About 12 years ago, we got a big order for 16 panels for Oxford area, from Butts we got a 30% discount so my 75watt panels came out at around £270 each. I bought 2 then and already had 2 x 70 watt panels, I think the deal still exists, the Pete Nash deal of 30% discount. anyway about 5 years ago, my engine packed up, it was Christmas time. I lasted right through to the spring with only the solar panels and regulator for my power. even I was quite amazed, had to put the colour tv away and get out the black and white and made sure I did not have lights on when not needed, (I don't anyway) used a portable radio, probably the most power thirsty thing I used was the pumps. So your right Ernie, as I proved myself, mind you some people have them fitted flat on the roof. This is only any good here in the uk during the middle of the summer, mine are always angled towards the sun and during the winter they are angled quite high, to get as much sun onto them as possible.
Pete was trying to design a frame that would move to follow the sun during the day etc but don't think he ever got round to it.
I am seriously thinking of adding more panels especially with diesel prices as they are now but as Chris says With all my power thirsty things I have on my boat now, I would probably have to cover half the roof with them (expensive) Some people can get a grant for them up to about £2500 for a house installation, so may be possible for a boat.