Solar photovoltaic (PV) panels are the most cost effective renewable energy source for most zero energy homes. Because installing a PV system is the most costly element of a zero energy project the number of solar panels required should be calculated only after all other energy-saving measures – outlined here in the Twelve Steps to Affordable Zero Energy Home Construction and Design – have been modeled to minimize energy use. The goal is to reduce the use of solar panels as much as possible to supply the remaining energy needs of the home at the lowest cost possible. Since the price of solar photovoltaic systems has been dropping this is becoming easier.