A proper and effective startup of the steam-assisted-gravity-drainage (SAGD) well pairs, where both horizontal wells inject steam, is a key parameter to SAGD performance. In the past, a number of investigations have been conducted on the optimum startup strategy and operational procedures; however, fewer studies have modelled the heat requirements of the process. This study proposes an effective practical method that correlates the heat flux inside the horizontal wellbore with the temperature outside the wellbore wall. A transient conductive-heat-transfer equation in a cylindrical coordinate system is adopted for two different startup strategies: (a) variable heat flux (steam circulation in the well pair) to maintain a constant desired temperature at the wellbore wall and (b) constant heat flux (electrical heating of the well pair) to gradually increase temperature and finally reach a desired temperature on the wellbore wall. A novel and simple mathematical technique is presented for each startup approach to evaluate the instantaneous and cumulative heat requirements to keep the wellbore hot. Excellent agreements between the results of the proposed techniques and outputs of a commercial reservoir simulator are demonstrated. Accurate quantitative evaluation of heat requirements for the startup phase of SAGD enables optimum planning for steam requirements and practical design of any other heat sources employed in the process.