I am currently running Ubuntu 18.04 which ships with a 4.15.x series kernel, but as reported around the web the 4.17 series have improved power management and can apparently extend battery life.
I have been running this latest series for a couple of weeks now and the battery life on my Lenovo laptop is greatly improved, coming very close to or even the same as the battery life I experienced using Windows out of the box.
I've never really needed to upgrade the kernel a particular distro ships with beyond the usual updates, except in this case which I explored due to the abysmal battery life I have always experienced on any of the distributions I have tried.
Though not as common as Debian, for example which moves at a much slower pace, any time I researched manually upgrading kernels for Ubuntu I've always seen posts and comments generally discouraging it, and I guess depending on how it is done it can be error prone and could potentially lead to a non-bootable system. Though I am quite familiar with a lot of Linux, there are still a lot of things that I don't know and don't necessarily have time to research before hand or fix especially when I am still needing to get a lot of work done.
The best solution I have found for this is an official Ubuntu mainline kernel repo which contains automated builds of the latest supported releases: http://kernel.ubuntu.com/~kernel-ppa/mainline/ here you can simply download the necessary .deb files (ensuring you get the -headers, -image, -modules components) and just dpkg -i linux*.deb them.
Update 2018-09-29 I don't recommend this or do this anymore, for a specific and likely Ubuntu-only reason, that the mainline kernel isn't compiled with user namespace support, and I have been dabbling with this functionality with certain Docker containers, so this is something to be aware of. Also see this guide for general recommendations.