This paper presents a methodology that uses the Monte Carlo Method (MCM) to estimate the behavior of economic parameters which may help decision, considering the risk in project sustainability. In order to show how this methodology can be used, a Grid-Connected Photovoltaic System (GCPVS) of 1.575 kWp, located on the roof top of the laboratory building of the Grupo de Estudos e Desenvolvimento de Alternativas Energéticas – GEDAE, at the Universidade Federal do Pará – UFPA, Belém – Pará – Brazil, and operating since December 2007, is analyzed. This system was chosen because it was the first GCPVS installed in the Brazilian Amazon Region, being the first risk evaluation on using a renewable energy source connected to the grid in the Region. This work also presents a similar treatment for the case of a stand-alone photovoltaic system (SAPVS) installed in the remote Santo Antônio Village, municipality of Breves, Pará, Brazil, considering the risk of investment assumed by an investor in power generation projects with similar characteristics or using other renewable energy sources. The last case allows a better assessment for other important applications of renewable energy in the Amazon Region, where the demand for energy is growing, but is still costly and often not a priority in government actions.