The Kelvin scale is not known by many people outside the field of science. In everyday English, "k" or "K" (capitalised) would usually be understood as thousand and is informal. If we need to be more precise to avoid confusion, then it is normal to use the standard abbreviation for the unit e.g. km = kilometre, kg = kilogram (54 km, 48 kg) If you were using the Kelvin scale in your writing, then it would probably be formal writing; and so k or K would be too informal to use in place of the full figure e.g. 8,000

Officially, k but in practice both are used. Of course I am writing from the United States where we still use miles and pounds and gallons.

thanks Gavin, I forgot it.

"Kelvin" not "Kevin". I'm not sure what the answer is :)