SAN FRANCISCO — An environmental group accused three major television manufacturers Wednesday of misleading consumers and regulators about how much energy their high-definition screens devour by designing them to draw less power during government testing than in ordinary use.

The Natural Resources Defense Council concluded that the TVs made by Samsung, LG Electronics and Vizio saddle households with an extra $120 million in electricity bills each year and generate tons of additional pollution.

The added expense works out to about $10 to $20 per household annually over the anticipated decade-long life of the typical widescreen TV.

Both Samsung and LG Electronics disputed the findings.

“It appears that some major manufacturers have modified their TV designs to get strong energy-use marks during government testing but may not perform as well in consumer homes,” said Noah Horowitz, senior scientist and director for the NRDC’s center for energy efficiency standards.

Samsung and LG did not break any laws, according to the report, but rather exploited weaknesses in the Department of Energy’s system to measure electricity usage.

But the behavior “smacks of bad faith,” Horowitz said.

The findings were based on an analysis of high-definition TVs with screens spanning at least 55 inches made in 2015 and 2016. The estimates on electricity costs are based on high-definition TVs with screens 32 inches and larger.

The study concluded that Samsung and LG exploited the testing system to get better scores on the yellow “EnergyGuide” labels that appear on sets in stores. Those scores often influence TV buyers looking to save money on utility bills.

Samsung and LG sets have a dimming feature that turns off the screens’ backlight during the 10-minute video clip used in government tests, according to the study. But that does not typically happen when the sets are being used in homes to watch sports, comedies, dramas and news programming.