The 970 has a TDP of ~145W for the reference design. This would be max load power draw for the card. Custom boards will be a little higher, maybe 25W or so. During idle, it's likely to be 20w or so. Of course, these numbers likely don't include factoring in the efficiency of your PSU, so you want to add some there. In other words, if the card can use up to 145W, it needs to pull more than that from the PSU and some is lost in the AC/DC conversion.