The new power system provides important support for the large-scale integration and efficient consumption of new energy into the power grid. New energy has entered an era of competition with traditional energy, and entire power systems need to be renovated and upgraded to enhance related auxiliary investments. From a systemic perspective, considering the low effective capacity of wind and solar new-energy power generation, it is necessary to increase investment in backup power capacity, flexible power supply, grid connection, and distribution, as well as other operational management system resources, in order to ensure the safe operation and power balance of the power grid. In this study, we focus on the comprehensive benefit evaluation of new investments in the power grid under the new power system, starting from the demand and classification of new investments in the power grid under the new power system. We then focus on typical project investments under the construction of the new power system, determining cash inflows and outflows within the investment cycle, establishing a full life-cycle economic benefit model that adapts to typical investments, and further analyzing the social benefits promoted by typical new investment in the power grid. We construct a social benefit model for various entities involved in power grid investment from the perspective of all members of society, distributed users, and power generators, covering multiple benefits such as economic benefits, reliability benefits, environmental benefits, driving economic benefits, and employment promoting benefits. Finally, we conduct an empirical analysis based on the typical scenario of promoting distributed photovoltaic technology in this new type of power system, targeting full spontaneous self-use. There are three modes of self-use surplus grid-connection and full grid-connection, and the comprehensive benefits of supporting investment in the power grid, such as economic benefits and social benefits, are calculated using the full life-cycle economic benefit evaluation method and the social benefit evaluation evaluation method. This study provides an important reference for solving the problem of missing and difficult to quantify evaluation systems for the new investment benefits of the power grid in the new power system.