 Solution

Use a conventional car consumption of $6$ litres per $100$ km and the supplied fuel-energy scale of $9$ kWh/litre. Driving $20,000$ km then consumes approximately $1200$ litres or $10,800$ kWh of fuel energy. If an energy-crop route delivers a net usable fuel power of $0.5\mathrm{W}{\mathrm{m}}^{-2}$ averaged over the year, its yield is $4.38\mathrm{kWh}{\mathrm{m}}^{-2}\mathrm{y}{\mathrm{r}}^{-1}$, requiring about $2500{\mathrm{m}}^2$ per car.

For an electric car consuming $0.18$ kWh/km, the annual electrical requirement is $3600$ kWh. With an annual incident solar energy of $1000\mathrm{kWh}{\mathrm{m}}^{-2}$ and module efficiency $20\%$, the module-area estimate isAllowing for spacing, charging and storage losses increases the solar land requirement, perhaps to several tens of square metres, while crop yield and processing losses change the biofuel estimate. Even with such corrections the difference can be many tens to over a hundredfold. The comparison includes the electric drivetrain's lower required input energy rather than incorrectly demanding equal fuel heating value from both routes. Agricultural land competition, biodiversity, fertilizer and indirect land-use change also matter for biofuels; solar infrastructure still needs material and seasonal-system assessment.