India, seeking to reduce electricity shortages, set up a new power market, in which transmission constraints sharply limit trade between regions. I use confidential bidding data to estimate the costs of power supply and simulate market outcomes with more transmission capacity. I find that the returns to building transmission hinge on market conduct. Under a competitive model of supply, transmission investments roughly breakeven. Under a strategic model, the same transmission expansion increases market surplus by 19 percent, enough to justify the investment, because low-cost sellers increase supply in response to a more integrated grid.