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If you are carrying your Apple MacBook (Early 2015) in your car, you can keep it powered with a suitable car charger like the Belkin USB-C Car Charger + Cable, since the MacBook supports USB Power Delivery. 

To see the charging activity between these two devices, we will use the software from Granite River Labs to capture the USB Power Delivery Protocol transactions.

We see here that the Belkin USB-C Car Charger + Cable is able to supply 15 watts (5 volts @ 3 amps) and 27 watts (9 volts @ 3 amps) over the USB Type-C connector. 

The Apple MacBook (Early 2015) only requests 12 watts (5 volts @ 2.4 amps) of charging from the Belkin USB-C Car Charger + Cable. While this is much lower than the 29 watts you get from the Apple 29W USB-C Power Adapter, the resulting slower charge is better than nothing and still useful during long road trips.

The actual current pulled by the MacBook from the Belkin charger is around 2.2 amps with the MacBook fully charged.

The Apple MacBook (Early 2015) takes approximately 4 hours 30 minutes to fully charge from the Belkin USB-C Car Charger + Cable.