On the edge of a bucolic field in Princeton, N.J., an eco-friendly office building recently opened its doors. Plants festoon the roof, a living wall is planned for the lobby, and rainwater storage tanks supply the building’s needs. In the parking lot there are wind turbines, solar panels and electric vehicle charging stations.

It is the picture of a sustainable future, one in which society’s insatiable demand for electricity can be met without polluting the planet.

The same cannot be said of the building’s tenant, NRG Energy.

The biggest independent power producer in the country, NRG sells electricity to utilities, companies and individual homes. To generate all that wattage, it burns enormous amounts of natural gas, coal and oil, making NRG one of the country’s biggest polluters.

It isn’t trying to muck up the planet; that’s just the nature of the business NRG is in. The electricity industry is the biggest source of greenhouse gas emissions in the United States, according to the Environmental Protection Agency. In 2014, NRG was the fourth-largest emitter of carbon dioxide among the country’s power producers.