Question about RAVPower Rp-Pb055

I bought the part for my Surface Pro4... via the inverter it's about 1.5x, via 12V interface it didn't work, although I found an appropriate charging cable on Amazon... The bottom line: It works, but not as well as I had hoped...

I fear that 27000 : 6000 was simply calculated here. The 27000 mAh are reached at 3.7V. The conversion to 220V and then back to 12/15V (depending on the power supply unit) or internal 7.5 V should result in some conversion losses. According to https://www.pocketnavigation.de/2016..., the RAVPower is just over 70 Wh. Our 3 Surface (pro 3/4/2017) have battery capacities of 38-45 Wh. This means that they should roughly correspond to the Macbook 12 in the table, which then theoretically only results in about 1.4 charges. Am I making a mistake here? Who can contribute practical experience here?

The power supply unit of the surface/Windows is the problem because of the integrated ms charge control.

Depends a little on the Surface version you have.
If you assume a capacity of approx. 6000 mAh for the Surface Pro, then approx. 4 charges are possible...