Indian factories, smelters and households are being hit by blackouts and rising energy costs as the impact of plunging coal stockpiles at the country’s power stations ripples through the economy. The weakest monsoon since 2015 has curbed hydropower output, while hotter-than-normal weather has buoyed demand for air conditioning and farm irrigation. That has seen coal-fired power stations, which supply about two-thirds of India’s electricity, rapidly burn through their inventories. Smaller...