This paper's motivation is to contribute to the growing literature on contingent valuation in developing countries. In a new survey in Rwanda valuing people's contribution for electrification, an innovative design is proposed, giving the choice to respondents between two scenarios : one in which they would contribute their time and another in which they would contribute their money. Results show that people prefer to contribute in time rather than in money. A great majority of people who have electricity would accept to contribute for the electrification of others. The potential contributors are willing to contribute 37,250 Frw ($ 55) and 66 days on average per year for five years.