Since ROI is the goal, let's run the numbers on the machines that were mentioned. Antminer S21 (200 TH/s, \~3,500W) — "sweet spot" pick * Hardware cost: \~$690–$900 used (Compass Mining/eBay listings, May 2026) * Power draw: 3.5 kW × 24h = **84 kWh/day** * At $0.05/kWh (surplus solar scenario): \~$4.20/day electricity cost * Daily revenue (minerstat, May 2026): \~$5–6/day * **Net daily profit: \~$0.80–$1.80/day** * **Estimated ROI: \~14–36 months** at current BTC price and difficulty Antminer S19 XP (140 TH/s, \~3,010W) — budget/experiment pick * Hardware: \~$800–$900 used * Daily revenue: \~$5.25/day (minerstat) * Power cost at $0.05/kWh: \~$3.61/day * **Net daily profit: \~$1.60/day** * **Estimated ROI: \~18–24 months** assuming current conditions hold If the solar surplus is genuinely free (off-grid batteries being topped up), the S21 used at \~$700 gives a realistic 12–18 month payback *if* BTC holds around current levels. You can verify your exact numbers at [**asicprofit.com**](http://asicprofit.com) or [**minerstat.com/hardware/antminer-s21**](http://minerstat.com/hardware/antminer-s21) just plug in your actual kWh rate and it'll spit out daily/monthly profit live.