The chain is simple: system size in kWp gives annual generation in kWh, and your car's efficiency in miles per kWh converts that to miles. A south-facing, largely unshaded array in the UK commonly yields somewhere in the region of 850–1,000 kWh per installed kWp per year.
Efficiency varies more than people expect. A small hatchback may return 4 miles per kWh in summer; a large SUV in winter can drop below 2.5. Charging losses of roughly 8–12 percent also sit between the roof and the wheels.
The honest caveat is that generated miles are not the same as driven-on-solar miles. Only the share you actually put into the car counts, which brings you back to timing, tariff and whether you have storage.
Use any calculator — including the one on our homepage — as a directional estimate. A survey of your specific roof is what turns an estimate into a figure worth planning around.