Smart monitors already exist. This project aims to develop an affordable and open-source monitor that can be easily connected to most homes. It is my understanding that it would likely be mounted within the breaker housing. The device constantly reads the kWh through this connection and updates the user's account via a WiFi connection. Setting up the network connection can be done a number of ways (broadcasted WiFi network, bluetooth, micro-usb connection, etc.). A small on-board battery and memory storage may prove useful for times of no power or internet outages.
An API can be developed with the following entities: users, devices, data entries, and goals. Once the data is captured from the household device, the possibility for apps, social competition, statistics, goal-setting, etc. is unlimited. The web architecture and implementation is far more important than the device as it is what drives users to make changes based upon the monitor's data. Gamification would be a primary tool in driving usage. The website could even host forums or features demonstrating how to reduce electricity usage on a household basis.
In addition, the site would have an api accounts for developers to use the data for their own projects involving Nest or other home automation/IoT devices.