MacOS – the font the battery percentage uses

I'd say that what's really being measured there is average CPU usage per application. Then some maths is being done in the background to calculate based on CPU usage how many watts per hour are being consumed per app. Knowing current battery capacity/charge would make that pretty trivial in terms of calculation.

On a portable computer, similarly, if an app is really driving your CPU, then your battery is gonna run out faster.

I'd recommend getting hold of something like iStat Menus which can show you realtime CPU usage in the menu bar as well as a bunch of other stuff. Specifically, the CPU monitor in iStat breaks down CPU usage per application.

Get Onyx, go into Cleaning -> Fonts, check all, Execute.

If that doesn't work, check your disk for errors. If it still doesn't work, your fonts are are missing or corrupt - reinstall OS X.