Net metering is not the same thing as only paying for what you use. Net metering means you could push 4 kWh during the day and draw 4 kWh overnight and get metered as zero consumption from the grid. This pushes a storage and distribution cost onto the utility and its other customers. Paying you at wholesale for the daytime push and charing you at retail for the nighttime draw moves those costs back to you.

What others are saying above is that you could generate 4 kWh during the day and consume it on premises, i.e. with air-conditioning or other day time appliance use, and still have zero consumption from the grid. This is also the point of the powerwall and any other on-premises energy storage.