Joules are a measure of energy. But that is all irrelevant
to MOVs. For MOVs, joules defines life expectancy. More
specifically, how fast the MOV will degrade. The manufacturer
defines MOV failure mode: degradation - not vaporization.
Joules is the parameter that defines that life expectancy.

Tony Hwang wrote:
Hi,
Joule is not a measurement of power it is unit of energy.
Let me put it this way, if you get a direct hit nothing uch will
work.
Telling from real world experience.
And real protector for whole house costs a lot. And surge current
is much faster than breaker or fuse.
Tony