# How does a solar cell its relative thermal coefficient "alpha" expressed in 1/K relate to %/K?

Some times I see the relative thermal coefficient of solar cells Isc (called "alpha" or α) written as 1/K, other times as %/K.

I think that %/K can be seen as 0.01/K, but I am not sure.

Is it correct that 0.000555 1/K = 0.0555 %/K?

PS: absolute Isc,stc of the solar cell = 149.6 mA and I would like to output temperature compensated irradiance with formula:

G = (Isc,meas ÷ (1 + αIsc (Tcell-Tref))) × (Gref ÷ Isc,ref)

thus G = (Isc,meas ÷ (1 + αIsc (Tcell-25))) × (1000 ÷ 149.6)

I think that αIsc needs to be filled out here as 1/K (=%/K divided by 100) and not as %/K, otherwise the temperature correction seems too high.

You need to know the actual value being converted to convert between systems.
That is the 1/K and %/K values do NOT have a constant ratio - it depends on the actual value under consideration. eg

• 1A change in 100A = 1%.
• 1A change in 40A = 2.5%
• 1A change in 10A = 10%

For variable "N" where N may be watts or amps or volts:

N/K means absolute change in N per degree K (or per degree C)

%N/K means percentage change in N per degree K change.

The conversion depends on the current (or voltage or power)
For eg I_25C = current at 25 degrees C.

• %N/K value = (100 / I_25C) x N/K value

• N/K value = %N/K value x (I_25C / 100)

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So eg "40A - 0.2A/K" means current would decrease by 0.2A for every degree K (or C) that temperature rose. So eg 50K rise = 50 x 0.2 = 10A so current would decrease from 40A to 30A.

"40A - 0.5%/K" would be the same as the above as 40 x 0.5/100 = 0.2A.

The same applies to figures for Voltage or Power where applicable.

• @Russell_McMahon I already knew that %/K is the relative form. I didn't know that 1/K is its absolute counterpart. Thanks for making me learn. For me, your answer can start with "1/K means absolute..." and as second sentence your example "For eg I_25C...". The rest is more background knowlegde and could be the last paragraph of the answer. Nov 8, 2019 at 8:48
• What I really needed is: %/K = 1/K ÷ (I_25C÷100); your answer helped me to get there. Nov 8, 2019 at 8:59
• "1/K" is the wrong unit in the case of absolute numbers - it has to be "A/K". Nov 8, 2019 at 9:26
• @asdfex Read carefully. 1/K is a contraction for "per degree K". And if one did this (I now have) you'd also want %A/K. || If one wished to do it real-proper-like you'd want to put X/K where X is the variable being considered - which in this case is usually current or Voltage or power (all of which have temperature dependencies. ). || Sometimes aiming at understanding trumps pedantic correctitude. Sometimes not :-). || I've edited the post to try and convey N/k and %N/k concepts better. That it will not please all is a given. Nov 8, 2019 at 13:32
• Nope, that's wrong. The absolute value is an addition as in 40A - 0.2A/KT and needs the 'A' unit. The relative value is a factor as in 40A * 0.1%/KT and therefore does not carry the 'A' unit. (T is the temperature) You did exactly this in the last section of your post! Nov 8, 2019 at 13:49