What does balance-of-system mean

BOS components include the majority of the pieces, which make up roughly 10%-50% of solar purchasing and installation costs and account for the majority of maintenance requirements. Essentially it is through the balance-of-system components that we: control cost, increase efficiency, and modernize solar PV systems.

Costs are steadily decreasing with regard to solar panels and inverters (formerly 50% of solar PV investment costs).