The millions of backup generators installed at commercial and industrial sites across the country make up one of the largest sources of distributed energy. But most of it is fueled by diesel generators that are too dirty, noisy and inefficient to run during non-emergency times. Still, a small but growing number of natural gas-fired gensets in the market are clean enough to run outside of the strict parameters set for diesel in many U.S. jurisdictions, albeit with some big exceptions like California.