With continuously increasing penetration of variable generation, the power system needs are changing and so is the need for ancillary services (AS). In time of high generation, PV and wind will be required to participate in AS provision. In this matter, the integration of forecasting confidence intervals into system operation planning is a much more relevant factor than the natural variability itself. This paper introduces a new straightforward approach to generate day-ahead confidence intervals, adjust them to intraday, and adapt this approach to the provision of ancillary services. As a result, the authors quantify the costs of AS provision by PV. The cost is expressed as unused PV generation due to curtailment which is needed to ensure the provision of upward reserve services with a certainty that is acceptable for the TSO. Furthermore, the paper quantifies how far the provider of ancillary services can benefit from larger sizes and wider spreads of its portfolios and how far the reduction of lead contracting times of the AS can reduce costs of unused electricity. Results on the loss due to the contracted reserve power are crucial to determine the competitiveness of PV as an AS provider, therefore the authors make several recommendations on the needed market changes to achieve this. The work presented in this paper has been conducted in the framework of the European funded project, REserviceS.