A method is proposed for estimating the electricity consumption (and associated carbon emissions) of a defined electrical-lighting configuration in an office building, accounting for the daylight contribution from windows and rooflights. Heat gains due to lighting for an average day in each month may be used to aid assessments of the effect of lighting systems on the cooling load, known to be high for office environments. For a typical 6-storey office building, annual energy savings for lighting of 56-62% and a reduction in CO2 emissions of nearly 3 tonnes are predicted by changing the lighting and daylighting specifications for a defined "2005" scenario to those of a low-carbon "2030" scenario. The associated reduction in peak lighting-load, and hence heat gain due to lighting, is 3 W/m2. © 2007 Elsevier Ltd. All rights reserved.