Does anyone know why Linux laptop battery life is so bad? Is it a case of devices needing to be turned off that aren't? Poor CPU scheduling?

It's ACPI - most laptops ship with half-broken ACPI tables, and provide support for tunables through windows drivers. It's convenient for laptop manufacturers, because Microsoft makes it very easy to update drivers via windows update, and small issues with sleep, performance, etc. can be mostly patched through a driver update.

Linux OTOH can only use the information it has from ACPI to accomplish things like CPU power states, etc. So you end up with issues like "the fans stop working after my laptop wakes from sleep" because of a broken ACPI implementation.

There are a couple of laptops with excellent battery life under linux though, and if you can find a lunar lake laptop with iGPU and IPS screen, you can idle around 3-4W and easily get 12+ hours of battery.

In addition to the other comments, its worth noting macOS started adding developer documentation around energy efficiency, quality of service prioritization, etc. (along with support within its OS) around 2015-2016 when the first fanless usb-c macbook came out: https://developer.apple.com/library/archive/documentation/Pe...

Think I'm arguing its both things where the OS itself can optimize things for battery life along with instilling awareness and API support for it so developers can consider it too.

> Does anyone know why Linux laptop battery life is so bad?

It's extremely dependent on the hardware and driver quality. On ARM and contemporary x86 that's even more true, because (among other things) laptops suspend individual devices ("suspend-to-idle" or "S0ix" or "Modern Standby"), and any one device failing to suspend properly has a disproportionate impact.

That said, to a first approximation, this is a case where different people have wildly different experiences, and people who buy high-end well-supported hardware experience a completely different world than people who install Linux on whatever random hardware they have. For instance, Linux on a ThinkPad has excellent battery life, sometimes exceeding Windows.

My Dell XPS had pretty good battery life on linux. Probably better than on windows. But Dell sells the XPS wiht linux preinstalled. So I assume it has a lot to do with the drivers. Many notebooks have custom chips inside or some weird bios that works together with a windows program. I'd say laptops are more diverse than desktop PCs with of the shelve hardware.

A big part of it is chipmakers deprecating S3 sleep in favour of Modern Standby.

Install powertop, the "tunables" tab has a list of system power saving settings you can toggle through the UI. I've seen them make a pretty big difference, but YMMV of course.

I ran into this problem on a Slimbook some years ago now. I found that my battery drained way too fast in standby, and I remember determining that this was some (relatively common) problem with sleep states, that some linux machines couldn't really enter/stay in a deeper sleep state, so my Slimbook's standby wasn't much of a standby at all.

But that's just one problem, I bet.