Elliptic curve cryptography (ECC) has been adopted in many systems because it requires shorter keys than traditional public-key algorithms in primary fields. However, power analysis attacks can exploit the power consumption of ECC devices to retrieve secret keys. In this paper, we propose an efficient window-based countermeasure that is secure against existing power analysis attacks. Compared to previous countermeasures, our method has low memory overhead, requiring only a table of w+1 entries when the window size is w bits. It also has better performance than many algorithms that perform one point addition or subtraction for every bit in the scalar.