Don,
Knowing your boat is on a
mooring I will give you a run down of how I size for
mooring re-charge systems. If you want larger for "cruising" then you'd need to go larger.
For mooring
charging I size like this:
Ideally you want the
alternator, or
generator, to handle "bulk" and some absorption
charging and bring the bank back to 80-85% state of charge. From there you can let the
solar panel do the rest and bring the bank back to 100% SOC, when you're not there during the weekdays.
If you want to satisfy cruising needs such as
refrigeration, instruments, AP etc. then your array can get quite large. You did not specify so I will give an example of hos I size for a mooring re-charge.
The faster you can bring the bank from less than 100% to 100% the less sulfation you will have and as a result the longer
battery life you'll get.
I'll use an example of a bank of 300 Ah's for a "mooring recharge".
The last 20% of capacity of a 300Ah bank is 60 Ah's. However, you ideally need to take charge inefficiencies into account too so you'll really need to put back in about 70Ah's +/- to get back to that elusive 100% state of charge that boats on moorings need.
For moored boats the
panels are usually left flat when you're not there so that you can capture "most" of the sun. In a land based
solar array the
panels are fixed, the property is not moving like a boat does, and can be angled at the sun for the best performance. For this reason alone land based solar calculations rarely if ever apply or translate well to boats. On boats the panels are rarely oriented at the sun for optimum solar gains and the stick,
rigging and other appendages get in the way on swings or at different times of the day causing shading which can drastically limit array performance..
Having the ability to "aim" the panels at the sun, on a boat, is not usually a workable solution when swinging on a mooring, or at
anchor if cruising unless you are there or are attentive to it, so, panel position is very often a compromise and most leave them flat.
If you are at a
dock you can rig the panels and aim them more appropriately but not on a mooring, like you are, or off cruising where your boat will swing at will unless you pay really good attention or the
wind & tide always come from the same direction, not so with 10+ foot tides... If you might someday
wind up at a
dock then articulating panels can help the output.
Because of these aiming restrictions I find, after lots of monitoring here in the North East, you can figure on about 3.5 - 4.5 hours of full rated output per day on average. In the summer our insolation numbers are actually pretty good up here.
Some folks use 5 hours per day in the Northeast but after lots of monitoring of my own panels, and customers, I found that to be a little to generous. Some days it will be more some less but here in
Maine 3.5 - 4.5 hours at full output, is an average sizing number that seems to work best.
So, a 300 Ah bank @ 20% down = 60 Ah's that need to be returned + charge inefficiency = 70 Ah's total needed for a "full" bank. Note that I don't care what your load calculations are because you are doing this with no loads, when you;re not even there, other than some parasitic loads. if you were a live abord this would be different but I don't think you are as you live in Merrimack, NH which is about an hour to your boat..
A 2.5A output panel X 4.5 hours = roughly 12.5 Ah's/Day returned to the bank. If you have phantom/parasitic loads, like a
propane sniffer or other "always on" loads, subtract those and this is your "net" average/day.
Next divide 70 amp hours (or what ever your bank is) by 12.5 and you can see that it will take approximately 5.6 days to go from 80% SOC to full on a 300 Ah bank with a panel capable of 2.5A. This is in PERFECT conditions though.
I personally feel that's a little long, especially accounting for
weather, so would prefer to see a panel in the 3.5A minimum range on the example 300Ah bank. Bigger is always better but this becomes a "real estate",
budget and let's admit it, an aesthetic compromise with many boats and boat owners. A 3.5A panel shaves a full day off the time it takes to hit full when compared to a 2.5A panel, 4.5A faster yet and a 7.5A panel you're full in just about two days from where the
alternator stopped..
For cruising figure the maximum amount of time you want to run the
engine or gen set per day, or every other day, then base your array size on the difference needing to be made up while accounting for loads. If you only want to run your
engine for an hour per day, when you hit 50% SOC, and your alternator can get you from 50% to 70% in an hour then you have a 30% deficit to make up plus charge inefficiencies. You can size your panel to make up this deficiency if you know your average loads, which you do. There are always worst case and best case scenarios too and a best case is not always going to happen..
For your bank I would think a
single 140W
Kyocera, or other comparable panel, (AltE has great prices, are in Webster, MA and can ship one to
Maine UPS for $23.00 in one day at ground rates BTW) would be a good base to start with. Going with two 140W panels would be really nice but a
single 140W should top you up fine during the week. I just ordered two
Kyocera 140's last week from AltE and they were here the next day (with ground shipping) for short
money and it they are a premium panel..
It's all in what you want the system to do and you really did not specify, so I answered with what most boaters here in Maine want to do with mooring stored vessels, and that is get the banks back to full, within a few days, from about 80% SOC...
I'll pre-duck now to avoid Don's lecture on why my input may not have been appropriate to his original question...
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