That item clearly shows you how many amps it draws, and that's what you need to know, as it is current which matters to the cable, not wattage.So how do I work out the watts of other devices like my DVR.

Here is a Pic of the power label

View media item 66021

Thanks

You only use wattage to calculate amps if you have to - no point in trying to calculate wattage when you already know the load in amps.

That label shows the maximum current it draws. As it is a switch mode power supply that will only happen when used on the minimum design voltage, 100v. When used on 240v it will draw considerably less current.

You cannot simply add the current rating of all your attached appliances anyway. If the phase angles are different the total will be less than the sum of the individual loads. In extreme cases with an inductive and a capacitive load the total current will be less than either individual load on its own.