From my research into building a hybrid narrowboat, my guess is that you either spend that £5k on an engine or a cocooned diesel generator. They will cost roughly the same, give or take. So my view is its best to have that diesel burner able to propel my boat as well as charge my battery bank (in addition to my solar). That's why I've decided to go parallel hybrid rather than full electric. I'll miss out on about £200/yr discount on my licence fee, but I think that's a small price to pay for peace of mind.