A reconciliation of the beginning and ending amounts of unrecognized tax benefits (without related interest expense) is as follows (in thousands):
Our effective income tax rates were 9.5%, (84.0)% and 9.3% for the years ended December 31, 2019, 2018 and 2017, respectively. Our effective tax rates were below the statutory rate primarily due to the tax windfall benefits from employee stockbased payment transactions, foreign derived intangible income deductions and research and development tax credits claimed, partially offset by the impact of non-deductible meal and entertainment expenses and state taxes.
We recognize a valuation allowance if, based on the weight of available evidence, both positive and negative, it is more likely than not that some portion, or all, of net deferred tax assets will not be realized. Due to the uncertainty of realization of  and development tax credits totaling $0.3 million, we established a valuation allowance of $0.3 million during the second quarter of 2019, which remained at $0.3 million as of December 31, 2019. As of December 31, 2018, based on our historical and expected future taxable earnings, we believed it was more likely than not that we would realize all of the benefit of the existing deferred tax assets. Accordingly, we did not record a valuation allowance as of December 31, 2018.
We apply guidance for uncertainty in income taxes that requires the application of a more likely than not threshold to the recognition and de-recognition of uncertain tax positions. If the recognition threshold is met, this guidance permits us to recognize a tax benefit measured at the largest amount of the tax benefit that, in our judgment, is more likely than not to be realized upon settlement. We recorded unrecognized tax benefits of $0.6 million, $0.8 million and $1.0 million for research and development tax credits claimed during the years ended December 31, 2019, 2018 and 2017, respectively.
As of December 31, 2019 and 2018, we accrued $0.2 million and $0.1 million of total interest related to unrecognized tax benefits, respectively. We recognize interest and penalties related to unrecognized tax benefits as a component of income tax expense.
We are not aware of any events that make it reasonably possible that there would be a significant change in our unrecognized tax benefits over the next twelve months. Our cumulative liability for uncertain tax positions was $3.1 million and $2.8 million as of December 31, 2019 and 2018, respectively, and if recognized, would reduce our income tax expense and the effective tax rate.
We file income tax returns in the United States and Canada. We are no longer subject to U.S. income tax examinations for years prior to 2016, with the exception that operating loss carryforwards generated prior to 2016 may be subject to tax audit adjustment. We are generally no longer subject to state and local income tax examinations by tax authorities for years prior to 2016.
As of December 31, 2019, we had federal net operating loss carryforwards of $4.9 million, which are scheduled to begin to expire in 2030. As of December 31, 2019, we had state net operating loss carryforwards of $1.7 million, which are scheduled to begin to expire in 2027. As of December 31, 2019, we had federal research and development tax credit carryforwards of $5.2 million, which are scheduled to begin to expire in 2038. As of December 31, 2019, we had state research and development tax credit carryforwards of $4.3 million, which are scheduled to begin to expire in 2021. The federal net operating loss carryforward arose in connection with the 2013 acquisition of EnergyHub. Utilization of net operating loss carryforwards may be subject to annual limitations due to ownership change limitations as provided by the Internal Revenue Code of 1986, as amended.
Table:
 |  | Year Ended December 31, | 
 | 2019 | 2018 | 2017
Beginning balance | $2,801 | $1,973 | $681
Additions based on tax positions of the current year | 718 | 857 | 718
Additions based on tax positions of prior year | 18 | 147 | 373
Decreases based on tax positions of prior year | (253) | — | —
Additions resulting from acquisitions | — | — | 277
Decreases due to lapse of applicable statute of limitations | (219) | (176) | (76)
Ending balance | $3,065 | $2,801 | $1,973


Question: How many years did the beginning balance exceed $2,000 thousand?
Answer: 1
Calculation: 2019