Battery life on upgrading to the iOS 7 from iOS 6 will definitely be worse because of the new design. The usage of colors on the iOS 6 are a lot more darker compared to iOS7 being completely white.
Let us take a bit of physics into consideration, we can say that white color drains most battery while displaying on the screen, because every wavelength of visible light must be emitted to produce white light. More wavelengths = more energy needed to produce light, so that consumes more power from the battery. Moreover, blue light has higher energy than red light (e=hf), therefore those users who are looking to save as much power as possible are suggested to pick a dark red wallpaper.
Apple has not designed the iOS 7 to be power efficient this time. You can try to improve battery performance by inverting the colors of the display under Accessibility settings. Then again, most of the power on a smartphone is consumed by the display, doing what you can to reduce its power consumption helps greatly.
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