While reviewing Journler, I stumbled upon a funny command - Toggle Low Light Display. This is the same as System Preferences > Universl Acces > White on Black and is obviously meant for writing in the dark, but it made me wonder if it would also help to conserve battery power. The reasoning being that in a standard black-on-white configuration we're burning pixels that really don't have to be lit at all. Granted, the majority of power consumption in a TFT comes from backlighting, which has nothing to do with the video signal, but I was convinced that there would be a difference. The only question is - how big of a difference?

To set up a consistent environment, I compiled an Automator workflow that triest to simulate a typical writing scenario - grab a few HTML pages, find some words, create a new document in TextEdit, paste the text, pause for 5 mins, check email, repeat (I created the loop by adding an Open Workflow step that opened the same file). The display in this case is a 20” ACD. I also have a bunch of USB devices, but they're not used during the test so they should all be constants. Display Sleep was disabled and brightness was at maximum. The Mac was left to charge overnight between the test runs.

I run the script twice, first with Black-on-White and then the opposite and measure how long the battery lasts, from being totally charged until it completely blacks out.

Luckily, Automator comes with a cli interface making it easy to measure the time. This is not 100% accurate because to stop the timer, I had to reconnect the power cord and kill automator:

> time automator /Users/filipp/Desktop/Loop\ test.workflow/

The results: Black on white: 54m55.852s White on black: 52m39.918s

So there's a difference, but it's the exact opposite - WOB actually gave a shorter battery life. I guess this could happen if the Mac is actually doing more graphics processing for the inversion and eating more power as a result. The difference is also so small, that there's really no need for another test.