Photovoltaic (PV) installations in the building sector are expected to play a crucial role in Switzerland's efforts to achieve decarbonization targets. Nevertheless, most analyses on PV system design prioritize economic factors, and overlook the impact of angle-dependent PV generation, dynamic grid greenhouse gas (GHG) emissions, and export limitations on their emission abatement potential during operation. Our study sheds light on the cost- and emission-optimal orientation and sizing of PV systems in buildings, factoring in hourly grid GHG emission intensity and curtailment measures. We employ mixed-integer linear programming (MILP) and illustrate the methodology for a case study building under different scenarios, aiming to minimize costs and emissions by considering a combination of rooftop and façade PV systems, and a battery. We show that assuming a constant annual grid GHG emission intensity may overestimate annual emission abatement potential of PV generation by a factor of two, emphasizing the need for dynamic grid GHG emission intensity. When considered, cost-optimal solutions favour larger systems that maximize total production. In contrast, emission-optimal solutions increase self-consumption via inclusion of battery systems. Curtailment due to export limitations reduces the cost and emission benefits of excess generation (from feed-in tariff payments and carbon credits), prompting a trend towards smaller PV installations with steeper panel tilts. Thus, the emission abatement potential of PV generation heavily depends on the option to export excess electricity production.