Monitor your energy usage and try to diminish it! Energy Monitor allows you to save your electricity and gas usage, but also allows you to save your electricity production (through for example solar panels). Energy Monitor consists of advanced graphing options to display and compare energy use over the years. Track how much your energy consumption decreased (or increased) over the past few years. This software is completely free and without ads.
Energy Monitor is written in Java, and thus completely cross platform. It's available for Windows, macOS and Linux. You only need Java installed (which is also free and can be downloaded here).
This product's source code is available here.