I'd like to come back to this thread and update it. My 20kg "portable" battery+inverter solution remains valid, after a winter spent carrying the battery up and down the flat, you end up in spring with huge biceps or a displaced disk on your spine.
So i had a look again at
@WelshGas post, since he's been around Calis way longer than me, and I must admit I like his solution (solar panel) better. Got fed up of carrying the battery up and down the flat and an additional problem was that leaving the tailgate slightly open in winter in the evening, you get humidity in, an additional problem to address.
I will need to wash the Cali often at the petrol station in winter to get rid of the salt, and I didn't like the idea of puncturing the hose that contains the tailgate wires due to warranty.
So a fixed solar panel on the roof wasn't for me despite its many advantages.
I got a portable, foldable 120w panel, which i leave behind the windscreen, hooked to the front LB under the seat.
Welshgas was right, it does charge the LB even in winter and a much more practical solution.