a) there are powerbanks which have this feature - you should of course only use such ones in this context
b) a car battery or a so-called Powerstation would be a simple example 
c) @Rocketman has a special setup which is not comparable to a usual Ecowitt weather station installation - also, the WN90LP is not a LAN or WiFi device - he depicts its data on a non-Ecowitt console with some DIY setup and a WiFi signal extender for the RS485 communication which is used there
have you looked into the WS6210 option (see also our WiKi --> my signature) which is very much made for remote locations ?
admittedly more expensive than a GW1100
Thanks Gyvate.
Yes, I am familiar with the Ecowitt range. I am just picking potential ideas to avoid having to go with the WS6210 which is pricey indeed and takes all the fun out of a DIY setup 
Gyvate is correct . My WN90LP is all powered via battery and solar panel , I have a Hongyuv HY-Display console which polls at very high speed rate ( 2 seconds updates ) I also run a Greatcontrol isolated hub running 2 masters which I have one master connected to the HY Display console and another connected to a laptop running Weather Display and Cumulus MX .