I should have said output voltage but If you can find a battery bank that has at least an output of 15V you should be able to charge.input voltage would be the minimum required for your power adapter to be able to work. 100V isn’t something you’ll find on a battery bank so you’ll have to use a battery bank that has a cable for the Surface.
Hereis one that has the actual charger designed for the Surface line.
BatPower ProE 2 ES7B Portable Charger External Battery Power Bank for Surface Laptop, Book, Book 2, Surface Pro 6 5 4 3 2 RT or Surface Go, USB QC 3.0 Fast Charging for Tablet or Smartphone -98Wh https://www.amazon.com/dp/B076P4P93T/ref=cm_sw_r_cp_apa_i_ksC4CbS3WXN79
I do recommend this as I do have one, this can replace your original charger as well. Although it's 100$ it is worth the investment!
I've run/charged my SP3 from a portable power bank with no issues. If you have any battery time concerns, this is a solution. There are a number of options on Amazon, like
BatPower ProE 2 ES7 Portable Charger External Battery Power Bank
with Surface connectors.
this one its great, designed really with the SP line mind. 98wH
https://www.amazon.com/gp/product/B076P4P93T/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
this one is built specifically for it. I'm going to get some velcro t to attach it to the kickstand on the back of my surface and then wire manage it.
Because it's the simplest solution, AFAIK.
The other solution would be something like this ( https://smile.amazon.com/dp/B076P4P93T/ref=cm_sw_em_r_mt_dp_U_0sWqDbNE10RGA) which I'm even less fond of.
If the SP6 had USB-C, like Go, then you can charge it directly with a C-to-C cable.
Cheers.
Weight is going to be your main issue here. Your dimensions are also fucking wild, you can get a generator that size!!
They do have portable power banks:
https://www.amazon.co.uk/BatPower-ES7B-Portable-External-Smartphone-26K-Black/dp/B076P4P93T
Or you could just get a portable mains charger:
https://www.amazon.co.uk/gp/aw/d/B07CKSPVM8/ref=sspa_mw_detail_0?ie=UTF8&psc=1