This question is rather complicated since your laptop's power consumption fluctuates a lot depending on if you're gaming or simply browsing the Internet on the laptop. This affects the time and percentage Omnicharge can charge your laptop. Remember, this is a small pocket-sized power bank, and laptop charging expectations should be set accordingly, as laptop batteries will often be significantly larger. The Mobile 12,800 is primarily designed to be a portable power bank, which has the ability to charge laptops. While laptop charging is not the primary function of the battery, it is great for topping up the laptop battery.
So to simplify the question, we use a more objective method by comparing Omnicharge's battery capacity to the laptop's internal battery capacity. Also considering the efficiency loss of Omnicharge's conversion circuits, the equation of charge percentage is:
Omnicharge's battery capacity / laptop's internal battery capacity * Omnicharge's conversion circuits efficiency
Take the Omni Mobile 12,800 and the 13'' MacBook air (2019 model), the Mobile 12,800 has a 46Wh capacity and the 13'' MacBook Air has a 49.9Wh internal battery; the Omnicharges conversion efficiency is around 90%. Put these numbers in the above equation and we get:
46Wh / 49.9Wh * 90% = 0.83
This means that the Omnicharge can charge the 13'' MacBook Air around 83%. This is assuming that the laptop is turned off or in sleep mode. Operating the laptop while charging will impact this calculation.
Remember that you might get significantly different results in real life. Let's say that your laptop battery is already fully charged, and you are only using the Omnicharge to extend the battery life. Since the laptop battery is already full, the power draw will be significantly lower, which also means that the Omnicharge will output less heat, and will, therefore, have higher efficiency.