S17 or S19 with a price that (hopefully) ROI under a year at 5c/kWh.
if you have free power a 50th s17 makes $3.61 a day if your power is 5cents it uses 56 kwatts or 56 x .05 or 2.80 net profit of 81 cents a day so .81 x 365 = $300 usd for an s17
a s19 makes 6.86 a day it uses 80 kwatts or 80 x .05 = 4.00 so profit is 2.86 a day
365 x 2.86 = $1044 usd for a s19
good luck finding those prices