The TCH has an electric "booster" for the heater. If you don't have ECO enabled, the TCH will use the 12V 1KW electric heater to aid the heater core until the ICE comes up to temperature. If you have ECO enabled, the 12V electric heater is disabled, and the ICE will shut off at a lower coolant temperature.
The ECO mode also runs the AC in a lower power setting to save energy. When I was in Arizona last summer, I found the AC performance acceptable in ECO mode in 110F weather once the car was cooled down. However, if the car was hot, ECO took too long to cool the car.
Our Prius factory repair manual indicates that the 1KW electric heater is only included for north of the border cars for Canadian sales. Additionally I seem to remember it's in the airflow path to the interior windshield surface and is therefore probably only activated or effective in defrost/defog/demist mode.
In any case 1KW equals only about 3000BTU and is therefore only a pittance when it comes to heating the passenger cabin.
"The PTC heater is installed in the radiator in the heater unit and operates when cooling water temperature is low and normal heater effectiveness is insufficient.
The air conditioning amplifier switches the circuit in the PTC relay and operates the PTC heater when the operating conditions (cooling water temperature is below 55°C (131°F), setting temperature is MAX. HOT, air outlet damper position is FOOT or FOOT/DEF, and blower switch is not OFF) are met."
I normally just set the temp and fan, so I can't say how noticeable the extra heat of the PTC heater is.