According to a report released by the National Renewable Energy Laboratory (NREL), rooftop solar could meet almost half of Massachusetts' electricity demand. For the country as a whole, NREL estimates a suitable rooftop potential for solar of 1,118 GW. The 1,432 TWh this amount of solar could produce, would account for 39% of the United States' electricity sales.
The study by the National Renewable Energy Laboratory says the estimated potential from rooftop solar has been revised upwards by more than 80 per cent since the last study in 2008, mostly because of improvements of module efficiencies, building availability and solar modelling.
A cluster of New England states – Maine, Vermont, Rhode Island, New Hampshire, Connecticut and Massachusetts – could generate more than 45 per cent of their electricity needs from rooftop solar PV, despite the below-average solar resource of these states.
The NREL says a common feature of the top six states – in terms of potential PV generation as a percentage of total sales – is the significantly below-average household consumption, suggesting the role an energy-efficient residential sector could play in achieving a high penetration of energy from rooftop PV.
NREL points out that these are not predictions, just potential output. But is also says its own estimates could be highly conservative.
Firstly, it uses an average module efficiency of 16 per cent, when most modern rooftop systems have efficiencies of around 20 per cent. If this average was used, it would lift its figures by around 25 per cent, meaning that rooftop solar had the potential to meet half the country's electricity demand (and nearly 90 per cent in California).
The study also did not take into account the enormous potential of ground-mounted solar, nor did it take into account other installations such as car parks, or integrating solar PV into building facades. Only 26 per cent of "small rooftop" spaces were considered suitable for solar PV.