The energy consumption for water heating accounts for approximately 20% of the CO
2 emissions in the residential sector in Japan. Despite the high expectation of low CO
2 emission, energy-efficient (EE) water heaters are not yet widely used. This study elaborates the cost-benefit of EE water heaters and derives the marginal abatement cost curve. We assume three cases; high efficiency without changing the energy source (case 1), high efficiency by shifting to electrification (case 2), and high efficiency by choosing the energy source (case 3). The result shows that CO
2 emissions can be reduced by up to approximately 63% in the cases 2 and 3, whereas approximately 25% in the case 1. Moreover, the optimization analysis reveals the cost-effective breakdown of water heaters under the CO
2 constraints. The larger the rates of CO
2 emissions reduction are, the more the share of the EE water heaters, especially the share of CO
2 refrigerant heat-pump-type water heaters remarkably increases.
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