TEC undervolting charts

Attention: This post is not accurate, but it will hopefully be revised in the future to resemble some form of factuality.


Since peltier cooling power is nonlinear, calculating cooling power at voltages other than VMax can be difficult. To make life easy, I've assembled a chart of cooling powers for some standard TECs using the formula... snipped ;)
YAY REWRITE! OK, much thanks to shadowing on this one :-). the new formula is P=(I^2)R. P = wattage, I = amperage, R = resistance.
First, I derive the resistance from the manufacturer-provided numbers. then, using these, i make I x and get the wattages for each amp.
OK, since I was dead-tired when I was updating this whole chart last night, I didn't bother to give you guys the real formula used (derived from P=(I^2)R), so here it is (as it appears on my TI-83+):
\y1=(((x/16.1*8.0))^2)*(80/(8.0^2))
thats how it would look for the 80watt TEC listed below.
here is the article that, after reading through it several times late last night, yielded the equation.
Much to my surprise, my amperage draw approximations were pretty much dead on; it was the wattages that made no sense at all :)

Keep in mind, these numbers are "rough guidelines" at best! Don't follow them too closely. I'd like to update this sometime soon with numbers that I'm more confident about, but I don't see that happening any time soon.

80 watt / 16.1 VMax / 8.0 IMax
5 volts: 5.7 watts; 2.48 amps
6 volts: 8.9 watts; 2.98 amps
7 volts: 12.8 watts; 3.47 amps
8 volts: 17.4 watts; 3.97 amps
9 volts: 22.8 watts; 4.47 amps
10 volts: 28.8 watts; 4.96 amps
11 volts: 35.6 watts; 5.46 amps
12 volts: 43.0 watts; 5.96 amps
13 volts: 51.2 watts; 6.45 amps
14 volts: 60.1 watts; 6.95 amps
15 volts: 69.7 watts; 7.45 amps
16 volts: 80 watts; 7.95 amps

120 watts / 24.6 VMax / 7.9 IMax
5 volts: 3.5 watts; 1.60 amps
6 volts: 5.5 watts; 1.92 amps
7 volts: 8.0 watts; 2.24 amps
8 volts: 10.8 watts; 2.56 amps
9 volts: 14.1 watts; 2.89 amps
10 volts: 17.9 watts; 3.21 amps
11 volts: 22.1 watts; 3.53 amps
12 volts: 26.7 watts; 3.85 amps
13 volts: 31.0 watts; 4.17 amps
14 volts: 36.4 watts; 4.49 amps
15 volts: 42.2 watts; 4.81 amps
16 volts: 48.5 watts; 5.13 amps
17 volts: 55.2 watts; 5.45 amps
18 volts: 62.3 watts; 5.78 amps
19 volts: 69.8 watts; 6.1 amps
20 volts: 77.8 watts; 6.42 amps
21 volts: 86.2 watts; 6.74 amps
22 volts: 95.0 watts; 7.06 amps
23 volts: 104.3 watts; 7.38 amps
24 volts: 114.0 watts; 7.70 amps

169 watt / 16.4 VMax / 16.1 IMax
5 volts: 15.7 watts; 4.90 amps
6 volts: 22.6 watts; 5.89 amps
7 volts: 30.8 watts; 6.87 amps
8 volts: 40.2 watts; 7.85 amps
9 volts: 50.9 watts; 8.83 amps
10 volts: 62.8 watts; 9.81 amps
11 volts: 76.0 watts; 10.79 amps
12 volts: 90.5 watts; 11.78 amps
13 volts: 106.2 watts; 12.76 amps
14 volts: 123.2 watts; 13.74 amps
15 volts: 141.4 watts; 14.72 amps
16 volts: 160.9 watts; 15.70 amps

172 watt / 24.6 VMax / 11.3 IMax (got the numbers from silverprop)
5 volts: 7.1 watts; 2.3 amps
6 volts: 10.2 watts; 2.8 amps
7 volts: 13.9 watts; 3.2 amps
8 volts: 18.2 watts; 3.7 amps
9 volts: 23.0 watts; 4.1 amps
10 volts: 28.4 watts; 4.6 amps
11 volts: 34.4 watts; 5.1 amps
12 volts: 40.9 watts; 5.5 amps
13 volts: 48.0 watts; 6.0 amps
14 volts: 55.7 watts; 6.4 amps
15 volts: 64.0 watts; 6.9 amps
16 volts: 72.8 watts; 7.3 amps
17 volts: 82.1 watts; 7.8 amps
18 volts: 92.1 watts; 8.3 amps
19 volts: 102.6 watts; 8.7 amps
20 volts: 113.7 watts; 9.2 amps
21 volts: 125.3 watts; 9.6 amps
22 volts: 137.6 watts; 10.1 amps
23 volts: 150.4 watts; 10.6 amps
24 volts: 163.7 watts; 11.0 amps

226 watt / 15.2 VMax / 24 IMax
5 volts: 24.5 watts; 7.89 amps
6 volts: 35.2 watts; 9.47 amps
7 volts: 47.9 watts; 11.05 amps
8 volts: 62.6 watts; 12.63 amps
9 volts: 79.2 watts; 14.21 amps
10 volts: 97.8 watts; 15.78 amps
11 volts: 118.4 watts; 17.36 amps
12 volts: 140.9 watts; 18.94 amps
13 volts: 165.3 watts; 20.52 amps
14 volts: 191.7 watts; 22.10 amps
15 volts: 220.1 watts; 23.68 amps

320 watt / 14 VMax / 26 IMax (these numbers were given to me by NoL, so if they're wrong, yell at him, not me)
5 volts: 40.8 watts; 9.3 amps
6 volts: 58.7 watts; 11.1 amps
7 volts: 80 watts; 13 amps
8 volts: 104.5 watts; 14.9 amps
9 volts: 132.2 watts; 16.7 amps
10 volts: 163.3 watts; 18.6 amps
11 volts: 197.6 watts; 20.4 amps
12 volts: 235.1 watts; 22.3 amps
13 volts: 275.9 watts; 24.1 amps
14 volts: 320 watts; 26 amps

OK, now, because wintsch labs gives conflicting information on this bad boy, theres 2 charts. I don't know which one is right, because wintsch labs is wierd. Here's the 32.8 IMax one; the second is the 25 IMax one.
437 watt / 26.7 VMax / 32.8 IMax
5 volts: 15.3 watts; 6.14 amps
6 volts: 22.1 watts; 7.37 amps
7 volts: 30.0 watts; 8.59 amps
8 volts: 39.2 watts; 9.82 amps
9 volts: 49.7 watts; 11.05 amps
10 volts: 61.3 watts; 12.28 amps
11 volts: 74.2 watts; 13.51 amps
12 volts: 88.3 watts; 14.74 amps
13 volts: 103.6 watts; 15.97 amps
14 volts: 120.2 watts; 17.19 amps
15 volts: 137.9 watts; 18.42 amps
16 volts: 156.9 watts; 19.65 amps
17 volts: 177.2 watts; 20.88 amps
18 volts: 198.6 watts; 22.11 amps
19 volts: 221.3 watts; 23.34 amps
20 volts: 245.2 watts; 24.56 amps
21 volts: 270.3 watts; 25.79 amps
22 volts: 296.7 watts; 27.02 amps
23 volts: 324.3 watts; 28.25 amps
24 volts: 353.1 watts; 29.48 amps
25 volts: 383.1 watts; 30.71 amps
26 volts: 414.4 watts; 31.94 amps

437 watt / 24 VMax / 25 IMax
5 volts: 19.0 watts; 5.2 amps
6 volts: 27.3 watts; 6.2 amps
7 volts: 37.2 watts; 7.3 amps
8 volts: 48.6 watts; 8.3 amps
9 volts: 61.5 watts; 9.4 amps
10 volts: 75.9 watts; 10.4 amps
11 volts: 91.8 watts; 11.5 amps
12 volts: 109.3 watts; 12.5 amps
13 volts: 128.2 watts; 13.5 amps
14 volts: 148.7 watts; 14.6 amps
15 volts: 170.7 watts; 15.6 amps
16 volts: 194.2 watts; 16.7 amps
17 volts: 219.3 watts; 17.7 amps
18 volts: 245.8 watts; 18.8 amps
19 volts: 273.9 watts; 19.8 amps
20 volts: 303.5 watts; 20.8 amps
21 volts: 334.6 watts; 21.9 amps
22 volts: 367.2 watts; 22.9 amps
23 volts: 401.3 watts; 24.0 amps
24 volts: 437 watts; 25 amps


I hope this chart is useful for somebody out there. If my equation for calculating these values is flawed in any way, please let me know! All of these TECs (except the 320 watt) are units sold either by dangerden or by frozenCPU and all numbers are taken from their specs.

Very nice and helpfull ;)

Can you include the power the TEC draws at certain volts? And maybe a TEC reated at 226.1w at 12v? (if you have the time for it)

Thanks in advance :toast:

Problem is that the power the TEC draws depends on the temperature differential over it...

So the hotter your waterblock then less current it will draw (lower DT) but worse temps.

Vice versa, reduce your water temp (and increase DT) then the TEC will draw more power.

you god :) that really helps when you cba to do the calculations... or find tec specs to do them :p:
do you have a table for a 320W 15V pelt?

updated with amperage approximations. anyone have the right info for the DD 172 watt pelt? their site just clones over the 120 watt pelt's specs.
I'll add the 320 watt pelt soon; got a link to a supplier? I can't seem to find any very easily.

Thermal Enterprises model CP1-12726

IDK if they have a website, if you email the guy named thermalenterprises on ebay he may be able to get the values for you. I am very interested in using one of these @5v.

Good to see im not the only one using big pelts at diff voltages ;)

EDIT:

Dividing amperage by the voltage you are running at is not a good way to predict amperage......How are you calculating that?

Just spot checking one example with the kryotherm software, you will be drawing 3.24A from the 80w unit at 5v. Would be interesting to see what is causeing this.

Qc seems to be about right though.

got a 320 here too, been looking around for a 5-10v variable PSU for it.. :D

Quote:

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Originally Posted by ls7corvete

Thermal Enterprises model CP1-12726

IDK if they have a website, if you email the guy named thermalenterprises on ebay he may be able to get the values for you. I am very interested in using one of these @5v.

Good to see im not the only one using big pelts at diff voltages ;)

EDIT:

Dividing amperage by the voltage you are running at is not a good way to predict amperage......How are you calculating that?

Just spot checking one example with the kryotherm software, you will be drawing 3.24A from the 80w unit at 5v. Would be interesting to see what is causeing this.

Qc seems to be about right though.

---

for amperage i was just doing (voltage)/VMax*IMax.The results do seem kinda fishy though.

yea they arent matching with kryotherm so I would say that they are questionable.

Very Nice Info you got there. Can anyone verify this to make sure all the info is correct and no issues are to be had anywhere.

I ask as I want to make this a sticky, but most certainly want to make sure all the info is correct and there are no annotations that need to be added before I do.

The Imax is a bit wrong I think.

For example the Imax of the 80W pelt at 5v is 2.48A. Then it uses 12.4w at 5v? that must be wrong. I don't think your way to calculate the Imax is correct.

Quote:

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Originally Posted by Bloody_Sorcerer

..................

437 watt / 26.7 VMax / 32.8 IMax
5 volts: 148.3 watts; 6.14 amps
6 volts: 174.3 watts; 7.37 amps
7 volts: 199.1 watts; 8.59 amps
8 volts: 222.6 watts; 9.82 amps
9 volts: 244.9 watts; 11.05 amps
10 volts: 266.0 watts; 12.28 amps
11 volts: 285.9 watts; 13.51 amps
12 volts: 304.5 watts; 14.74 amps
13 volts: 321.9 watts; 15.97 amps
14 volts: 338.1 watts; 17.19 amps
15 volts: 353.0 watts; 18.42 amps
16 volts: 366.8 watts; 19.65 amps
17 volts: 379.3 watts; 20.88 amps
18 volts: 390.6 watts; 22.11 amps
19 volts: 400.6 watts; 23.34 amps
20 volts: 409.4 watts; 24.56 amps
21 volts: 417.0 watts; 25.79 amps
22 volts: 423.4 watts; 27.02 amps
23 volts: 428.6 watts; 28.25 amps
24 volts: 432.5 watts; 29.48 amps
25 volts: 435.2 watts; 30.71 amps
26 volts: 436.7 watts; 31.94 amps
............................................

---

Impressed with accuracy of 437 table. I'm using a 437W at 19.9V. Current draw is 24.4A.


EDITED for brevity

I'm fairly confident that my wattage numbers are correct, but obviously something is wrong with the amperages. Currently, I'm looking for a better equation to calculate them. Still missing IMaxes for 320watt and 172watt TECs.

for the 320, its 26 amps.

gah, these amperages are driving me crazy! I had been trying a method in which i took the % efficiency of the peltier (watts drawn / watts cooled) and multiplied that by the new watts cooled and then divided by the new voltage, which seemed to work mostly, until i tried it on the 437watt tec,where it went absolutely bonkers, suggesting that it draws 50-plus amps at 12volts, something i highly doubt. I'm also starting to think that it's amperage figure is totally wrong, too. I doubt that a 437watt tec draws 875 watts at VMax.

they lose efficiency as voltage increases.. it might be near to that.

:clap: Thanks Sorcerer! :clap:

bloody when ur 98% confident its all right shoot me a PM and I will sticky it. until then its a WIP.

you can try kryotherm calculator for the amps.

I think amperage is gonna vary too much to make a chart.

I'm declaring this done. Amperages are all approximate. huge thanks again to shadowing :-)

What about 350 watt peltiers? But great stuff man. Thanks a bundle.

w00t! updated! finally got numbers for 172 watt TECs (from silverprop's e-store) and used numbers from NoL (who got them from eBay) for 320 watt TECs, so those could be wrong :-P
this was supposed to get updated last night, but i left my TI-83+ in school...
if anyone wants them, I'll but the exact equations i used for each TEC in.

where can i find a 320w pelt in the usa

Ebay thermoelectric enterprises

When were the values changed?

The wattage values are completley wrong, they need to be changed, they seemed pretty close before.

The old wattages were physically impossible. Originally, if you multiplied the amperage and the voltage, you'd get a lower wattage draw than they could cool. I since found a graph by a manufacturor (lost the page... stupid me.) as well as an article in the Journal of Science Education that told me otherwise. If you wish to manually test all the numbers, be my guest.

Quote:

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Originally Posted by Bloody_Sorcerer

The old wattages were physically impossible. Originally, if you multiplied the amperage and the voltage, you'd get a lower wattage draw than they could cool. I since found a graph by a manufacturor (lost the page... stupid me.) as well as an article in the Journal of Science Education that told me otherwise. If you wish to manually test all the numbers, be my guest.

---

lower wattage draw than they can cool huh, well I guess what you are reffering to is a COP of greater than 1. This IS entirly possible and is the main reason that we undervolt TECs. In my article on overclockers.com I was getting a COP of around 3 . Please search around on the efficiency of TECs at different voltages and repost.

As for manualy testing TECs at different voltages I just may do that, it would involve several hundred dollars for a variable transfomer though so I an hesitant at this time. A better approach would be to download the kryotherm software and plug in the variables and see what happens, there is many charts in that program that will demostrate this affect to you.

The data is there you should find it easy enough.

Quote:

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Originally Posted by Bloody_Sorcerer

for amperage i was just doing (voltage)/VMax*IMax.The results do seem kinda fishy though.

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If your 320W undervoltage table is referring to the Thermal Enterprises CP1-12726, suspect it needs a major rework.

Referring to Page 2 of the Thermal Enterprises documentation on the CP1-12726, the graph in the upper left hand corner plots Qc(W) vs dT(C). Look at the intersection point of the I=26A plot with the y-axis scaling of Qc. The value indicated validates the Qcmax noted on Page 1 of 243.5W. This device has a maximum cooling rating of 244W (Th = 27C), in round numbers, at a maximum input power consumption level Pcmax of (15.4V*26A)W.

The 320W Qcmax rating is at a Th = 50C. This would require a Pcmax level considerably greater than (15.4V*26A)W.

Quote:

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Originally Posted by Premmer

If your 320W undervoltage table is referring to the Thermal Enterprises CP1-12726, suspect it needs a major rework.

Referring to Page 2 of the Thermal Enterprises documentation on the CP1-12726, the graph in the upper left hand corner plots Qc(W) vs dT(C). Look at the intersection point of the I=26A plot with the y-axis scaling of Qc. The value indicated validates the Qcmax noted on Page 1 of 243.5W. This device has a maximum cooling rating of 244W in round numbers, not 320W which I believe is hype, at a maximum power consumption level of (15.4V*26A).

If there is a 50mm square 320W Qcmax peltier in the marketplace, it's not this one.

---

Hmmmm is there one? I still believe this is the most powerfull 50mm.

Quote:

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Originally Posted by ls7corvete

I still believe this is the most powerfull 50mm.

---

That may be but lets refer to the device by its actual Qcmax rating, 244w.

I emailed Thermal Enterprises and posed the following question.
"Your ebay advertisement refers to the device as 245-320w. Your specs clearly state that the maximum Qc of the device is 243.5w . Why do you use the 320w reference in your advertisement? It's misleading to the public".

Their response was as follows.
"We simply follow the same common practice as most other TEC manufactures, we establish the watt rating of our devices under two different operating conditions. The lower wattage rating is established with hot side temperature at 27 deg C. The higher wattage rating is established with hot side temperature at 50 deg C".

After researching the peltier rating subject, I have to agree with Thermal Enterprises response. It is up to the end-user to determine which set of peltier parameters would be most applicable for their operating conditions.

In summary, when peltier maximum cooling power wattage ratings (Qcmax) are expressed by two values as is the case with Thermal Enterprises devices, the lower rating equates to a Th = 27C ambient condition and the higher rating to a Th = 50C. In any well planned CPU peltier/water-cooled system, the Qcmax that would be most representative of their system's cooling performance would be the vaule at Th = 27C.

Quote:

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Originally Posted by Premmer

That may be but lets refer to the device by its actual Qcmax rating, 244w.

I emailed Thermal Enterprises and posed the following question.
"Your ebay advertisement refers to the device as 245-320w. Your specs clearly state that the maximum Qc of the device is 243.5w . Why do you use the 320w reference in your advertisement? It's misleading to the public".

Their response was as follows.
"We simply follow the same common practice as most other TEC manufactures, we establish the watt rating of our devices under two different operating conditions. The lower wattage rating is established with hot side temperature at 27 deg C. The higher wattage rating is established with hot side temperature at 50 deg C".

---

Interesting, I do think that most applications are going to be running with the TEC closer to the 320w rating than the 244w rating though.

I always assumed that the temperature range we are dealing with in WCing loops was not extreme enough to affect the performace of the TECs, just when I thought I knew enough to give good answers to users in the forums this kind of thing has to come into play.

interesting, I guess I gotta play with kryotherm to see how this affects things.....

FYI, Wintsch Labs also has 2 ratings on their big peltier.

Th = 50C
Qmax = 437.3W
I(Qmax) = 32.8A
I(dTmax) = 25.2A
Umax = 26.7V
dT = 74.5

Th = 27C
Qmax = 376.8W
I(Qmax) = 30.4A
I(dTmax) = 23.7A
Umax = 24.8V
dT = 66.0

The advertised maximum cooling rating of the Dominator Pro when first released was 360W. After a brief period of time, it was advertised to be 437W with no hardware change to account for it other than quoting its rating based on the Th = 50C spec which I believe is nothing more than technical hype.

In general terms, Qmax @ Th = 27C, which is most likely the most realistic rating to use as a measure of expected performance, is in the range of 60% to 62% of the maxiumum input power for the 226W, 245W and 360W peltiers.

That is all confusing :D

Can we conclude which peltier is the MOST POWERFUL at 12V?
(I'm assuming 320W? What about the 350W?)

Quote:

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Originally Posted by Stocky

That is all confusing :D

Can we conclude which peltier is the MOST POWERFUL at 12V?
(I'm assuming 320W? What about the 350W?)

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Yes, the "320W" is still the most powerful 50mm pelt because even if it is really only a 245W pelt (which what's being argued above), it's still more powerful than the 226W pelt.

The 350W is even more powerful, but it's a 62mm pelt. (That won't be properly covered by a waterblock like the Maze4-1 or Swiftech MCW5002.)

Quote:

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Originally Posted by Revivalist

Yes, the "320W" is still the most powerful 50mm pelt because even if it is really only a 245W pelt (which what's being argued above), it's still more powerful than the 226W pelt.

The 350W is even more powerful, but it's a 62mm pelt. (That won't be properly covered by a waterblock like the Maze4-1 or Swiftech MCW5002.)

---

The 350W Qcmax is at Th =50C.
At Th = 26.8C:
Qcmax = 270.5W
dTmax = 68C
Vmax = 15.4V
Imax = 30A
Pcmax = 462W

So from the above logic, there's no point looking at the TEC's wattage and thinking it will perform anywhere near this well when connected to a PC PSU with only +12V max. A 89W TEC typically becomes about a 53W TEC at 12V, correct? On the plus side less power/heat, but on the bad side much less cooling capacity, about 40%.

Quote:

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Originally Posted by GFORCE100

A 89W TEC typically becomes about a 53W TEC at 12V, correct?

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Nо, it becomes 69W. The right formula is

Qc=Qmax*I/Imax.

If we assume linear current dependence on voltage, I=V/R, with constant R (this is close for most TECs, at least 15V TECs), then we can rewrite the equation as

Qc=Qmax*V/Vmax

For 89W/15.4V TEC, at 12V:
Qc=89*12/15.4=69W

Quote:

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Originally Posted by alexio

The Imax is a bit wrong I think.

For example the Imax of the 80W pelt at 5v is 2.48A. Then it uses 12.4w at 5v? that must be wrong. I don't think your way to calculate the Imax is correct.

---

Not only is a TEC a heater/cooler, it also is a thermal pile generator. The generator counters the TEC effect. Increasing the internal resistance, there by lowering the current drawn by the TEC. I could get nowhere near the current spec for a 169w TEC, with my set up.
44C on the hot side and
-1.0C on the cold side.
only air loading the cold side.
ambient temp at 25C
slot A dual fan heat sink on the hot side.
I was pulling less than 7 amps
at 13.4 volts

245 watt / 15.2 VMax / 26 IMax
5 volts: 125 watts; 8 amps
6 volts: 137.5 watts; 9.8 amps
7 volts: 162 watts; 11.6 amps
8 volts: 177 watts; 13.2 amps
9 volts: 182 watts; 14.3 amps
10 volts: 204 watts; 16.2 amps
11 volts: 213 watts; 18 amps
12 volts: 225 watts; 19.8 amps
13 volts: 238 watts; 21.2 amps
14 volts: 242 watts; 22.6 amps
15 volts: 247 watts; 23.8 amps

*Measured in delta T @ 0 degrees C

This was taken from Custom Thermoelectric Online Store. I have one on the way now and will post back how accurate or inaccurate this actually is.

That would be appreciated. What with the mini revival of interest in TEC's due to running better COP's, actual collected data could be of value.

Quote:

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Originally Posted by THE JEW (RaVeN)

That would be appreciated. What with the mini revival of interest in TEC's due to running better COP's, actual collected data could be of value.

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There is some good info here but for the most part these charts are useless. The heat transfer is greatly affected by the differential and hot side temp, as are voltage needed for a given current. These are semiconductor devices and have non-linear characteristics so none of the simple formulas being used here are going to give anything near correct results. Anyone who is serious about using TECs needs to learn how to read TEC operating curves.

I have designed using TECs for years, but always up near the .5 to .8 CoP level (CoP is basically power transferred over power wasted so a CoP of 1 means for example you burn 100W to transfer 100W, for a total heat load of 200W on the hot side). Recently, because of feedback from other posters here, I took a good look at moving very low on the curve. You can achieve big CoP ratings (3 or higher) with a differential under 5C, but for a differential over 15C, nothing over CoP of 2 is possible. So high efficiency TEC use requires WC on the hot side and operation way down on the curve. If you need max performance in a small package, you will pay the price in power wasted. Sometimes that's what you need but there is no magic bullet to get high power at high efficiency - you need lots of TECs run low in their voltage curve to do that.

Anyone interested in understanding the way TEC design using the curves works should go to http://www.ferrotec.com/products/the.../highPower.php
pick a TEC and play around with the curves. Default hot side is 50C but you can plug in whatever you want. Also you can used advanced settings and get performance for a certain heat sink performance. You will understand a lot more about how these things work if you put an hour or two into that exercise. Ferrotec does not have TECs like most of those being discussed but they have similar ones - the performance will also be similar, except the absolute values will be scaled based on the Ferrotec unit you are looking at.

Yeah, i realize now that basically all the numbers i came up with back when i first wrote this are flat out wrong.

I might have a bit stuped question.

If Vmax is 31,5v, Imax is 16A and Qmax is 300W, is it possible to calculate the resistance?
And what is the Qmax and Imax at 25 and 20V?

Quote:

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Originally Posted by DonNiger

I might have a bit stuped question.

If Vmax is 31,5v, Imax is 16A and Qmax is 300W, is it possible to calculate the resistance?
And what is the Qmax and Imax at 25 and 20V?

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oh course you can but unfortunately TEC's are semiconductors and that is why they don't follow Ohm's very well and you won't gain much by calculating the resistance it is far better to use manufacturers performance charts...that's what they are there for but even so you will only get a ballpark working figure.

I assume the TEC's you refer to are the 26316.

At 25v ballpark figures - Qmax 270w Imax 12.5A

At 20v Qmax 240w Imax 10.5A

The TEC is the same (HT)

Thanks! I am limited to 3-26v and 11A (16A fused) constant power. At 20v, it sounds good enough.