A/C is *EVIL* 1111

The impact can vary quite a bit depending on what your commute is like, and what the temperature is set to. To reduce the impact, I suggest raising the temperature; you don't need the cabin to be cool, you just need cool air to be blowing on you. I often start the temperature out at 78-80°F (or whatever it takes to run the fan at around 4 bars - not really fast, but not really slow either). I rarely drop the temperature below 75°.

If you spend a lot of time stopped or at slow speeds where the engine wouldn't normally be running, that makes for a big drain on the battery (which is why you saw it at only 2 bars), which it resolves by charging the battery for the brief times when the engine is on. Also, driving aggressively to charge it certainly won't help your fuel economy. Let the computer take care of the battery; it'll charge it before it gets too low (even if you're stopped, as long as the car is in Ready - so on and not in Neutral), and it has to run the engine in the morning when you first turn it on to warm everything up anyway.

Also, if you don't let the A/C system run in auto (e.g. you adjust the fan speed manually), that may cause inefficiency; the amount of power used to cool directly depends on the set temperature. So if you have the temp set to "LO" but manually adjust the fan to 1 bar, you're using tons of energy to make a tiny bit of air very, very cold. It's more efficient to run the fan at higher speeds, making lots of air just pretty cold. So I'd recommend using the temperature setting in full Auto mode to indirectly adjust the fan, if you're not already doing that.

 

The energy to run the compressor has to come from somewhere!
One salesman tried to tell me it was free electricity that ran the system.
I did not buy from him!

 

What mode did you use? That is one particular situation where the differences in behavior with ECO could be apparent.

 

Macman pretty much covered it.

Stop driving around aggressively. LOL

Read this and apply as much as you can. How to Reduce MPG loss Because of A/C Use | PriusChat

Sadly, when temps reach 100F you are at the critical point where A/C is a must (for most people) and tire rolling resistance is at its lowest. So while it should be the best time to try to high mpg you must try and keep cool. Or you can just be like me and do a little cross flow cooling and deal with the heat. My trip home (47miles downhill) was 85.4mpg and it was 99F outside.

 

Everything the others have said is spot on. I would emphasize pointing ALL the ducts right at your neck, mid section or whatever part of you needs cooling. You will be surprised how high you can set the temp at, and how low you can go with the fan. Of course, if you have passengers, you can forget this as they won't understand. But this strategy will greatly reduce power consumption and hence improve mpg.

If you're going a steady 55 mph, the draw from the A/C will be almost unnoticeable. If you are in stop and go and in direct sunlight...you lose.

 

No, but I don't have a Gen 3.

Just keep in mind Car and Driver: Mileage? No, it's Your Gallonage that Really Counts | PriusChat. The difference in gallonage over say 100 miles of 65 mpg vs. 42 mpg is 0.84 gallons. The difference for 20 mpg vs. 17 mpg is greater at 0.88 gallons.

Although the 65 to 42 mpg drop sounds huge, would you complain as much if you dropped from 20 to 17? Would the latter seem more palatable and less noticeable?

 

Try living where I live. It is 100+ degrees mostly 5 months (May-Sept) out of the year and sometimes on some days in April, Oct too.
It sucks.

You will find F8L post useful.
I use AC all the time and I am averaging on my last 5 tanks (05/23-07/07) 53.1 mpg.

 

I doubt this is right, but as soon as I have an android and the Torque app this is an area I plan to look into.

Talking about cooling ---
I *think* that the temperature setting determines how much power the AC uses to cool the air entering the cabin, based on the difference between the cabin temperature and the setting. Fan speed is clear. You can cause inefficiency by forcing the AC to markedly cool down a small amount of air; think carnot cycle.

The larger question is whether AC is run in circ mode. I do not think there is a problem running the fan at any speed you want in re-circ mode, but I personally use auto mode in non recirc mode.

 

You monitoring batt temps during that? Just curious what temps you've seen.

 

I have been trying but I'm not 100% sure I am using the correct pid. On Torque I chose HV battery temp 2. Maybe I should have chosen 3? Temp inside the car was between 99.3F-100.4F. Outside temp was between 93F and 99F by the time I got down the hill. Battery temp started off at 86F and ended at 93F. Interestingly, when I got inside the car the temps were as follows:

Ambient temp: 93F
Cabin temp: 99.3F
HV battery temp: 86F

This was prior to taking the sunshade down and I took the readings with the door closed. I park in full sun and it was just after noon when I left. Of course I had the solar roof vent going as usual. I'm still getting used to using Torque so I didn't log the data. I just kept an eye on it. FWIW I used my usual techniques and ended the trip at 85.2mpg over 48.7miles (stopped to pick up a movie) and 66.3mpg on this tank (193.8miles).

 

Hmm...I suppose it could be that low. But I rarely see batt temps that low, especially when it's in the 90's outside. Maybe the solar ventilation really is that awesome!

 

Maybe I have the wrong PID? There were only 3 options for HV battery temp so I chose the middle one. Which one do you use?

 

I use the middle one too. It's probably accurate. Just shows you how good the solar ventilation is.

 

It's supposed to be 104F tomorrow so I'll track it again. I'm on too good of a tank to use A/C though.

Hmm, my girlfriend just said we have to go to Chico tomorrow to pick up her son. There goes my tank. Unless I offer to pay for her gas.... Hmmm LOL

 

I have a ScanGauge, so it's what I've observed (from the admittedly relatively-rare occasions when I use A/C). I think your second paragraph is correct, and is the same as what I was trying to describe, so I'm not sure what you're flagging as incorrect in my statement.

Basically, the A/C power is always "Auto". For best efficiency, it's probably best to leave the fan in Auto too; if the car is at the set point, the fan might be running too fast (wasting power moving lots of air), and if the car is above the set point, the fan might be running too slow (wasting power cooling just a tiny amount of air).

As for recirculate/fresh, I think that's a fairly straightforward choice. To the first order, you want recirculate on if the inside air is cooler than the outside air. However, the A/C is also drying the air as it cools, so you actually want recirculate on starting when the inside air is maybe 5-10°F warmer than the outside air, since the hot dry air inside the car holds less heat than the less-hot moist air outside. Of course, if you live in a dry climate, this effect is minimal. But maybe I'm missing another complication in that decision?

 

Pfft! While AC hurts, it's frigid Winter that destroys mileage!

 

Hmm... I've never noticed a disappointing MPG whenever I use my A/C. Then again the outside temp rarely goes above 79 here by the coastline and I have covered parking so the car is never too hot whenever I jump in.

Perhaps that's the dealbreaker: do you hop in a cool car to begin with or is it baking inside the car whenever you leave it under the sun?

 

The closer the cabin temp is to your A/C temp setting the less the A/C has to work so the less of a mpg loss you suffer. In my testing the mpg hit was only 2mpg instantaneous when outside/inside temp was within 2degrees F of my desired A/C setting. Always remember this, to do work you must use energy. Reducing air temperature from a hotter state to a cooler one is work. The closer you can get the cabin temperature to your A/C setting the less work needs to be done.

 

That's because your fuel efficiency is attacked from multiple angle instead of just one.

Heater/defroster use
Increased air drag
Increased tire rolling resistance
Increased ICE warmup times
Increased time ICE must run to maintain operating temperature
Increased oil thickness at startup
Increased wet, windy, snowy weather
Etc. etc. etc.