This paper presents the first comprehensive hourly-resolution scenario study of the Indian electricity system with a view to investigate the transition from fossil to renewable energy-based power generation. Currently, India is 76% dependent on coal for power generation, however ambitious goals exist. Energy organisations including International Renewable Energy Agency and The Energy and Resources Institute, have presented scenarios but neither with support of dynamic simulation nor with assessment of optimal renewable energy contributions. In this paper, the optimal electricity system is analyzed in comparison with these. Results show that an optimal scenario could supply the hourly electricity by (i) increasing production capacity to about 615 GW, which is a 98.3% increase compared to the 310 GW in 2016, (ii) improving capacity factor of wind power and photovoltaic to 27% and 21% respectively, (iii) increasing wind power production to 433 TWh/yr (from 46 GWh/yr in 2016), solar photovoltaic to 290 TWh/yr (12 GWh/yr in 2016), river hydro to 42 TWh/yr (7.7 GWh/yr in 2016), biomass and nuclear power of 15 GW each. The costs of the identified optimal scenario are lower than of the previous scenarios. Finally, simulations show that inclusion of optimal proportion of biomass and nuclear could avoid any import dependency. (C) 2019 Elsevier Ltd. All rights reserved.