Using approximations it took 1 year and 3 months for the savings of the Ecobee 3 to pay for itself in our test environment. Purchase price was $300 because of additional sensor (3 total). Our test environment is a detached one story house with building size of 2501-3000 sq. ft.. 3 bedrooms and age between 16-20 years. Depending on your structure and heating and cooling specs expect anywhere between 1-3 years for the device to pay for itself, not including rebates and other perks from energy and insurance companies. Seriously if you are still using that old stuff upgrade now to save yourself some moolah!