It's the manufacturing process of the solar panels and replacement being factored in. Obviously, power generation is carbon free during the panels lifecycle but manufacturing today produces co2
I’m not sure of the answer on this website, but if you’re going that route you’d also need to factor in the carbon cost of the building materials for power plants of any type. Steal and concrete aren’t carbon free either.
Their data sources are quoted when you click on the power source for each country.
They do. For nuclear, they also include cost of decommissioning the power plant completely.