Why do people want to spend money ?
When I am out which isn't often, I use my old car batteries from my Renault Scenic Diesel, when I get a new battery every three years I keep the old one, I have five now, and camping/scope/away I take three in the back of the car, along with a converter, they last a week including charging my phone/laptop/tablet, lighting the tent
During last winter, and we are in a small town/large village not the outback, they lit the house running low energy lights (20W = to 100W) in the lounge, bedrooms, etc for the full 12 hour power cut AND ran oil heaters
Cost ZERO
They have never ever run flat.
Oh I also have these and they are exceedingly good for 2-3 hours, but blasted expensive, just convenient
https://www.celestron.com/products/powertank-lithium
https://www.widescreen-centre.co.uk/...tery-pack.html
GREAT for phones/gopro etc
https://www.amazon.co.uk/Veho-Portab...gateway&sr=8-8