You can use the "Charge Stats" page in the app, but it only shows the last 31 days so you'd have to be religious about screenshotting it every 31 days.
The gold-plated solution is to get an electrician to install a single circuit meter in your switchbox on the charging circuit, like this: Lanx Single Phase KWH Meter.
The gold-plated solution is to get an electrician to install a single circuit meter in your switchbox on the charging circuit, like this: Lanx Single Phase KWH Meter.