Mileage intermittently drops by over 20% with A/C on lately

When you run the A/C try to keep it at a higher temperature like 77 and on one of the 2 lowest fan speeds.

As far your other question the A/C is not belt driven in the 2004+ and is all electric. This is good as it does not require the engine to run while stopped at a light in order to stay on.

 

I notice the same phenomenon here in Phoenix. Today its 114 and I set my AC to 78 in Auto mode. The system cools the car down and the fan speed drops to an almost imperceptable speed. I've noticed when the fan is running very high, likely the compressor is also and the drain on the battery is substantial. One time at a drive through, the battery gauge dropped from 5 blue bars to 2 pink bars over the 10-mins I was working my way through the line. So, this is evidence of the high drain on the battry to run the compressor at high speed.

Conversely, when the fan has dropped to a very slow speed, likely the compressor has also and I barely notice a drop at all on the battery.

So, my answer to you, mfa-prius, is what you're experiencing is normal. Under high demand from the AC there's a large load on the battery and the system will run the engine a bunch to keep the battry charged and hence, lower gas mileage. Last winter I was getting 60+ mpg but this summer its dropped to 50+. That's about a 15% drop. But, I'm still doing much better than the EPA numbers.

 

it could be that you are driving shorter routes then you used to as well. The battery usage to propel the car is much greater while the the vehicle is in its warming up cycles (ie the first 5 minutes) so you add the warming up + the A/C use and you are using alot of battery

 

With high temperature 90+ and high humidity, the AC compressor will draw 4KW/hour. Just doing some simple calculation, typically the Prius will need 250Wh/miles driven, so for 4KWh is equivalent to 16 miles driven in an hour.

The power will come from the ICE and this power is robbed from the FE,
this is like losing 16 mpg with the AC on. This is an approximation to explain why the drop in mpg.

 

You might need to check everything over just to be sure. 0W30 oil, air filter, tire pressure, change transmission oil, fan on low and recirculate, coolant drain and fill, and most of all have your wife push at stop and go traffic.
Your AC needs a full complete check up. Did they actually do a freon pressure check and an AC compressor cycle test or just shove you out the door? Sounds like a compressor/electric motor bogging down to me.
Get a second opinion.

 

Have you checked the cabin air filter? Perhaps its clogged, forcing the ac to run harder to keep you as cool, since the air isn't flowing as fast...

 

Keep that wife pushing man!
Not that you care but I never run the AC unless my wife is in the car.