There's no real way to specify this. The Prius tries it's best to keep the battery state of charge (SOC) at six bars in the 2004-2009 and 3/4 full in the 2001-2003. When the SOC is lower or higher than these points the Prius will either run the engine more or use the battery more to get back to the "ideal" point. This is varied by the amount of power called for by the driver and other items such as engine temperature, cabin heat, defrost, or air conditioning requirements.
Note that the battery icon only covers the usable SOC of the battery, not the total SOC. This is because the Prius never allows the battery to cycle fully so that battery life is extended. A graph showing the battery SOC for the 2004-2009 Prius is at
http://privatenrg.com/
To get additional instrumentation you can purchase a Scangauge (
http://www.scangauge.com/ ) which reads the ODBII and CAN buses in 2004 and later Prius.
How low does the battery get before the engine kicks in on my toyota prius?
Linear Mode
