Another way is to count the power sources it uses. The pci-ex motherboard slot provides 75W. An 8 pin pci-ex connector supplies up to 150W. A 6 pin pci-ex connector supplies up to 75W. Your Gainward requires an 8 pin AND 6 pin, plus the motherboard slot itself.
Next, you don't want your power supply running at 100% load. If your videocard requires 220W, you don't want to use a 220W power supply. You'll want to check reviews of the power supplies you're looking at to see the ideal load level but staying around 80% load can't hurt. So then for a 220W part you'd want ~300W.