I am having serious doubts that the Dell R815's will be profitable in the long term.
u/tevador put together "The profitability of RandomX" calculations for RandomX Hash Rate for profitability of a given rig:
https://www.reddit.com/r/MoneroMining/comments/dxp2h9/the_profitability_of_randomxFirstly, for those who are not aware, the profitability break-even network hashrate H (i.e. the network hashrate at which your electricity costs are equal to your mining rewards) is given (approximately) by the following equation:
H = 30000 * h * R * X / (p * e)
h ... the hashrate at which you are mining [H/s]
R ... the current Monero block reward [XMR]
X ... the current Monero exchange rate [USD/XMR]
p ... the power consumption of your mining device [W]
e ... the price you pay for electricity [USD/kWh]
For example, my Ryzen 1700 mining machine has a break-even point of 30000 * 4250 * 2.1 * 62 / (70 * 0.18) = 1.3 GH/s (assuming a block reward of 2.1 XMR and 62 USD/XMR).
Three of the variables (h, p, e) are specific to your mining setup, the block reward R is more or less predictable and the price of XMR X is unpredictable.
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Using today's price of $51.92 XMR
https://bitinfocharts.com/monero This is what I have measured for four Dell R815 Servers on one PDU.Two with quad 6348 and two with quad 6378 Opterons.
43890 H/s for RandomX
2880 watts (12 Amps * 240 VAC) avg of 720 watts per server measured at the panel.9.0c per KWH
30000 * 43890 * 2.1 * 51.92 / (2880 * 0.090) = 553.8 MH/s
So if the hash rate for RandomX goes to the 1.3 GH/s that is estimated when it settles after the fork these Dell R815's will again be turned off.
I would then only be mining on my various Dell T5600/T7600 Workstations.
I would also look into an electric plan where electricity is free for nights or weekends to mine on the Dell R815's.
Or I would just resell the Dell R815's for a slight profit as most of them only cost me around $250 each.
Those servers are very inefficient it seems. But you would expect that given how old they are.
You are at 15.2 hashs per watt which is pretty bad for any CPU you'd want to mine with. Seems to me, you would want the highest Hashs per watt for longevity of mining. Efficiency is king here.
15.2 is very poor considering even Intel CPU's will be much better off. i5-8400 would be around 28-30 hashes per watt.
This guy on Reddit managed 56 Hashes per watt with a Ryzen 3900x pulling 253 watts from the wall.
https://www.reddit.com/r/MoneroMining/comments/dynz46/getting_14200hs_on_ryzen_3900x_miningp0rn/Another guy managed 8245 H/s with Ryzen 3700x pulling 155 watts from the wall. Thats 53 hashes per watt.
Ryzen 3600 is good for 60 hashes per watt! CPU cost less than $200 and a cheap board is $50. Just need $50 of good RAM and literally any PSU.
I think if a person wanted to make a mining farm today, it would need to be Ryzen 3600 (or 3600X), Ryzen 3700X or 3900X. Resale will be very good with these CPU's so you can mine until its not profitable and then resell for most of your money back or build into rigs to resell at a small profit.