Sustainable power systems of the future will be characterised by significant penetration of renewable energy sources. Planning and operation of these systems to ensure reliability and security of supply will require application of smart grid techniques and technologies in which demand response (DR) will play a key role. Planning and design of generation projects based on renewable energies, such as wind power projects (WPPs), should therefore be predicated on the assumption that they will operate in a smart grid environment taking into consideration the special characteristics of the generation technologies and the DR potential of the system. The main factor affecting the planning and design of WPPs is the wind resource, especially the mean annual wind speed variability (MAWSV) and the long-term wind speed uncertainty (LTWSU). Additionally, DR and wind power penetration can also affect the planning and design of WPPs when the load is able to respond to fluctuations in electricity prices driven by the availability of wind generation. This paper proposes two simple approaches to analyse the impact of MAWSV and LTWSU in the planning and design of WPPs in a smart grid environment. Both approaches are illustrated using a United Kingdom based case study. The results show that the impact of both factors increase with significant penetration of wind energy in the system, but DR tends to mitigate such impacts. It is concluded that, by addressing the MAWSV and LTWSU, project planners can design more economically attractive WPPs, thereby attracting investments.