Does prepayment enhance consumer responsiveness to energy prices when advanced technologies are unaffordable? We exploit two overlapping Indonesian policy reforms—a nationwide prepaid electricity meter conversion and a 35% tariff increase affecting some customer groups but not others—to estimate how prepayment affects price elasticity. Using eight years of household billing data and a difference-in-differences design, we find prepaid users exhibit price elasticities two to four times larger than postpaid users. This differential grows over time: prepaid elasticity rises from -0.14 to -0.47 after seven years, suggesting sustained behavioral adjustment. Survey and choice experiment evidence indicates price salience drives this differential responsiveness, with positive willingness-to-pay confirming welfare gains. We estimate prepaid metering reduces deadweight loss by 1.5% and CO2 emissions by 6% relative to postpaid systems. These findings suggest that payment structure reforms represent a scalable, low-cost approach to improving environmental policy effectiveness across developing countries.