Here’s a tip app note (PDF) from ON Semiconductor on selecting specs for an TVS diode.
There are two important properties of a TVS device during an electrical stress. The TVS must be able to self-protect and survive the stress current, and the TVS must clamp the protected line to a low voltage during the stress. Power ratings of TVS devices do not provide a good measure of clamping effectiveness because this metric is artificially enhanced by a higher, less desirable, clamping voltage. It is true that TVS devices with high peak power ratings are often larger diodes and may therefore also have low dynamic resistance and low clamping voltage but there are exceptions, and one such exception has been highlighted in this note. It is therefore important to always consider the dynamic resistance or the clamping voltage at specific currents levels, rather than focusing solely on the peak power rating when selecting a TVS device.