Wasting/saving energy is not as simple a calculation as that. When energy is at peak use (say in the winter, around 5:30/6:00 when everybody returns from work), it becomes more expensive for energy companies to procure the requisite energy needed. During these peak times, they often have to buy energy from other companies at a higher cost or find less efficient means of generating it (e.g. coal) to meet the demand. Getting an accurate picture of estimated energy demand can actually save a lot of money/energy for customers.
I work at Nest and it may be a little unclear at the moment (we're working on it), but The Nest API does not guarantee that if you send an ETA of 15 minutes (docs: https://developer.nest.com/documentation/eta-guide), the home will heat to the appropriate level in 15 minutes. It's actually filtered through our algorithms to determine the best course of action to not waste energy but also provide the appropriate level of comfort.
I work at Nest and it may be a little unclear at the moment (we're working on it), but The Nest API does not guarantee that if you send an ETA of 15 minutes (docs: https://developer.nest.com/documentation/eta-guide), the home will heat to the appropriate level in 15 minutes. It's actually filtered through our algorithms to determine the best course of action to not waste energy but also provide the appropriate level of comfort.