Many of our numbers are calculated from multiple sources. Each calculation and source is in our spreadsheet at xls.CO2List.org That spreadsheet gives complete transparency about where the data come from and what calculations were done to bring all items into comparable units.
Hydroelectricity: Hydroelectric reservoirs convert some of the carbon in the area they flood to methane. Methane has 25 times as much global warming effect as CO2, so hydroelectricity which converts carbon to methane can have major global warming effects.
DRIVING Speeds of cars greatly affect their fuel consumption and therefore the amount of CO2e they release per mile.
Ecoinvent.org is a source of data for many large companies. They charge 1,800 Euros for access and provides very little public explanation of data or methods.