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Old 23-08-2010, 15:24   #1
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Help Me Size My Solar Array

I'm trying to size a new set of solar panels, and would love to hear what you guys are really getting out of your system. So here are the questions:

1. Where are you?
2. What's the size (wattage) of your array?
3. What do you really see in terms of watt-hours per day?

I've calculated my needs at approximately 120Ah/day without the fridge and 260Ah with the fridge.

I'm waffling between a 400W system and a 275W system. I'm guessing that I'll average 75% efficiency for about 8 hours - for 400W, that's 200Ah.

We'll be cruising in the tropics.

Is my guess of 75% for 8 hours in the ballpark?

I guess the secondary question would be how many think the fridge is necessary... without it, 275W is probably enough...

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Old 23-08-2010, 15:31   #2
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The 75% efficiency is a good starting point for daylight operation but you've got to add 15-20% efficiency loss for the battery system.

If you have the space I'd opt for the 400W system. If you had 2x 200W panels one could go down and you'd still be about even on recharging. It seems to me that many cruisers add more power requirements as they go along than take away. Having nearly fully charged batteries will prolong their life as well.

My solar panel experience has been in the US SE and Caribbean including the Bahamas.

Capt. Douglas Abbott
USCG/MCA IV/M.I./C.I. 500-ton Oceans
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Old 23-08-2010, 15:45   #3
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I've got 260W solar backed up by 200W wind, and find it sufficient to run both a fridge and a freezer (separate units). My system does not include a watermaker at the moment.

At latitude 38, during the summer months, the solar array alone consistently generates 90 AH per day. I don't get this type of performance during the winter months because my panels are fixed horizontally, forming something of a solar bimini, so they suffer at this latitude when the sun is at lower angles.

Regardless, I think your numbers might be too high. My own panels, which are new in the last year, are running about .65 efficiency.
cruising is entirely about showing up--in boat shoes.
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Old 23-08-2010, 16:15   #4
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260 AH with the fridge seems an awful lot. I have a fridge/freezer and usually use less than 100AH per day for everything. The fridge takes most of this. The CS insulation is pretty good, what are you using all those amps on? I'm just wondering because you'll be using half of a 600 amp bank in a day! How big is your bank ? I don't think a good fridge should use 140 amps daily, even in the tropics. Even if it runs 20 hours per day it'll be about 100 amps. And yes, for most of us a fridge is very necessary.
Rick I
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Old 23-08-2010, 16:29   #5
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my fridge uses 6A when it's on. i'm being conservative and figuring in the tropics, it'll be on all the time (24*6 = 144Ah right there). i'd love to be wrong about the duty cycle...

other big ticket items:

VHF: 2A * 24 = 48Ah
Autopilot: 2.5A (my guess on an average) * 24 = 60Ah
Computer: 4A * 5 = 20Ah

i could see us using pretty minimal power at anchor... all lights are LED's (including the anchor light) and we don't have a watermaker.

underway though, i can't imagine using a whole lot less than the 120Ah i mentioned, even without the fridge.

or maybe i'm just being too conservative?
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Old 23-08-2010, 16:34   #6
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just looked at the specs on the VHF - maybe 2A is high. i'll have to check next time i'm on the boat.
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Old 23-08-2010, 16:35   #7
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300 Watts here

I've got 2X150 24v panels paralleled through a MPPT 24v to 12v controller to a 375 AH AGM battery bank

I'm located in the tropics and I haven't had any issues yet. The fridge does run 20 hours a day during the summer months

I also supplement with an Ampair 100

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As I sit, a swirling sea of passion gives it's poems in waves underneath me.
The whispers of the sun in my eyes, a silence within.
Rhythm of the surf, drums of the sea. Thoughts tumble and toss about the deep blue abyss inside me, where the love of you dwells.
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Old 23-08-2010, 17:06   #8
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from practical experience i estimate that 260w would run my boat with little or no other source of battery charging. that would be 2 130 watt panels. i currently have one 130 watt panel and have to otherwise charge it about every three days. i have an adler barbour fridge which is the primary drain on the batteries (450 amp/hours) but even that only runs about half the time - i'm guessing about 60 amp hours a day. and this in florida and the bahamas.

don't expect the panels to supply 100 percent of your charging. the sun doesn't shine every day. my alternate means of charging are either the engine alternator or a honda 1000 powering a battery charger.
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Old 24-08-2010, 05:07   #9
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Solar panels should cover the space allotted. Install as many as you practically can.
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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Old 24-08-2010, 12:42   #10
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I think you need to spend some time with your battery monitor and see what the devices you are using ACTUALLY consume. If my VHF took 48 amp hours I think I would switch to a hand held unit and buy AA batteries... Go use your boat for some time and see what you are actually consuming in a 24-48 hour period. My fridge set up is not so great but it takes about 60 amps in a 24hr period on the chesapeake in high 90* temps. We run cabins lights, stereo, inverter for the coffee pot and bean grinder and computer, anchor light, electric flush head(s), FW pump, radar, and I normally wake up to about -80amp hours on the meter. Autopilot depends a lot on what sort of seaway, calm chesapeake its 8-10amps when running, rough offshore 15-18amps when running. I have improved things some with Alpenglow cabin lights, LED anchor light, only 1 pot of coffee. But I also dont leave the VHF on, I dont like hearing all that crap on the radio all day. Autopilot will likely be a big draw but you wont be using it sitting on the hook in paradise. I dont mind running the engine on passage some to offset the AP, its nice to make hot water and get a shower at sea.

We are setting our boat up for extended live aboard tropical cruising and I am hoping to afford (4) 135watt panels. With them I hope to make 180amps on a really good day. I am hoping can get my 24hr consumption to stay within 100amps when on the hook and I am using an 800ah bank.

Anyway my point here is that the approach of reading the manuals and estimating how much time you might use a device is not the best way. Far better to use the real world and gather some epirical data on YOUR usage before spending on a theoretical system.

Of course Gord is right too, put on as many of the damn things as you can afford...
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Old 24-08-2010, 13:00   #11
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I just brought a 170w panel from Budget marine.. they have a sale this month.... it cost 771 usd. I took it home sat it on top of the bimini and just hooked it up to the old wires form a previous panel through a regulator and the Xantrax showed 9.2 amps positive from the one panel. I think i'm getting three more... 4 x 9 = 36 amps per nice sunny hour.... maybe 5 or 6 a day 180 to 200 amps a day ( if i'm lucky ) .... = cold drinks..... and i'm thinking of a tanning bed in the salon to go with the dingy in cinema.........( similar to a drive in ....! ) ok well at least cold drinks and maybe a freezer?

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