Complex sentence: Historically, the United States had been the leader of installed photovoltaics for many years until 1996, when total capacity amounted to 88 megawatts, more than any other country in the world at the time.
Split into simpler sentences: Historically, the United States had been the leader of installed photovoltaics for many years. Its total capacity amounted to 88 megawatts in 1996, more than any other country in the world at the time.