Would need to know volts, not clocks of your card. Even then it can be tricky as the card wont stay 100% load, and therefore highest voltages, all the time. Just pick up a kill-a-watt meter and plug it into your wall, then plug computer into that. Measure at stock and your OC. That will give you a real accurate measurement, which you can then check while gaming, idle and basic use, as your power consumption will vary depending on your tasks.
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