This methodology outlines the path for establishing business-as-usual (BAU) emission scenarios water and wastewater utilities could exhibit in the mid-term if the current management and practices were to continue in four easy steps. The approach is created on the basis of the “Energy Performance and Carbon Emissions Assessment and Monitoring” (ECAM) tool. Based on the projected future values of key parameters, the variables that are necessary to be inputted into ECAM – the ECAM inputs – for the computation of GHG emissions can be quantified for a certain point of time. This step is facilitated by the “Tool of Projecting ECAM Inputs for GHG Emissions as BAU Scenarios (PEIGE)” in Excel format, which automatically calculates future values once users have entered the current ECAM input values and BAU trends. BAU scenarios can help to understand the impacts of adopting a low carbon policy and can serve as a technical component to inform/decide strategic planning on climate change, emissions mitigation goal setting and long-term climate policy design.