NRG Energy Inc. had the biggest jump in short interest relative to shares outstanding among SNL-covered power and gas companies between July 14 and July 31.

The percentage of the company's shares held short relative to shares outstanding increased 283 basis points to 6.86% as of July 31.

SolarEdge Technologies Inc. topped the list of power and gas companies with the biggest decreases in short interest in the second half of July. The percentage of the company's shares held short relative to shares outstanding dropped 281 basis points to 11.20% as of July 31.

Spark Energy Inc. topped the list of most shorted power and gas companies between July 14 and July 31. The company saw short interest slip 48 basis points to 29.96% of shares outstanding.

Arch Coal Inc. was the most shorted coal company during the period. The company saw short interest fall 261 basis points to 14.32% of shares outstanding.