Climate change and global warming are currently the biggest threat to human society. In Iranian households, buildings expend 33% of the yearly energy consumption. The potential of renewable energy sources in Iran is high and Iranian government is taking steps to increase the use of renewable energy sources. This paper examines the use of renewable energy in two major city buildings in Iran. In this paper, solar radiation in two cities (Tehran and Yazd) is analysed. The average annual sum of solar irradiation in Yazd is 2200 kWh/m2 and in Tehran, the amount is 2000 kWh/m2 . The major task in this paper is to quantify the impact of geographical region and to evaluate the viability of implementing renewable systems in buildings in Iran. To reach this end, the annual average solar radiation is calculated in each city. Hourly Analysis Program is used to design each building and calculate their respective heating and cooling loads. Photovoltaic solar systems and solar-thermal systems are designed for both of the buildings. It is seen that, although Yazd has hot and arid climate compared to Tehran, the cooling and heating load in the building in Tehran is higher, and the higher solar radiation in Yazd reduces the number of solar panels required for the designed building. However, the difference is not significant. It is assessed that in order to have more efficient systems, the grid infrastructure in Iran should be upgraded to enable applying large-scale renewable energy systems for buildings in a neighbourhood.