A US environmental group has accused three major television manufacturers 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 US households with an extra $US120 million ($A157 million) in electricity bills each year.
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 centre for energy efficiency standards.
The report called to mind elements of the Volkswagen emissions scandal in which the automaker duped US regulators by using sophisticated software to turn on emissions controls when the cars were being tested and turn them off during real-world driving.
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.
The behaviour ``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.
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.
The reason the TVs perform differently during tests is because the US government relies on an 8-year-old clip that stitches together 260 separate scenes that rapidly change, keeping the backlight off. But longer scenes, such as a football game, would result in the picture staying brighter, drawing more electricity than in government tests.
LG spokesman John Taylor wrote in an email that the company was ``confident that our products are being tested properly and are delivering energy efficiency in real world use.''
Samsung said the government-tested ratings are based on the factory settings for the picture - something that most consumers do not change. That is consistent with the report, which assumes that two-thirds of consumers leave those settings in place.
Vizio did not respond to requests for comment.
