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Old 18-04-2019, 12:16   #31
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Re: How Many Watts of Solar?

We started the season with 3 flexible 80 watt panels. That was barely enough to keep up with our cold-plate refrigerator/freezer system. Then a hurricane-force squall tore away the flex panels. We finished the season with an old household 80 watt panel. -Some solar is better than no solar.
Since we have the room with no shadows on our davit arch, we will install 3 mono 175 watt panels before next season.
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Old 18-04-2019, 12:25   #32
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Re: How Many Watts of Solar?

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Originally Posted by sailorboy1 View Post
WOW!

I doubt my bilge alarm has used 1 AH in its whole life (I test it once in a while)
Your correct...my error, garbage in garbage out.

In addition, I found a good template to use at Pacific Cup 2012 Sample Energy Budget.

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Old 18-04-2019, 13:02   #33
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Re: How Many Watts of Solar?

Do not forget shading. I have three panels split by the boom so one panel is almost always shaded, (except at high noon) therefore I get less than half the 390 watts installed. If these were installed over the davits, clear of the boom, this would not be an issue.
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Old 18-04-2019, 15:37   #34
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Re: How Many Watts of Solar?

Motion30 gets my full tick.
As much as you can comfortably install with multiple regulators and a battery bank to match.
Leprena has 840 watts through 3 regulators into 1,000 a/h of VRLA batteries, the regulators cycle on most days.
Good budgeting of power use will allow you to survive happily. If you can can run the heavy users ( water making, cooking preserves, water heating, hand tools ) to suit power production, you can expect to have power available for the more important functions most of the time.
I apologise if your cash budget looms.
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Old 18-04-2019, 16:42   #35
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Re: How Many Watts of Solar?

Agree with the many other posters who have fleshed out the charging system. Max out your solar, but also carry other charging sources, including a good main engine alternator.

Our electrical system consists of a 320 ah battery bank. This is on the small side, but our demands are modest. Still, I will be increasing our capacity at the next battery-replacement cycle.

Our charging systems consist of:
  • 3-stage AC (shore power) charger for when we’re at the dock.
  • 65 amp alternator attached to the main diesel.
  • 1000 watt portable gas generator (Yamaha).
  • 400 watts of solar run through a MPPT controller.
  • 400 watt wind generator (Silentwind) run through its own controller.
These final two generators produce the vast majority of our amp needs when off the dock. And in fact, solar probably accounts for ~85% of our needs. But wind is a nice compliment, and often excels during the times when solar suffers.

So yes, a diverse portfolio of charging systems is the best plan for a cruiser who wants to limit the amount dino-juice charging.
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Old 18-04-2019, 17:16   #36
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Re: How Many Watts of Solar?

Two schools of thought I see and read about:
1. A careful energy budget down to two decimal places for amp hours.
2. Put up as much solar as you can afford (space and dollars).

Problem with item 1. You can only estimate your actual usage after using a similar system for a while, if you're new or the system is getting a makeover, it's hard to tell exact numbers. But we try. Then you fudge factor another 20-50% just in case. Too much guessing.

I recommend option 2. If you get enough, you can skip the wind generator. As to people not hating their wind generators...
I can't stand my wind gen...
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Old 18-04-2019, 17:47   #37
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Re: How Many Watts of Solar?

I skipped many of the answers above to try to give you a totally different perspective. The simple answer to you question is “More than you can fit.” But think about this.

We are in the Caribbean. We have wind and solar. Because we are in the trade winds the boat lays E-W, give or take, all the time. We are here in the winter seasons when the sun is in the South. Our wind generator is mounted on the Starbord side of the arch, thus it shades our starboard panel a lot. IF we were here in the summer months this would be less of an issue. IF we were in the prevailing westerlies, northern hemisphere, it would be less of an issue.

The point is that how many watts of solar can be less important than how they are placed, and that depends on your location and usage pattern.

It gets worse.

With our boat on a dock in Delaware the wind generator was about useless. But here in the trades it puts out a reliable, if modest, output 24/7. The wind blows even when the sun don’t shine. But then there is that shading issue. Dang!

Now if you have 12VDC panels, that is panels inherently designed for 12VDC batteries, then you can go with a simple controller. Ours charges our batteries to 14.2 volts and maintains them there. IF you go to higher wattage panels, then you need a MPPT controller that converts the higher panel voltage (lower current) to a lower charge voltage (higher current.). i find it useful to think of the panels as constant current sources, our 310 Watt panels will put out something like 60V peak voltage at 8 amps. If I hook them to a lead acid battery it will drag the voltage down to about 14.2 volts but still at 8 amps. In real life I can get about 2/3 of the rated power, best conditions. My controller reports that the best I get is about 215watts out of a 310 watt rated panel with a MPPT controller. Or 110W with a normal controller. In each case the panel is putting out about 8A. So there is an argument for more smaller “natural” 12vdc panels over “bigger” panels. There are advantages and disadvantages to each that are too expansive to go into here. My apologies if I’m confusing.

The bottom line is that there are additional considerations above the ones mention in the above posts. It all comes down to your particulars; how, when, and where you use the boat. And how the boat is configured. The only pure given, and there are caveats to that, is that you should generally try to reduce electric use as much as possible. But a few years ago one very stingy and obsessive poster figured out that, for him, a microwave cost less than using the cooker because he had a big well serviced electric system that was essentially free while propane cost bucks.

Great luck with your quest.
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Old 19-04-2019, 00:38   #38
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Re: How Many Watts of Solar?

Try to get to at least 300 watts. But one mistake many make is to not have a big enough storage battery bank. On my 40ft mono hull I have 6 235amp hr 6vt batteries and one group 27 deep cycle 12vt for the engine. All the solar in the world won’t do you any good if you have no place to store the energy.
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Old 19-04-2019, 01:06   #39
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Re: How Many Watts of Solar?

I have 2 x 240 W Kyoceras marine panels above bimini and have had them since 2012. I have 38 foot boat. Given Lake Ontario I sail 6 months of the year max. During that 6 months most days are daylight more than 12 hours. I leave solar power on during winter storage. Keeps batteries powered and also dehumidifies boat with controller fan.
So I have fridge, freezer, TV, all electronics incl radar, AIS. chartplotter, auto pilot, etc. All lighting on boat is LED. Fridge and freezer are high efficiency. I do not have microwave though. My hot water comes from engine cooling. So I have adequate electricity with my panels and on a sunny day am fully charged by 900am. Obviously electronics are mostly used during day when there is adequate sunshine. This set up works fine for me and I never plug in except if in a port I do not reside at and the plug in is free. Just my two cents worth.
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Old 19-04-2019, 09:02   #40
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Re: How Many Watts of Solar?

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Try to get to at least 300 watts. But one mistake many make is to not have a big enough storage battery bank. On my 40ft mono hull I have 6 235amp hr 6vt batteries and one group 27 deep cycle 12vt for the engine. All the solar in the world won’t do you any good if you have no place to store the energy.
That sort of ratio is fine if you regularly get input from sources besides the panels.

Or if you use a good SoC meter to ensure you don't take out as much as solar replaces on average.

Need to get back to 100% full very frequently for longevity, and with a solar-only setup with only 300W panels (~100Ah per day), draining a 600+Ah bank over 50% is a deep hole to climb out of.
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Old 19-04-2019, 09:08   #41
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Re: How Many Watts of Solar?

It would be **very** rare for solar to get a bank back to true 100% Full before noon, unless LFP chemistry.

True 100% Full is what's required for longevity, can't take the controller dropping to Float as meaning anything,

needs to be verified with an ammeter, adjusting the controller AHT until current tapering below .005C or whatever your bank mfg specs for endAmps.

Most lead banks take 6+ hours, even if the source puts out more current than the maximum amps accepted by the bank.

So by 9am? Only in latitudes where the sunrise is before 2am. . .
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Old 19-04-2019, 10:58   #42
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Re: How Many Watts of Solar?

It was such a basic simple question the OP asked considering how many people cruise and know their power needs. So I’m amazed at how all the “experts” have worked to make it hard and confusing.

For a 36’ or there about you need at least 100ah of usable battery capacity and enough solar for 150ah a day in normal use. Anything more is a plus, but there’s no reason to turn your boat into a power plant.

Now I could be wrong. But My wife and I been cruising full time for 2.5 hours. The first 2 years with 440ah of batteries and 290w solar. We rarely went below 75% state of charge, but were power short 50ah/day that was easy to make up with engine/generator. Now we have 640w solar and will be fully charged by around 2-3pm and our solar charger says solar is supplying 150-170ah/day.

We run a larger freezer and refrigerator, watch 2-4 hours of video/ streaming a
day, listen to music, play on our computer, charge our phones and tablets etc.
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Old 19-04-2019, 11:12   #43
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Re: How Many Watts of Solar?

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It was such a basic simple question the OP asked considering how many people cruise and know their power needs. So I’m amazed at how all the “experts” have worked to make it hard and confusing.

For a 36’ or there about you need at least 100ah of usable battery capacity and enough solar for 150ah a day in normal use. Anything more is a plus, but there’s no reason to turn your boat into a power plant.

Now I could be wrong. But My wife and I been cruising full time for 2.5 hours. The first 2 years with 440ah of batteries and 290w solar. We rarely went below 75% state of charge, but were power short 50ah/day that was easy to make up with engine/generator. Now we have 640w solar and will be fully charged by around 2-3pm and our solar charger says solar is supplying 150-170ah/day.

We run a larger freezer and refrigerator, watch 2-4 hours of video/ streaming a
day, listen to music, play on our computer, charge our phones and tablets etc.
Well with 2 and half hours of cruising experience you clearly have more cruising experience than some self professed experts.
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Old 19-04-2019, 11:25   #44
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Re: How Many Watts of Solar?

The way I did this was to simply start using solar.

I bought a 65 watt panel for $76.00 and a PWM controller for $12.50 and used that for a couple years then went from there

Most boats already have an alternator so you can get by but my boat did not.

It had a pull start outboard with no alternator.

Now I have 135 watts of solar panels, 3 PWM Controllers, and 1 Victron 75/15 MPPT Controller

I'm using both an MPPT Controller and a PWM controller with the panels at this time.

The next change will be 6 volt batteries close to 200 ah each. I have two 12 volt 80 ah batteries in parallel now.

Solar systems are rather fun to play around especially when at anchor when there isn't a lot going on

I have no fridge or windlass. I guess the lights and the inverter are the biggest power draw

I used to run a 120 volt fan all night on the inverter, but now I have bought a much more efficient 12 volt fan so I'll probably only need 100 watts or so of solar panels to keep my batteries charged

The inverters will now just be for power tools and for charging my phone, and laptop
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Old 19-04-2019, 11:36   #45
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Re: How Many Watts of Solar?

Ok there has been so much posted its really getting confusing for some.

Here is my personal opinion ( as an installer)
first you need an idea as to how much power you will use . Size battery bank accordingly ( for this you want to plan about three times your expected usage to allow for days when solar is not good)

( there have been a couple of sites with budget prep papers posted Already)
next imo you will want about a watt per amphour of battery capacity for charging.
This equals to be about 100 watts per 100 ah capacity. (50 ah usable power for Fla for longevity)
this is if solar is your only charge source.
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