I guess this has already been covered but need some advice.
I am going to sail from Lake Macquarie in October, solo.
My old boat has two batteries with a switch my plan was to run My house battery to power my auto pilot. I have been told that it should take around 6 to 7 hours? I will have power to start my engine when I switch over to my other battery but don't want to run my house battery flat. So Funds won't allow a wind vane so I want to know what size or watts solar panel I will need to run to keep my house battery charged when I run my auto pilot and gps.

To figure this out you need to calculate your power budget.

Start by measuring (or getting it from the data sheet as appropriate) the power draw of everything you want to have on. Radio, nav lights, auto pilot, etc, etc.

Then multiply this by the number of hours you want to run everything (so nav lights might only be 12 hours (overnight) while the radio would be on 24/7)
This gives you the your amp draw per 24 hours.

To last indefinitely you need to put this energy back into the battery in about 6 hours (you limit of useful sunlight).

So solar panels are specified in watts, you have calculated amp hours. To convert the the output of a solar panel to amp hours simply divide the wattage of the solar panel by the voltage output. This gives you the maximum best case amperage out of the panel and I would work on only getting 75% of that value.

At this point it's simply a case of making sure that 6 hours times solar amperage output is equal to the 24 hour amperage usage from the battery.

You are likely to find that indefinite running will require a surprisingly large amount of solar panels.

If you can cope with a limit on the number of days you can take a smaller number of solar panels. To work out how many days you can last do the calculations as above. Then take the deficit and subtract that from the capacity of battery. You can safely run your house battery down to 50% without damaging the battery.
The number of days you can last for is 50% of battery capacity divided by power deficit.

Hope that helps.

Moppo said..
Sorry yes from Lake Macquarie to port stevens.
st 4000+ auto pilot
2 x 150 amp batteries
As this is my first outside trip I will only be traveling during the daylight hours even if the winds aren't favoable I will motor.

Why are you planning on using the auto pilot the whole time?

Moppo said..
Sorry yes from Lake Macquarie to port stevens.
st 4000+ auto pilot
2 x 150 amp batteries
As this is my first outside trip I will only be traveling during the daylight hours even if the winds aren't favoable I will motor.

I have that set up and use my mate "Hans"[auto pilot] all the time as you will.

I have found that with a 100 watt solar panel you will have no problems.
I believe the new flexible panels are even more efficient that the ridged one I have [c2013].