Also known as the let-through voltage, this specifies what spike voltage will cause the protective components inside a surge protector to divert unwanted energy from the protective line/equipment.
The standard let-through voltage for 120 V AC devices is 330 volts.
This number defines how much energy a Metal-oxide varistor surge protector can theoretically handle safely in a single event, without failure or causing more damage.
Surge protectors don't operate instantaneously. Surges usually take around a few microseconds to reach their peak voltage, and a surge protector usually have a nanosecond response time.
If a surge protector is properly installed, for every joule absorbed by a protector, another 4 to 30 joules will be dissipated harmlessly into the ground.