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The Anker PowerCore+ 26800mAh PD with 27W PD Portable Charger Bundle has both a USB Power Delivery based portable battery as well as a USB Power Delivery charger to supply power to other USB Type-C based phones, tablets, and notebook PC's. We'll use the 30 watts based portable battery portion to charge the USB Power Delivery enabled Google Pixel phone over USB Type-C.

From the USB Power Delivery negotiations, we can see how well the Anker battery charger powers the Google Pixel on a fully depleted battery. When directly connected with a USB Type-C to Type-C cable, the charger can be seen to offer an initial 15 watts (5 volts @ 3 amps) and 30 watts (15 volts @ 2 amps) charging power to the Google Pixel.

The Google Pixel proceeds to request 15 watts (5 volts @ 3 amps) and is accepted by the Anker battery charger.

The Anker battery charger later advertises it supports the following power profiles: 15 watts (5 volts @ 3 amps), 27 watts (9 volts @ 3 amps), 30 watts (12 volts @ 2.5 amps), 30 watts (15 volts @ 2 amps), and 25 watts (20 volts @ 1.25 amps). Interestingly 30 watts (12 volts @ 2.5 amps) wasn't listed as one of the USB Power Delivery profiles that Anker claims to support on its casing.

The Google Pixel doesn't take advantage of the higher PDO's offered by still settling on 15 watts (5 volts @ 3 amps) that was initially established with the Anker battery charger.

From 0% battery, we can see that the Google Pixel is able to pull around 15.3 watts (5.3 volts @ 2.9 amps) from the Anker battery charger while the phone is off. Since 15.3 watts is higher than the 15 watts established during negotiations, the phone is actually over pulling power from the Anker PowerCore+ 26800mAh PD which should not be happening. This is also much higher power compared to using the phone's own charger.