We develop an energy calculator within an online teaching platform to aid citizens and decision-makers understanding an energy system. The model, designed in order to be easily adapted to any energy system at national or regional scale, is presented and applied to the case of Switzerland. Although autonomous on a yearly balance, Switzerland today already relies on foreign imports to face higher electricity demand in winter months. The decision of the country of phasing out nuclear power by 2034 will have as a consequence to further increase this seasonal electricity deficit. The Swiss-EnergyScope.ch project is a contribution to the public debate escalated concerning Switzerland's future energy strategy, helping citizens to take an active part in it by associating numbers and facts to opinions and choices. The online platform mainly consists of an energy calculator, enabling users to evaluate the effect of a list of possible choices on the energy future of the country. An online wiki and a MOOC will allow users to acquire basic knowledge on energy and to be guided through the learning process and the use of the calculator itself. The robust conceptual design strategy adopted to model the Swiss energy system allows for the contextualization of the key issue of electricity supply within the framework of the overall energy system, conveying a holistic view in which technologies affecting both heat and electricity (such as heat pumping and cogeneration) can be easily integrated. The monthly approach used for the calculation and display of data allows highlighting the central role of seasonal fluctuations in supply and demand. Overall, the designed tool and the associated learning experience allow popularizing energy issues, demystifying the complexity of a national energy system without oversimplifying it.