Joules.
You have to check what the surge protectors state on the package.
Most of the ones found in stores are 200 or 600 Joules that protects you for squat.
A lightening strike near your home?
""An average bolt of lightning, striking from cloud to ground, contains roughly one billion (1,000,000,000) joules of energy"". A surge protector from Lowes is not going to stop that!
Other outside causes for a power surge.
""Every day, destructive electrical spikes and surges — caused by lightning or industrial and automobile accidents with utility poles — pass through the electrical lines and into your home.""
""These power surges or spikes can be caused by many things including the most obvious one – lightning. Surges can also be caused by birds and animals, falling trees, or vehicles crashing into power poles. Whatever the cause of these power surges, the results can be devastating""
I've had 2 transformers go by my house. One was an animal and another from lightening. And the third was lightening to ground. NONE of the hardware store surge protectors in my house worked for that level of voltage spike. Lunched 2 TV's and the amplifiers. All had surge protector plugs.
Just saying you wouldn't know if there was a car crash into a pole that spiked the voltage and/or lightening which can happen a long distance from the building and still cause a failure at your location.