There's a lot of great content on PrepLounge on how to structure a market sizing e.g.
https://www.preplounge.com/en/consulting-forum/how-would-you-estimate-the-market-size-for-vegan-shampoos-in-the-uk-9150
https://www.preplounge.com/en/consulting-forum/i-am-trying-to-solve-a-market-entry-case-but-the-answer-framework-makes-me-confuse-6379
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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