To this end, the main idea of this study is to develop a risk assessment model of an ESCO by consideration of the energy savings market structure as a decision-making tool for determination of the appropriate time period by the market regulator for certificates sale. Overall, studies about ESC market can be categorized into three main stages, as shown in Table 1. The early studies were mainly about the fundamentals of market, which include the studies of the market's performance mechanism, the relationship of the market players, compatibility of the market with energy saving regulations and policies of the country, etc. The studies of the benefits and costs of the market is the second category that includes analysis of the needed resources for creation of the market, cost of publishing, and the exchange of the certificates, benefits of saved energy and certificates, etc. Sustainability and better performance of the market is the third category of studies which encompasses the issues such as price fluctuation, monopoly, demand, and supply matters that should be analyzed to consider appropriate solutions for them to avoid market failure. Although the previous studies concerning the ESC market are, in most cases, related to the ESCO activity in the market, the issue requiring more investigation is risk reduction in ESCO business \begin{table}
\begin{tabular}{c c l} \hline \hline  & \(\bullet\) & Key structural elements of the market [21] \\ Market Fundamentals & \(\bullet\) & Market’s compliance with the country’s regulations and policies [22] \\  & \(\bullet\) & Market’s players and their roles [23] \\  & \(\bullet\) & Potential, challenges, and further development strategies of the market [24] \\ \hline Market Benefits and Costs & \(\bullet\) & Transaction cost, benefits of saved energy, and certificates, etc. [25] \\ (Economically, Environmentally, etc.) & \(\bullet\) & Emission reduction and clean production [26] \\  & \(\bullet\) & Energy security increase [27] \\ \hline Market Sustainability & \(\bullet\) & Market share of the players and monopoly formation [13] \\ and Better Performance & \(\bullet\) & Certificates pricing models and price fluctuations [28] \\  & \(\bullet\) & Market trend, status and solutions for improvement [14; 22] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main categories of studies about ESC market (source: categorized by authors). Figure 2: The benefits of Energy Service Company (ESCO) and energy consumer from energy reduced cost and certificates sale [20]. through determination of the appropriate time period by the market regulator for selling the certificates. Additionally, it is notable that different studies about ESCO risk analysis have been undertaken in the next section, but the main distinction between this study and previous work is the focus on risk assessment of ESCO in the ESC market structure as well as the mentioned related issues to the market regulator. To this end, the NPV function has been considered as a basic tool for developing the risk assessment model in this article, since it allows assessment of how the time period deemed by the market regulator affects the risk in ESCO business. In addition, all of the affective risk factors could be reflected in the NPV function as input cash flows (\(\chi_{i,t}\)) and output cash flows (\(y_{i,t}\)) as Equation (1) [29]. \[\mathrm{NPV}=\sum\
olimits_{t}^{\intercal}\frac{\left(\chi_{i,t}-y_{i,t} \right)}{\left(1+\mathrm{r}\right)^{t}}, \tag{1}\]  
where (\(x_{i,t}\)) include income items and (\(y_{i,t}\)) include expense items at time \(t\) in the period of T.