An owner of two existing commercial buildings is considering the possibility of retrofitting them for energy conservation. Both buildings are otherwise in good condition and are expected to last for another 30 years. Preliminary investigations indicate that technologically a solar energy system with 25 years of useful life is most suitable for building 1, and a heat recovery system with 18 years of useful life is most suitable for building 2. What planning horizon should be used for the economic analysis of these two projects?