There are a few technically feasible approaches to get about a megawatt of power in space using nuclear power or solar power. Russia has a project to develop about a megawatt nuclear reactor for space applications.
NASA Glenn Research Center, GRC, has several programs to advance near-term photovoltaic array development. One project is to design, build, and test two 20 kW-sized deployable solar arrays, bringing them to technology readiness level (TRL) 5, and through analysis show that they should be extensible to 300 kW-class systems (150 kw per wing).