There's a lot of great content on PrepLounge on how to structure a market sizing e.g.
Think through this problem from first principles which means there are different approaches you can take e.g.
- Residential vs Business solar panel owners/operators - assume a certain % penetration of solar power usage, estimate avg panels/user
- Landmass based - total roof size in a country, average % dedicated to solar panels, avg size per panel
- Energy usage basis - total energy used in a country, % renewable, % solar, kWH generated per panel, therefore inferred number of panels