone will ask for a recharge quicker. Cause the screen consumption is more or less a constant (less true with amoled and black screen that save a little).
My 1520 could last 2 days with 80% battery used by the screen, cause I don't ask it to do anything.
If I play game, display will drop to 60% because the process will use 30% (10% average on wifi/cellular) and the phone last only 6 hours.