Should one say 200 kelvins or 200 kelvin?
Please provide a source to the answer, as that would be much appreciated.
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You should say "two hundred kelvins", and write "200 kelvins" with a small 'k', or "200 K", with a capital 'K'.
The revised SI (November 2018) says
The kelvin, symbol K, is the SI unit of thermodynamic temperature; its magnitude is set by fixing the numerical value of the Boltzmann constant to be equal to exactly 1.380649 × 10-23...J K-1[joules per kelvin]
The US National Institute of Standards and Technology (NIST) page about this says:
The kelvin unit is not expressed in degrees like Celsius or Fahrenheit are. It is used by itself to describe temperature. For example, “mercury loses all electrical resistance at a temperature of 4.2 kelvins.”
Technically, the unit is "a kelvin", and we pluralize them like any other unit - for example, the NIST uses kelvins ("mercury loses all electrical resistance at a temperature of 4.2 kelvins") - but it is still often written as "degrees Kelvin".
As someone whose training is in condensed matter physics and who has attended a lot of talks, seminars, lectures, etc. in which people talk about temperature dependence, I've only heard "kelvin" without the s. I don't know what the official SI rule is, but "kelvin" is definitely the de facto standard.
Edit: I checked a reference I thought might write this out (An Introduction to Thermal Physics by Schroeder), and in the one place I could find it he uses "kelvins":
For this reason it's usually wise to convert temperatures to kelvins before plugging them into any formula.
I stand by my answer in that if you go around saying "kelvins" in spoken language, you may look a little silly.