MWh per year / 3.3 = homes powered - this uses Ofgem's assumption that the average annual consumption of a home is 3.3MWh.
MWh per year x 0.38146 = tonnes carbon saved - Every unit of electricity generated by a renewable source embedded on the network offsets emissions at the generation level as well as losses on the transmission and distribution network. 0.38146 is the 2017 government estimate.
Cars off the road
Tonnes carbon / 4.7 = cars off the road - A typical passenger vehicle emits about 4.7 metric tons of carbon dioxide per year. This assumes the average gasoline vehicle on the road today has a fuel economy of about 21.6 miles per gallon and drives around 11,400 miles per year. Every gallon of gasoline burned creates about 8,887 grams of CO2. (source)