Lighting energy conservation measures are typically recommended in commercial bui1ding energy audits. Over 60% of the cost in Bonneville Power's commercial building energy conservation programs are related to lighting. To estimate lighting energy savings it is not uncommon to ignore detailed energy simulations which account for interactions of lighting with heating and cooling systems and simply multiply hours of use by wattage reduction. This paper investigates the potential error in performing simplified 1ighting calculations which ignore interactions. A series of computerized hourly energy simulatinos, using the DOE-2.1C program, were conducted on prototypical small office and small retail buildings. For different interior lighting levels, annual energy consumption and lighting energy consumption are estimated. The prototypical buildings are modelled with different configurations, each of which is modelled using the different interior 1ighting levels. The energy simulations are performed for four pacific northwest cities: Seattle, WA, Yakima, WA, North Bend, OR, and Missoula, MT. The same prototypical buildings were input onto ASEAM-2.1 and modelled using slightly different weather data. Results show that, regardless of the simulation model used, the error in performing a manual calculation to estimate lighting energy savings varies from a high of over 125% for a weather sensitive bui1ding in a co1d climate to a low of 0% for a weatherized building in a moderate climate. It is concluded that for these prototypical commercial buildings the impact of reductions in interior lighting power on total building energy usage is significantly different than a simple one-to-one, relationship. The impact depends on several factors such as: climatic location, model1ing assumptions, weather sensitivity of the bui1ding, and type of heating and cooling systems. These factors can best be included in a building specific simulation.