Is the car always less efficient with AC "on"

When the light is on, it indicates that the control logic can use the compressor if the conditions suggest that it would be beneficial. Even though you may not be using the compressor to cool the air, depending on the conditions, the vehicle may still choose to use the compressor to reduce humidity.

The compressor is driven by an internal electric motor. It is not belt driven. So when the vehicle chooses to use the compressor, it does not create more "drag" directly on the internal combustion engine. It does draw electric power, and eventually that electric power will need to be generated. So if the control logic chooses to use the compressor, it probably will *slightly* effect the overall fuel usage. Of course it will do it to improve comfort or prevent fogging of the windows, and the amount of fuel eventually used may barely be noticeable.

So to answer your question, the car may be slightly more fuel efficient in winter with the AC light off, but you may have higher humidity in the car which may lead to fogging or frosting in the inside of the windows.

 

In a Prius, it's best to think of "air conditioning" as exactly that -- a device and a process that will heat or cool the air, as well as reduce its humidity if needed, according to environmental conditions.

Most of us find that simply setting the climate-control system to "auto" and then selecting a comfortable temperature is all we need to do. In some conditions you may need to use the front or rear defrosters, but beyond that it's pretty much a "set it and forget it" arrangement.

A final note: The traction battery functions at its best and most efficient at temperatures that are comfortable for humans. There's a false economy in choosing to swelter or shiver in the hope of improving your fuel economy. In the long run, letting the climate control system do its (automatic) thing is the best way to go.

 

I just wanted to add that from a practical standpoint and for my city and driving habits A/C hurts mpg in the city, but its impact on the highway, where more extra charge is usually generated, is a few decimal points of mpg.

So in MD I only use A/C in the city when the the low falls below 32 F (0 C) or the high is above 90 F (32 C). On the highway I will set the Auto setting to 70 F in the winter and 74 in the summer.

 

I'm not so sure that it has anything to do with charge being generated on the highway vs. the city. It think it has more to do with small amounts getting lost in big numbers.

CITY:
If you average (after stoplights, stopsigns, traffic, slow speed limits, etc) 20 MPH and drive for half an hour, you will have covered 10 miles. If, with the A/C off, you burn through 0.2 gallons of fuel to travel that 10 miles, you will have averaged 50 MPG. If during that same 30 minutes you drive with the A/C on and find that the A/C burns through 0.4 gallons per hour to keep the chosen temperature, you will use an extra 0.2 gallons (for a total of 0.4 gallons). Having used an extra 0.2 gallons during the half hour drive, you are now averaging 25MPG

HWY:
If you average 60MPH and drive for half an hour, you will have covered 30 miles. If with the A/C off, you burn through 0.6 gallons of fuel to travel that 30 miles, you will have averaged 50MPG. If during that same 30 minutes you drive with the A/C on and find that the A/C burns through 0.4 gallons per hour to keep the chosen temperature, you will use an extra 0.2 gallons (for a total of 0.8 gallons). Having used an extra 0.2 gallons during the half hour drive, you are now averaging 37.5 MPG.

It isn't that less fuel was used to operate the A/C on the Hwy. In both cases of this hypothetical example you used an extra 0.2 gallons of fuel to operate the A/C for a half hour, but the city MPG drops to 25 while the HWY only drops to 37.5. The A/C hasn't used less fuel to operate, you've just traveled more miles in the same amount of time.

Of course I've exagerated the numbers here a bit for dramatic effect (the A/C probably uses quite a bit less than 0.4 gallons per hour to operate), and specifically chosen times and MPG values that are identical for both City and Hwy to demonstrate how speed (and therefore distance) may be what is causing Hwy use of the A/C to seem more efficient.

Here is an example of how Hwy MPG could appear better even if City A/C usage was more efficient:

CITY:
Without A/C - Drive for one hour averaging 20 MPH and 50 MPG (0.4 gallons used)
With A/C - Drive for the same hour averaging 20 MPH and 45.45 MPG (0.44 gallons used)

HWY:
Without A/C - Drive for one hour averaging 65 MPH and 48.15 MPG (1.35 gallons used)
With A/C - Drive for one hour averaging 65 MPH and 46.1 MPG (1.41 gallons used)

Notice that the CITY example only used an extra 0.04 gallons to operate the A/C for an hour while the HWY example used an extra 0.06 gallons to operate the A/C for an hour (The A/C system burned 50% more fuel on the HWY than in the CITY) and yet the CITY MPG numbers dropped 4.55 MPG from better than the HWY to worse! While the HWY MPG only dropped 2.05 MPG! Clearly in this hypothetical example it would be better to use the A/C in the City where it uses much less fuel, and yet just looking at the MPG numbers it would seem as if you'd be better off using the A/C on the HWY.

 

The term "AC" as used on the Prius display is confusing. It is indeed, as Vic says, conditioning the air. But we are accustomed to using the term in the context of summer cooling. I prefer the term climate control (CC).

Danny explains well that the ratio of CC power demand to ICE power demand is considerably higher at lower speeds. The Prius MPG Simulator (and user experience) demonstrates the effect of this. The simulator performs modeling using a variety of user-specified criteria, including speed, outside air temperature (OAT), automatic climate control (ACC), and the following combinations of window positions: all up, all down, two halfway down, and two diagonally opposed 1" down.

The chart below shows results with 90F (32C) OAT, with and without ACC set to 75F (24C), and all window position models. The models assume the following:

• Steady speeds
• Level terrain
• No wind
• 190' elevation
• 30.00 barometric pressure
• Smooth dry pavement
• Michelin Hydroedges inflated to 50/48 PSI
• E10 fuel
• 300# load, driver + cargo

There is another highly significant variable at work in cold weather: ICE temperature. I haven't run the simulator to create a cold-weather counterpart to this, but my hunch is it will show a greater low-speed difference between ACC-on and ACC-off. Even though CC is powered by electricity, it uses the ICE as its heat source. In more moderate temperatures, we expect the ICE to complete its warmup cycle fairly quickly, allowing the ICE to reach S4 temperatures and shut down when coasting, gliding, or braking. Running CC in cold weather delays ICE warmup, and even aborts it at slow speeds. The ICE remains in S2 as CC continuously robs it of heat; the only time ICE shutdown occurs is at a full stop. The defroster makes things even worse as it often forces the ICE to run even at a stop.

So a worst-case scenario for Prius fuel economy is short trips in stop-and-go driving at slow speeds in the winter with the defroster running at full blast. It's easy for trip FE to drop into the 30s or even the 20s in these conditions. It is because of this that certain fuel economy nerds () forsake CC completely in the winter until the cold becomes unbearable. We ... er, they rolleyes... also block the front grille and use an engine block heater.