Roofs that have high solar reflectance (high ability to reflect sunlight) and high thermal emittance (high ability to radiate heat) tend to stay cool in the sun. The same is true of low-emittance roofs with exceptionally high solar reflectance. Substituting a cool roof for a non-cool roof tends to decrease cooling electricity use, cooling power demand, and cooling-equipment capacity requirements. while slightly increasing heating energy consumption. Cool roofs can also lower citywide ambient air temperature in summer. slowing ozone formation and increasing human comfort. DOE-2.1E building energy simulations indicate that use of a cool roofing material on a prototypical California nonresidential (NR) building with a low-sloped roof yields average annual cooling energy savings of approximately 3.2kWh/m(2) (300 kWh/ 1000 ft(2)), average annual natural gas deficits of 5.6 MJ/m(2) (4.9 therm/1000 ft(2)), average annual source energy savings of 30 MJ/m(2) (2.6 MBTU/1000 ft(2)), and average peak power demand savings of 2.1W/m(2) (0.19 kW/1000 ft(2)). The 15-year net present value (NPV) of energy savings averages S4.90/m(2) ($450/1000 ft(2)) With time-dependent valuation (TDV), and $4.00/m(2) ($370/1000 ft(2)) without TDV. When cost savings from downsizing cooling equipment are included, the average total savings (15-year NPV + equipment savings) rises to $5.90/m(2) ($550/1000 ft(2)) with TDV, and to $5.00/m(2) ($470/1000 ft(2)) without TDV. Total savings ranee from 1.90 to 8.30 $/m(2) (0.18-0.77 $/ft(2)) with TDV, and from 1.70 to 7.10 $/m(2) (0.16-0.66 $/ft(2)) without TDV, across California's 16 climate zones. The typical cost premium for a cool roof is 0.00-2.20 $/m(2) (0.00-0.20 $/ft(2)). Cool roofs With premiums up to $2.20/m(2) ($0.20/ft(2)) are expected to be cost effective in climate zones 2-16; those with premiums not exceeding $1.90/m(2) ($0.18/ft(2)) are expected to be also cost effective in climate zone 1. Hence, this study recommends that the year-2005 California building energy efficiency code (Title 24, Part 6 of the California Code of Regulations) for NR buildings with low-sloped roofs include a cool-roof prescriptive requirement in all California climate zones. Buildings with roofs that do not meet prescriptive requirements may comply with the code via an "overall-envelope" approach (non-metal roofs only), or via a performance approach (all roof types). Published by Elsevier Ltd.