Worm gearboxes are not necessarily energy efficient, particularly worm and wheel gearboxes with higher ratios.
In terms of torque loss, a typical worm & wheel gearbox at 10:1 ratio would be around 87% efficient, whilst at 100:1 it would be around 50% efficient, based on 1400rpm input speed.
The addition of a helical stage to a worm gearbox improves the overall efficiency (helical gears are around 99-98% efficient per stage) and the torque capacity of the gearbox (worm-wheel gearing has a higher torque capacity).
Despite not being the most efficient type of gearing, worm and wheel gearboxes have other advantages, such as preventing back drive under load conditions when stopped (generally achieved at ratios above 40:1) which can be particularly useful when the application involves lifting, turning or it is a conveyor with an incline.