The problems with the electrical power system affect a key component known as an inverter, which boosts voltage from the battery and converts to alternating current, similar to what comes out of a household outlet.
The Prius inverters were overheating and frying internal high power transistors, leading to a loss of power that in many cases left owners stranded on the side of the road.

Toyota attempted to fix the problem by modifying the inverter software on model years 2010 to 2014, though it never disclosed how the software or its modifications would result in less overheating.
The fix did not prevent the overheating, and inverter failures continued.