Oh I wasn't necessarily saying its not an issue. I've heard of plenty of people who have found their battery dead... several reasons why that happens:
- Jammed float switch keeps pump on all the time (No solar system will compensate for that!)
- Lots of rain. See below - you need to know if full of water = damage or inconvenience
- Spray / waves. I think is significant
- Frequency of boat use. Can you get by with topping up a bit if not fully
I took a figure from the metoffice website but I can't find the page right now. Re-searching comes up with a different figure on their site! Wiki quotes different data still which is MASSIVELY higher.
Climate of the United Kingdom - Wikipedia, the free encyclopedia - some of the regular posters on here will be chaulking up 4,500mm in a year.
So 9 times the figure I quoted and I didn't point out the obvious that Poly did that you don't let the boat fill with a year's water and pump it out. So you don't get the efficiency either. But annually that might mean 36hours of running assuming we are shipping no spray. While thats an average of 3 hours a month not all months are average. Its also not unknown to get "freak" weather events where a months rain falls in a day. You'd need to decide what you want to happen in those situations. Will the boat be damaged by being half full of water?
The Met Office provide monthly data on a map and "helpfully" list a lot of places in winter with >250mm /month, and some with >800mm / quarter.
UK climate information - Met Office
I think realistically you might want to assume 500mm per bad month was a possibility - so 8Ah in a bad month from Rain. That will be the same month you don't visit the boat cause the weather is cr@p and the light is poorest so charging is reduced. 8Ah would still be fine. Doubling or even tripping that for inefficiency
would be fine. Doubling that again for spray might be stretching it.
So the other question is what size solar panel were you thinking? If its something permanently attached to A Frame or console then:
- Position not optimal for sun all year round - assume only 50% efficiency (See:
Optimum Tilt of Solar Panels) - but may be far worse than this if can't be mounted well even for best winter angle.
- Size is very limiting - 1.5W might be about the right size
In the heaviest rain months (winter) the light will be poorest. Maybe 6-8 hours of decent light and will be some very dull days. The 1.5W is perfect position, with bright sun. Advice I've seen seems to be to assume on a dull day its 50%. So bad position plus bad light takes you to 25% of 1.5W. So 375mW x 6 Hrs = 2.25W/Hr = 187mAh. Over 30 days that would add 5.6Ah of charge to the battery. Over the same period you would be drawing something like 24Ah if its heavy rain, not efficient because of on-off on pump and shipping spray. So you get a net loss of 18Ah from the battery. In a single month thats probably OK - but would question if a loss of 24Ah wasn't also OK. In two months thats 36Ah which is taking a 70Ah battery towards the "discharged" point. Without the solar panel you get there two weeks earlier.
Thats all conjecture. But I doubt 6 weeks vs 8 weeks matters...?
To really know you need to fill your boat to the point the float kicks in and measure the power consumption till the float switches off. Then calculate rain and spray required to activate that and how many times etc, and what power you need to replace that and therefore what solar panel to look at.
Anyway - My message was not so much meant to mean it wont draw that much power rather that rain might not be the biggest issue so calculating for rain rather than spray might not make sense...
