Pro Tip: Setting the climate 71F or lower uses less energy in hot weather

[hamtonp]

With hot Summer weather approaching, I thought to point out that setting the climate above 71F consumes about double the amount of energy. Intuitively, the thinking is setting the climate warmer means the a/c runs less often. The reality is that in Teslas, it creates heat to get the air to the warmer temperature.

For the graph below, I used ChargePoint for my testing. While the car is charging, you can log into your ChargePoint account using a browser and view the current charge rate. When the car is done charging, it uses the power from to charger to power its devices and functions. This is when you can monitor how much energy the car consumes. I used this opportunity to do some testing. The car is parked in a covered parking garage.

9:30 - Car completed charging 85% charged
9:50 - Climate on at 75F, car temp 92F, outside temp 92F, 2kW
10:45 - Climate off
11:10 - Climate on at 70F, car temp 88F, outside temp 94F, 0.88-0.97kW
11:30 - Climate off
12:05 - Climate on at 76F, car temp 80F, outside temp 96F, 1.8-1.97kW
12:20 - Climate on at 69F, car temp 76, 0.8kW
12:40 - Climate on at 75F, car temp 69F, outside temp 102F,1.59-1.92kW
1:00 - Climate on at LO, car temp 75F, outside temp 104F, 3.17kW and 2.65kW usage
1:09 - Climate on at 65F, car temp 57F, outside temp 104F, 0.87-0.89kW

 

[Frank99]

Wow, that’s some interesting info. Makes me want to go out and do some measurements to verify-nah, that sounds like work.
Good work.

 

[RayK]

1:09 - Climate on at 65F, car temp 57F, outside temp 104F, 0.87-0.89kW

I assume that this is a typo? That you meant to say 75F?

Or did your car actually get about 20 degrees colder with the A/C set to LO for 9 minutes?

Also, what was the fan speed used during these cool down periods?

 

[pt19713]

One thing you're forgetting is the car is using other accessories to cool the battery pack, depending what the battery pack temp was after you finished the charging session. With ambient temps around 90-95F in my area the last week, I'm seeing battery pack temps of 105-109F. The car is pulling 400-700 watts just to keep things cool. By the afternoon tests you've done, the active cooling may have finished and that would throw off the numbers compared to your late morning tests.

I monitor the battery draw with Scan My Tesla and I find that the with the cabin temp set to 69F and fan speed around 5-7, the A/C pulls about 700 watts. In the image below, with the car running, total battery drain was 1.36 kW. When I turned off the A/C, it dropped to 500 watts.

I can try testing it at 75F but I'd have to test later in the evening or during a day when the car isn't being driven. This type of test is better done when ambient isn't as hot since the BMS is constantly pulling energy to run the fans, pumps, valves, regulators, etc.

 

[hamtonp]

I assume that this is a typo? That you meant to say 75F?

Or did your car actually get about 20 degrees colder with the A/C set to LO for 9 minutes?

Also, what was the fan speed used during these cool down periods?

It was actually 57F. So setting it on LO did get the temp pretty low. Once again, this was in a covered parking garage.
The fan speed was just on automatic. I changed the temperature from the phone.

 

[hamtonp]

@pt19713
I would think that testing in lower temperatures would be trickier since lower temperature would cause the car to use the heater more.
You have a better tool. I would like to see what you come up with.
I'm going to dig into my Teslamate logs. I wonder if I can pull out some raw data.

 

[Candleflame]

this has been known for a while. I dont think the number is exactly 71 Fahrenheit or 21.5C, it sort of depends on your ambient temperature, but usually below 22C the car doesnt heat the air up. I would presume in high humidity and even lower temperature needs to be chosen i.e. 20C.

The AC really just has 1 level and that is ice cold. If you cool the air from i.e. 35C to 22C that is quite achieveable. But if you cool it from 26C to 22C by the nature the air ends up way too cold so has to be heated up again. This gets made worse if the air has to be dehumidified where i.e. there might be a need to have the AC running even though its cold outside (good example are the tropics where i live, 24C but need aircon at night even and usually the problem is that you have to put it to way too low so you end up freezing at night. Dehumidification mode helps a bit...

Hence there is no 'heater off' button in a model 3 either.

There are apparently A/C designs which are more efficient and can avoid using a heater but they tend to be sluggish to cool things down so dont make sense in a car.

What you can do to disable the heater completely is to set the temperature to LO and manually set the fanspeed to 1-3. Where I live it is quite humid, so as a result the air ends up being way too cold if you want to be comfortable. I counter this with the heated seats. I used to do this quite a bit on road trips as we only get 43kw charging here. However, the reality is that this only really saves maybe 2-3kwh. so its only like 3min longer charging. Not worth the hassle/discomfort.

 

[rrolsbe]

I live and drive in an area where humidity levels are usually very low so heat is rarely needed while using the AC. I set the HVAC/AC settings to manual, temp set to LO and modulate the cabin temp using the fan speed (voice command works great). My monitoring using SMT and a DC current probe around the HV cable, leading to the heater elements, showed no power delivered to the radiant heating elements. Using this method, I see very little range drop while using the AC. It is possible the AUTO mode is smart enough to only add heat when humidity levels are high?

 

[Candleflame]

I live and drive in an area where humidity levels are usually very low so heat is rarely needed while using the AC. I set the HVAC/AC settings to manual, temp set to LO and modulate the cabin temp using the fan speed (voice command works great). My monitoring using SMT and a DC current probe around the HV cable leading to the heater elements shows no power delivered to the radiant heating elements. Using this method, I see very little range drop while using the AC. It is possible the AUTO mode is smart enough to only add heat when humidity levels are high?

if you do motorway driving then AC does have very little impact. Even when its 45C or something AC only uses around 1-1.3 kwh or so.

Tbh even heating isn't too bad once the cabin is nice and warm. heating is just superinefficient when it has to warm up the entire interior of the car first

 

[Gasaraki]

With hot Summer weather approaching, I thought to point out that setting the climate above 71F consumes about double the amount of energy. Intuitively, the thinking is setting the climate warmer means the a/c runs less often. The reality is that in Teslas, it creates heat to get the air to the warmer temperature.

For the graph below, I used ChargePoint for my testing. While the car is charging, you can log into your ChargePoint account using a browser and view the current charge rate. When the car is done charging, it uses the power from to charger to power its devices and functions. This is when you can monitor how much energy the car consumes. I used this opportunity to do some testing. The car is parked in a covered parking garage.

9:30 - Car completed charging 85% charged
9:50 - Climate on at 75F, car temp 92F, outside temp 92F, 2kW
10:45 - Climate off
11:10 - Climate on at 70F, car temp 88F, outside temp 94F, 0.88-0.97kW
11:30 - Climate off
12:05 - Climate on at 76F, car temp 80F, outside temp 96F, 1.8-1.97kW
12:20 - Climate on at 69F, car temp 76, 0.8kW
12:40 - Climate on at 75F, car temp 69F, outside temp 102F,1.59-1.92kW
1:00 - Climate on at LO, car temp 75F, outside temp 104F, 3.17kW and 2.65kW usage
1:09 - Climate on at 65F, car temp 57F, outside temp 104F, 0.87-0.89kW

Huh? Why would the car create heat to raise the temperature when it could just open the vent and mix the outside air like every other car...

 

[Candleflame]

Huh? Why would the car create heat to raise the temperature when it could just open the vent and mix the outside air like every other car.

because this cant be used in recirc mode and also you let in humid air which has to be reconditioned and in case of having i.e. dry air outside it results in a very sluggish system to respond to temperature changes. Where is it gonna mix the air?

 

[Gasaraki]

Recirc is part of auto mode. The car will do recirculate if the inside temperature is way higher than the temp setting set by the user or it think it needs that to maintain the set temperature inside. If the temp set is close to the outside temp, it will open the vent to mix outside air. If outside is 90 and you set it to 75 vs. you setting 71, they both would be on recirculate but the 75 setting will just have a slower fan speed, which allows the temp inside to raise more.

I would never believe that Tesla would do something this stupid by running the heat to make 75 degree air instead of using the hot air that is already outside.

Also using the humid outside air doesn't matter because the AC is already running and dehumidifying whether it is using the outside air or not. You can't tell an AC not to dehumidify, it's part of the process.

The temp you set is not the temp of the air coming out of the car. The temp is what you want the inside car temp to be. The car will use the AC, fan and vents to try to bring the inside temp to what you want. Lowering the temp will not cool the car faster. People always think that. That is why when you first get in a hot car the AC runs with fans on full blast even though you have it set to 75, the air coming out is colder than 75.

 

[Candleflame]

Recirc is part of auto mode. The car will do recirculate if the inside temperature is way higher than the temp setting set by the user or it think it needs that to maintain the set temperature inside. If the temp set is close to the outside temp, it will open the vent to mix outside air. If outside is 90 and you set it to 75 vs. you setting 72, they both would be on recirculate but the 75 setting will just have a slower fan speed, which allows the temp inside to raise more.

I would never believe that Tesla would do something this stupid by running the heat to make 75 degree air instead of using the hot air that is already outside.

Also using the humid outside air doesn't matter because the AC is already running and dehumidifying whether it is using the outside air or not. You can't tell an AC not to dehumidify, it's part of the process.

The temp you set is not the temp of the air coming out of the car. The temp is what you want the inside car temp to be. The car will use the AC, fan and vents to try to bring the inside temp to what you want. Lowering the temp will not cool the car faster. People always think that. That is why when you first get in a hot car the AC runs with fans on full blast even though you have it set to 75, the air coming out is colder than 75.

it doesnt mix, it either draws it in or recirculates. auto can be used with recirc on or off. internal A/C is way more efficient than drawing in air from outside. cooling is just part of what an AC does. it's called air conditioning afteral not air cooling.

"Also using the humid outside air doesn't matter because the AC is already running and dehumidifying whether it is using the outside air or not. You can't tell an AC not to dehumidify, it's part of the process." - this is completely wrong. as per your own post above yo uare wondering why ai rcannot be mixed from outside. i told you if you are mxiing cold air with humid air you end up with air which needs to be warmed up as its getting too cold as it needs to be dehumidified first.

"The temp you set is not the temp of the air coming out of the car. The temp is what you want the inside car temp to be. The car will use the AC, fan and vents to try to bring the inside temp to what you want. Lowering the temp will not cool the car faster. People always think that. That is why when you first get in a hot car the AC runs with fans on full blast even though you have it set to 75, the air coming out is colder than 75" - this is irrelevant as there are A LOT more factors influencing temperature perception including humidity and temperature of the interior/glass as well as whether you have sun shining on you.

 

[pt19713]

In Auto mode, the recirculation symbol is a light/opaque blue, not the normal darker solid blue if you select it manually. Doesn't this mean it's going to use fresh & recirculation as needed?

 

[Candleflame]

In Auto mode, the recirculation symbol is a light/opaque blue, not the normal darker solid blue if you select it manually. Doesn't this mean it's going to use fresh & recirculation as needed?

yes, when it gets cooler (or warmer if you live somewhere cold) it will start drawing in air. i believe this is mostly just based on outside temperature and maybe humidity. For me it essentially never autoswitches to draw in air outside because its too humid and hot here even at night. However, in winter at nighttime it does often switch to outside air being drawn in. The magic cutoff seems around 23C or so. I guess at those temperatures it is more efficient to take the outside air and condition it than to use inside air - as the car needs to then turn on the heater to warm the air back up again.

The car afaik cannot mix outside with inside air. You either draw air in via the outside intake or via the recirc intake inside the cars.

 

[GoldCountryCA]

OP, what year is your car?

Anything I’m blathering on about from here on out is based on my experience with other heat pumps, not the Tesla specifically—though the concepts would be the same.

The way a heat pump works (or the refrigeration cycle generally) is the opposite of what most assume. When in cooling mode, it is not adding cold—rather the fan coil is absorbing heat into the refrigerant from the air moving through it. That heated refrigerant then rejects the heat into the outside air at the condenser. When in heating mode, it operates in the reverse fashion. If you have a heat pump at home, put your hand over the condenser fan. When in cooling mode, that air will feel hot as it is removing heat from the home. When in heating mode, that air will feel cool. We are never adding cold, only moving heat around.

The refrigeration cycle is running one way or the other. It cannot do both simultaneously. With a variable speed compressor it can ramp up or down to offer more or less, but still in the same direction. Older or cheaper compressors that are not variable speed achieve a temperature that is somewhere in the middle by simply cycling on and off. When a motor turns on, there is certainly an energy spike. However, I’m not sure this is what you are seeing. I haven’t looked too deep into the Tesla air conditioning system and I’m not sure if it utilizes a variable speed compressor or not.

TL;DR The heating and cooling system with a heat pump are the same system. It cannot heat and cool simultaneously.

 

[Candleflame]

OP, what year is your car?

Anything I’m blathering on about from here on out is based on my experience with other heat pumps, not the Tesla specifically—though the concepts would be the same.

The way a heat pump works (or the refrigeration cycle generally) is the opposite of what most assume. When in cooling mode, it is not adding cold—rather the fan coil is absorbing heat into the refrigerant from the air moving through it. That heated refrigerant then rejects the heat into the outside air at the condenser. When in heating mode, it operates in the reverse fashion. If you have a heat pump at home, put your hand over the condenser fan. When in cooling mode, that air will feel hot as it is removing heat from the home. When in heating mode, that air will feel cool. We are never adding cold, only moving heat around.

The refrigeration cycle is running one way or the other. It cannot do both simultaneously. With a variable speed compressor it can ramp up or down to offer more or less, but still in the same direction. Older or cheaper compressors that are not variable speed achieve a temperature that is somewhere in the middle by simply cycling on and off. When a motor turns on, there is certainly an energy spike. However, I’m not sure this is what you are seeing. I haven’t looked too deep into the Tesla air conditioning system and I’m not sure if it utilizes a variable speed compressor or not.

TL;DR The heating and cooling system with a heat pump are the same system. It cannot heat and cool simultaneously.

hm i dont have a heat pump but i would find it strange if the heat pump cant heat or cool at the same time.
from what i was told the model 3 does not have an auxillary heater. So if you drive in the humid tropics and its 23C at night and the air has to be dehumidified you need to warm it up otherwise itll become really cold.

Good example is my room right now. Nice and comfortable 23.5C in winter here, but its like 70% humidity. If i put on my AC to make this more bareable in dehumidify mode the temp goes down to 20.5-21C. Thats kinda cold but when I go to bed in a minute I will put it on as its just too stuffy sleeping.

 

[GoldCountryCA]

hm i dont have a heat pump but i would find it strange if the heat pump cant heat or cool at the same time.
from what i was told the model 3 does not have an auxillary heater. So if you drive in the humid tropics and its 23C at night and the air has to be dehumidified you need to warm it up otherwise itll become really cold.

Good example is my room right now. Nice and comfortable 23.5C in winter here, but its like 70% humidity. If i put on my AC to make this more bareable in dehumidify mode the temp goes down to 20.5-21C. Thats kinda cold but when I go to bed in a minute I will put it on as its just too stuffy sleeping.

The refrigerant is moving in one direction or the other via a reversing valve at the compressor depending on whether the system is in ‘heating’ or ‘cooling’. It cannot move both directions simultaneously.

If your home doesn’t have a heat pump, the heating and cooling are two different systems that may or may not be tied into the same duct work.

 

[Bill Price]

OP, what year is your car?

Anything I’m blathering on about from here on out is based on my experience with other heat pumps, not the Tesla specifically—though the concepts would be the same.

The way a heat pump works (or the refrigeration cycle generally) is the opposite of what most assume. When in cooling mode, it is not adding cold—rather the fan coil is absorbing heat into the refrigerant from the air moving through it. That heated refrigerant then rejects the heat into the outside air at the condenser. When in heating mode, it operates in the reverse fashion. If you have a heat pump at home, put your hand over the condenser fan. When in cooling mode, that air will feel hot as it is removing heat from the home. When in heating mode, that air will feel cool. We are never adding cold, only moving heat around.

The refrigeration cycle is running one way or the other. It cannot do both simultaneously. With a variable speed compressor it can ramp up or down to offer more or less, but still in the same direction. Older or cheaper compressors that are not variable speed achieve a temperature that is somewhere in the middle by simply cycling on and off. When a motor turns on, there is certainly an energy spike. However, I’m not sure this is what you are seeing. I haven’t looked too deep into the Tesla air conditioning system and I’m not sure if it utilizes a variable speed compressor or not.

TL;DR The heating and cooling system with a heat pump are the same system. It cannot heat and cool simultaneously.

Correct. There is much more going on in the refrigeration cycle than is explained here, such as Delta T, superheat, subcooling, expansion valve, and in the case of heat pumps, reversing valves.

The efficiency of a refrigerator or heat pump is given by a parameter called the coefficient of performance (COP).

The equation is:

where

• 
  is the useful heat supplied or removed by the considered system.
• 
  is the work required by the considered system.

In a worst-case scenario, the heat pump will supply as much energy as it consumes, making it act as a resistance heater. However, in reality, as in home heating, some of QH is lost to the outside air through piping, insulation, etc., thus making the COPHP drop below unity when the outside air temperature is too low. Therefore, the system used to heat houses uses fuel or resistance heating to heat the house when ambient temperature is below this gradient called the "balance point". Today's heat pumps are far more efficient than those used 20-30 years ago and even those used just 10 years ago. A COP rating of 4.0 for a heat pump today is commonplace in higher efficiency units, which means under ideal conditions, a heat pump can be 400% efficient or more. A heat pump this efficient has a "balance point" far below 0° F, providing heat when the ambient is as cold as -15° F.

 

[Candleflame]

The refrigerant is moving in one direction or the other via a reversing valve at the compressor depending on whether the system is in ‘heating’ or ‘cooling’. It cannot move both directions simultaneously.

If your home doesn’t have a heat pump, the heating and cooling are two different systems that may or may not be tied into the same duct work.

yeah heat pumps dont really exist here. There are dual cycle ACs which i guess do something similar but we dont have those here as it doesnt get cold enough. (i.e. here you wake up maybe once or twice in winter freezing for that odd cold night we get. and by cold i mean i.e. 15C. Feels worse in europe though because houses have no isolation). Weather here is sorta like Havana in Cuba with the difference that we get a colder winter I guess... Cant wait until I move down south next year haha. #los angles climate yas.

when you go into the desert here it can go down to 0 at night (and bloody 30 during daytime). When I lived there I did have a reverse cycle A/C but tbh I didnt use it much... just use a thick blanket in winter.


Anyway all of this doesnt explain how the newer model 3s use the heatpump to condition the air. I think what happens is that the car has a seperate air conditioner/dryer before the heatpump so it dries the air to the desired % and then just uses the heatpump to either warm or cool it, taking into account the humidity %.