By default, Construct does not use a wake lock. If you switch to a different app or lock the device Construct will suspend the game so that it becomes completely inactive. (If you want a wake lock it's currently only supported by third-party addons, and yes, tends to needlessly hammer battery life.)
Games are generally pretty battery-intensive: during gameplay the screen is continuously on, and both the CPU and GPU are working pretty hard to run the game, so a lot of the system hardware is powered up. I'm not sure there's much to be done about that. I suppose it could drop to half-framerate mode, but that makes the game look less smooth, and HTML5 doesn't have a good way to do that yet.