Пре­об­ра­зуй­те, если это не­об­хо­ди­мо, слово EXPENSIVE так, чтобы оно грам­ма­ти­че­ски со­от­вет­ство­ва­ло со­дер­жа­нию тек­ста.

Solar panels are quite expensive to install, but they are getting __________ every year, and besides, users save a lot of money later  — once the equipment has been installed, there is no operating cost because sunlight is free.