I decided to go for a low price to err on the side of caution and to avoid forum know-it-alls arguing my point but saying they can get diesel cheaper. At 90p a litre my point is even stronger. Also many boats drink more than 1 litre an hour.
Basically, there's a lot of tosh talked on this forum about battery charging and the received wisdom seems to be to charge for 2-3 hours a day 6 days a week and then charge for about 6 hours on a sunday. Even if you have solar for say 7 months of the year, that's about 20 hours a week for 20 weeks through the winter. That's 400 hours of battery charging a year (assuming no summer equalisation charges etc). At a guess I'd reckon about 200 of those hours aren't really to provide power as such, they're to keep your batteries healthy. 200 hours of battery charging is gonna cost (at 90p a litre and 1.2litres an hour) £216.
So you could either spend £216 annually to try and keep your batteries good (and enjoy lots of extra noise, fumes, vibration, engine wear etc.), or you could spend £240 on a set of 3 batteries and, if they last longer than a year, have a little party to celebrate your good sense. Link: https://www.limekilnchandlers.co.uk/3-x-110amp-leisure-batteries-collect-only.html
I'm sure there's some sense in carefully cossetting your batteries if you've just spent £2000 on 2v traction cells. But for the other 99%, it's probably a waste of money even trying. And when the red diesel goes, even more so.
Now just let me nip out for some popcorn.