According to various sources, it looks like the JBL output is rated at 40 watts per channel (peak). Realistically speaking, you're probably looking at 20 watts RMS or less.
JBL Synthesis Surround • The Prius JBL audio system comprises nine loudspeakers in seven locations. All speakers utilize metal cone/dome technologies to provide a precise, accurate representation of the source signal. • Its 7-channel JBL digital signal processing (DSP) amplifier produces 385 watts of pulsing power on demand, delivering quality power and headroom to properly drive each of the 9 specialized loudspeakers in the system. • This JBL audio system has been acoustically engineered for Prius’ unique cabin properties, accounting for a broad range of bass response to match the music selected, and specifically equalized to provide an unparalleled listening experience for all occupants. • All two-channel listening formats are enhanced by JBL’s awesome sound systems. • This JBL audio system is fully covered under the overall Toyota product warranty, and will be serviced by any Toyota dealership nationwide in the unlikely event it is needed.
That 385 watt figure is meaningless, as the amp can't deliver that amount of power continuously, or at a low enough distortion level to be tolerable (<1% THD). All you have to do is pull the amp out of the car and take a look at the puny output transistors and tiny heatsinks (not to mention the undersized switching power supply) and you'd know that 25 watts/channel is far closer to the actual usable amount of power. There's a reason why my 4x100 watt amp weighs about ten times as much as the JBL amp.
I believe this includes the answer, plus much, much more: "The JBL amplifier is separate from the radio; it lives under the passenger seat. It is rated at 45 watts per channel to each of 4 channels by Toyota." Prius JBL Audio system response, specs, and photos
Once again, any power figure that does not state the following conditions is meaningless: 1) Input voltage to the amp - 12.5 or 14.4 volts 2) Continuous power rating 3) All channels driven 4) Total harmonic distortion 5) Load impedance
