Batteries
Batteries are pretty much the most expensing individual component of any solar system. This despite the fact that battery prices have come down nearly 50% in recent years, and will continue to decline in the future because of competition and emerging technologies.
Competition is mostly being driven by the electric vehicle market, especially in the brutal Chinese market. Where as once the CEO of Toyota erroneously said there wasn't enough lithium in the world, China has recently been forcing mines to close in order to stabilize prices all along the supply chain.
Emerging technologies are mainly being driven by new substitutes for lithium, of which sodium is the main one which some companies claim will be 90% cheaper (at the cell level) than lithium, though there are certainly other technologies in play.
The primary concern with all these technologies is the question of energy density. Sodium has a lower energy density than lithium, though rapidly improving.
On the other end of the energy density spectrum are various approaches to solid state batteries, which Toyota has also been promising for years, but not delivering on. These batteries will eventual deliver very high energy densities enabling all sorts of new applications (such as electric air transportation) or very long range at much lighter weight in traditional applications such as EVs, but are very expensive due to difficulties in production. They will also deliver on safety improvements, especially compared to lithium turnery batteries with liquid electrolytes.