When buying a surge protector for a Dell laptop, is the joules rating the most important decision? For example, will a single plug protector rated at 540 J be enough, or should I look for a higher rating? I already have whole house surge protection, so this will just be additional insurance. Thanks!
That should be just fine.
Surge protection means you know where hundreds of thousands of joules dissipate. A 540 joule protector is near zero. Typically protects from surges that do not overwhelm protection already inside the laptop.
So where do hundreds of thousands of joules get harmlessly absorbed? In earth ground.
Two completely different type devices are both called surge protectors. An undersized protector has no low impedance (ie 'less than 10 foot') connection to earth. Another completely different device has that dedicated wire connection to an earth ground electrode. And then everything in the house is protected. And then a surge is not inside destructively hunting for earth via a laptop or any other appliance.
A minimally sized 'whole house' protector is 50,000 amps. So that hundreds of thousands of joules are connected to and therefore dissipate harmlessly in earth. Even that 540 joules protector would be protected by an earthed 'whole house' protector.