How does the new iPad's Retina Display affect its battery life?
Your question's wording is a bit unluckily.
There is no previous iPad with the same battery capacity. It does not really makes sense to talk of an impact of a new display to the battery-life. That's not a reasonable comparison because the battery has been updated as well.
The retina display of the iPad 3rd generation

The new iPad uses 2.5 times the backlight power of the iPad 2. This is due to the TFT transistors of the high-res LCD display blocking more light. (source)

This also contributes to the heat emission. The overall performance however is only little affected, because Apple compensated the increased power drain by adding a 70% bigger battery.


(pictures 1,2)