> American solar installer companies do seem to charge way more than European or British ones

One of the reasons for this is that in many parts of the US, solar has sadly been market segmented as a luxury product, just like other high efficiency products like heat pumps or EVs.

This is enabled by both the prevailing cultural attitudes about efficiency and renewables as indulgences for the better off, and industries that are happy to keep captive high margin markets of those customers, i.e. the continued lack of a US produced low-cost EV.

The American cult of individualism is also at play, wherein collective solutions are shunned vs private ones, which is why renewables and storage are so popular among off grid libertarian types.