Cost of capital (CoC) of renewable energy technologies (RET) is an important component in energy system modeling with a high impact on resulting model-outcomes and derived implications. Even though research has shown vast differences in CoC between countries, technologies and across time, it remains difficult to obtain credible values for modeling without time consuming and costly elicitation efforts. As a result, it is still common practice amongst energy modelers and scholars to bypass these differences and work with strongly generalized or unified CoC values instead. In this paper, we develop an open access tool to calculate the CoC by country, technology and year. We calibrate the tool by empirically using the largest available CoC dataset and systematically analyze the circumstances, under which the open access CoC tool provides more versus less reliable results. Our approach helps increase the accuracy and policy usefulness of energy models, particularly in data scarce regions of the world, thereby assisting policy-makers in making more informed decisions.