Step 1: A village needs 50 kWh of electricity per day. Compare (a) a diesel generator (3 kWh per litre, diesel ₹90/L) with (b) a solar array (5 peak-sun hours, system efficiency 15%, panel cost ₹50/W). Step 2: (a) Diesel needed = 50/3 ≈ 16.7 L/day → cost ≈ 16.7 × 90 = ₹1500/day ≈ ₹5.5 lakh/year, plus CO₂ ≈ 16.7 × 2.7 ≈ 45 kg/day. Step 3: (b) Energy per installed watt per day = 5 h × 0.15... more carefully: required peak power = 50 kWh/(5 h × 0.75 system derating) ≈ 13.3 kW. Step 4: Panel cost ≈ 13,300 × 50 = ₹6.65 lakh one-time; fuel cost zero thereafter; CO₂ zero in operation. Step 5: Payback vs diesel ≈ 6.65/5.5 ≈ 1.2 years — after that the solar electricity is nearly free for 20+ years. Result: Diesel is cheap to install but expensive forever; solar inverts the economics — high upfront, near-zero running cost. This crossover logic drives the global energy transition.