Solar PV is already the cheapest option to produce electricity in many world regions. Moreover, the price of utility-scale solar power plants in 2050, while highly uncertain, will decrease further with present-day learning rates and projected solar capacities. In this article, we analyze how possible future costs of solar and other characteristics affect the optimal share of electricity demand supplied by solar in different energy systems. We first use a simplified, open, hourly resolved, copper-plate model for four isolated regions, with only wind and solar generation and storage allowed, to identify the core dynamics while sweeping the cost of solar. Using this model, we show that future cost assumptions of utility solar affect energy systems in regions with comparable solar and wind potential compared to regions with much stronger solar or much stronger wind potential. Then, we use a multicountry, networked, sector-coupled model of the European energy system (PyPSA-Eur-Sec) to analyze not only the effect of solar character-istics in a comprehensive energy system, but also how our model assumptions themselves affect the optimal share of solar. We find that assumptions of system properties, such as transmission or sector-coupling can greatly affect the optimal solar penetration at the country level.