The growing penetration of renewable energy sources integrated through power electronic converters has led to a reduction in grid inertia. This fact affects power system stability and reliability, highlighting the critical role of reserves in managing power imbalances. This paper analyses two key reserve services: Frequency Restoration Reserves (FRR) and Replacement Reserves (RR). The analysis focuses on the dependence of these reserves on the level of intermittent renewable energy integration and system load. A case study based on the Spanish grid is evaluated, from extensive data provided by the Spanish transmission system operator. Potential dependencies between reserve variables and day-ahead hourly forecasted power generation are anaylised and discussed. Results show that reserves are nearly not used for 25 % of the time; and 50% of the times the use of reserves was 11.7% and 24.7% for upwards and downwards FRR ratios respectively. The proposed methodology is applicable to other countries and power systems, being available the necessary data or an equivalent data set.