What is the difference between e3 and e3lite?

The ecobee3 lite does not support accessories (HRVs, ventilators, humidifiers, and dehumidifiers). However, you still get all the same smart functions offered by the ecobee3, such as Smart Recovery and scheduling.

As of March 21, 2017, the ecobee3 lite now also supports wireless room sensors, which can detect the temperatures in the rooms that are occupied to help your ecobee3 lite be smarter about saving energy when you’re away. They also help manage hot and cold spots for comfort in the rooms you place them in, like your living room, home office, or bedroom.

For more details, explore the ecobee3 lite features.