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.
The best way to learn is by doing :) Post your approach here and we can provide feedback!
General Tips for Market Sizing
I would always recommend you to give it a shot and then post it here, this would help you the most.
This is a very typical market sizing case, where the key steps are:
Calculate the total population
Calculate the % of population that could be interested in this product (this could be done by clustering -the classical ones are age groups, purchaching power, etc.-)
Hope it helps!
It's a nice market sizing question.. First of all, it really depends on the country and what kind of energy is used in which proportions. I would start with that.