For me the electricity costs in this case are zero or so close to zero that it does not factor.

I put this on an existing server running in my house that runs 24/7 so I am already paying that cost.

The load this adds is as follows. Every 2.5 minutes for 4 seconds there is drive access and a blip from nominal cpu load to about 30% across all four cores.

Given this "server" is the guts of an old laptop with an i5 4200U which has a TDP of 15W at full chat the extra costs are probably not even noticed.

I will have to explore ROI on disks. My current plan is to build a mining PC with the followig

3 * 1050ti Kalmx (Fanless) to mine ETH
1 * Ryzen 1600 (Big fanless cooler if possible)(Mine monaro)
1-2 8TB HDD for Burst

My thinking is if I can get stable and quiet enough this running all three would replace an electric room heater I have, be lower electric cost that the heater and also turn a profit.

Not sure the ROI on it but given triple mine capability and double as room heater that it would be better than normal mining rig ROI :)