I could downvolt eth only to use ~80w stock clocks, GPU-Z (not at wall, but same card so it's relevant). ZEC uses similar. If you downclock to say 1000MHz instead of 1260, you get same speed on Eth (zec suffers), but you can lower voltage another 100mv or more, chopping off perhaps another 20w (haven't tested yet).
Sure you can downvolt to that extreme but your hash per watt suffers past certain points.
Anyways, good luck doing that 2 times a day per card if you're using the RX series if you're wanting to mine whatever is the most profitable.
Just FYI, on my Sapphire 470 Nitro 4Gb, I'm hashing at 150-152H/s per card with a hex offset of -162.5mV undervolt. The whole system uses around or less than 400W @ Wall, all 4 cards give me a nice and stable 600H/s. I'm currently hashing with 3 rigs at 1800H/s for 1260W, so that's a ratio of ~0.7W/H
What I noticed, is that past -150mV, you gain very little watts.
why? I'm getting on all of my 470s 165-175 sols running on 900mv - consuming ~108W/card