The integration of wind and solar generators into power systems cause “integration costs” for grids, balancing services, reserve capacity, reduced utilization of the capital stock, and more flexible operation of thermal plants. This paper proposes a market-based valuation framework to analyze and estimate these inte-gration costs. The framework exhaustively accounts for all costs that occur at the level of the power system. It is based on three inherent properties of wind and solar power: variability, uncertainty, and location specificity. Each property has a corresponding cost that can be estimated from numerical models or from observed market data. Surveying the literature indicates that at high penetration rates, say a wind market share of 30%, integra-tion costs can be in the same order of magnitude as wind generation costs. Many previous studies do not fully account for integration costs, underestimating the social costs of variable renewables.