Using defrosters vs AC for heating...which is more fuel efficient

As long as you have the temperature set to more than outside temperature that’s enough to keep the windows from fogging or freezing. So basically set it to one step above max cold. Just use higher fan speeds to keep the windows from fogging or freezing.

Always use fresh air mode in cold, newer recirculation mode. Check that cabin air filter is clean (change it). You can block the upper grill when you don’t use AC to cool. You can block most of the lower grill if temperature is below 7c 45F (or even warmer depending on your driving).

-Use rear window heater if mirrors fog up. No other way to keep them from fogging. And you do need them for driving.
-If the engine is colder than 40c or 104F use front window mode. If you don’t have engine temp gauge this just means couple of first miles. If it’s above freezing this uses a bit of AC but that’s still better than taking more away heat from the engine.
-If it’s warmer than about -14c or 7F switch to window+feet when engine stays above 40c or 104F. If you don’t have engine temp gauge this just means after couple of first miles. This allows the engine to shut down more.
-If it’s colder than -14c or 7F blower mode doesn’t really matter but use either front window or window+feet as anything else will cause windows to fog.
-As long as engine temp stays above 70c or 158F you can use as much heat as you like and not affect the fuel economy. If you don’t have engine temp gauge this is harder to know. Basically you can use more heat on longer highway drives.

In winter I use temp setting 16c (lowest except max cold) and fan speed 1 or 2 if driving alone. And may put the fan speed up to 4 with more people.

 

To answer your question, The A/C runs with the defrosters. Therefore more fuel is used.

 

Front window defrost which in a Prius is actually only blower direction does turn the AC on. But actual compressor only runs if outside temperature is above freezing. And even if temperature is warm enough for the compressor to turn on it will only run at very low power. So if the engine is cold bit of electricity used to turn the compressor is less energy than using more energy to run the engine more just to keep it warm.

 

you're in miami.