Hi @uuqqawst and welcome to the community.
Typed on a Chromebook
So there are fours dimensions to think about.
1) The battery life of your Chromebook. This is typically very good, 8-10 hours is common if not hammering media and apps. Good to know your Chromebook's internal battery size in Wh and how many hours you expect to get, to then be the basis of extrapolation.
2) Wattage. I will use an example of the Asus Chromebook C302. So say you bought a Chromebook with say 39Wh battery and it affords 8 - 10 hours means its battery is draining about 4W - 5W per hour. Remember that is it important.
3) External battery powerbank (your ask). So a Powerbank which is not Power Delivery will output about 10W-12W. SO compare to the 4-5W above, so your typical Powerbank will output at about double the consumption needs of power of your Chromebook. So you do not actually need 30W Power Delivery Powerbanks at all!
4) Watt-hours. Also known as Energy. A smaller 20000mAh Powercore holds about 74Wh of energy, about 30% is lost in conversions so your example Chromebook above is going to see about 1.3x recharges. IF you got 8-10 hours off the Chromebook's battery therefore you're adding 10-13 hours of recharge off this example Powercore to get 18-23 hours of total Chromebook time.
SO here are your options:
A) "reactive". You sit with a Chromebook and use it for hours til "oh what a surprise the battery is flat", it will be discharging and needing 4-5W and if you plug in a slower 10-12W Powercore it will slow the discharge and slowly recharge.
B) "proactive". You know you are about to enter a long Chromebook usage period, so you plug in your Powercore early, then that 4-5W demand and the 10-12W input is able to keep your Chromebook's discharge around nil so you begin with a charged Chromebook and you ended with a charged Chromebook.
Where you need to be sensitive to:
- being dumb. Do not react. Plan ahead.
- do not consider Wattage, "speed" as avoiding being dumb means most lower cost technology is perfectly fine
- the precise combination of your Chromebook and the Powercore you are considering because they do need to negotiate a common power which is not too slow. So ask in forums. Check, find someone who has a specific combination you seek.