This is a tough question to answer because it depends on how many peripherals you have connected to your iMac. You could base the required Joules rating on how much energy is being drawn by your machine and its peripherals and you might be fine. For example, if your CPU draws 300 watts and your monitor draws 200, then you could get away with at least a Joules rating of 500. That seems a little low to me, so to be safe I would double that number. That is the MINIMUM I would recommend. What you really want is as much protection as you can afford, so go with the highest rating of Joules you can find in a surge protector. Shoot for 3000 Joules or higher. This goes for both the iMac and PC.
But a decent surge protector is really only a basic form of surge protection for an iMac. Yours should include EMI/RFI line noise filtering and should be able to protect modem and network cables. Surges can travel on any electrical conduit, including phone lines and cable lines so it is important to protect these avenues.
Another good option is a device that allows easy placement of wall warts or power bricks (the AC/DC transformers PowerBooks use and some printers and other devices). Most modern surge protection devices are designed to share space between these large adapters and normal sized plugs.
While good surge protection can really save your iMac from damaging surges, it’s not full proof against such occurrences as blackouts. The most optimal choice in iMac power protection is a UPS, or an Uninterruptible Power Supply. Its job is to provide power to your iMac in the form of a limited battery backup so that you have enough time to save any work and shut down your computer properly.
One thing about a UPS is that many do not provide surge protection. So you have one of two options. Plug your UPS into a surge protector or buy an APC UPS. APC makes UPS devices with surge protection built in. This is what you want ideally.
Although there has been much contention on many forums on the subject of plugging a UPS into a surge protector, the fact of the matter is this: it can be done, but you jeopardize the functionality and protective operations of both devices. One thing you DO NOT want to do is to plug your surge protector into the UPS. The problem with this is surge protectors do not distribute power. This means that if some of the peripherals are plugged into the surge protector while plugged into the UPS, you risk premature or unwanted shut down of the attached equipment. You can, however, plug a PDU (Power Distribution Unit) into the UPS.
The problem with plugging the UPS into a surge protection device is similar in that other attached equipment may be more power bearing than the UPS which could draw power from the UPS and cause it to go to battery power. This in turn could cause an unwanted shut down of equipment.
How Much Joules Protection Is Needed For The iMac UPS?
Surprisingly, you don’t need much of a Joules rating with a UPS. This is because the device has Automatic Voltage Regulation which stabilizes unsafe voltage levels. The raw minimum of the CPU quoted above is sufficient. So, at least 300 Joules is sufficient for the iMac. APC UPS surge protectors provide 365 Joules.
Take a look at these APC units: