Matching solar panels to batteries to consumption

[Sauntress]

This question concerns an engineless wooden yacht of modest proportions (28' x 8' x 4'10") sailed fairly adventurously (meaning long distances) with minimal electrical power requirements. Specifically I would appreciate advice on the solar power needed where

1. The vessel is now fitted with two new MK50-12-SLD M batteries on which are written

12V-42.5AH/5HR
12V-50 AH/20HR

Which appears to mean 50 amp hours each? They have to run, all night, every night, the following

Masthead 25 watt 12 volt lamp (giving 2.1 amps?)
Compass light 0.1 amp
And very occasionally a cabin lamp, chart table lamp or galley lamp each at 12V x 10 watt (or 0.8 amps?) for a few minutes at a time (night lighting aboard Sauntres is by oil lamps)

The question is what solar panel set up do I need to keep that lot going indefinitely on passage. (I am replacing here, a 30 years old Unisolar MBC262 rated to produce 15 to 29 amp hours PER WEEK, but measures 15" by 26" which is a very useful size.
Thank you

[Vasco]

I know this isn't the question but the first thing I'd do is swap out the 25w bulb for an LED.

[newhaul]

OK here goes if you were a customer of mine I would recommend changing the lights to led which will reduce yur power needs by 75% to 85% . Then i would recomend upgrading to a 50 watt panel and controller . The panel you have now only produces 11watts .
This is basicly what I would install .
Renogy 50 Watt 12 Volt Monocrystalline Solar Bundle Kit | eBay

[Sauntress]

That is most helpful. I have looked at the product details and the one thing I cannot find is the dimensions?

[TeddyDiver]

So you use some 25Ah (*12v = 300wh) a day. For the batteries it's reasonable 72% DOD (10h rate~about 46Ah/battery) at the day break. To get that back during the day light hours I'd like to have at least 100w panel, maybe a bit more to be on the safe side when it's a cloudy..

Ps. Didn't actually count anything just aproximated the figures

[newhaul]

Originally Posted by Sauntress

That is most helpful. I have looked at the product details and the one thing I cannot find is the dimensions?

Aprox 21in by 24in by

[leftbrainstuff]

Simple math to start then you need to validate.

Some conservative assumptions:

1) useful battery capacity is 50 to 80% charged
2) it could take 2 or more days to recover charge
3) shading on a sailboat is problematic. Assume only half your panels are effective.

It appears that one of your batts is not deep cycle. The 5hr rating. Is that a UPS battery?

It's also prudent to reduce consumption first. If you repalce the mast head light you can reduce the consumption markedly.

You also did not add any budget for charging phones, operating radios, etc. Always good to be ultra conservative.

[Sauntress]

The batteries are identical bought at the same time from the same supplier. All I did was copy down exactly what is written on the side of the battery which I then incorporated in the post. I was puzzled by 12V-42.5AH/5R and then, below it the 12V-50 AH/20HR, by I am no electrician. What I do know is that they were very heavy for their size (which I believe is good) and not exactly cheap, for which read expensive. (If you go to the manufacturer's site you will see an illustration of the battery precisely as I have described it).

LED or not? A previous post said replace. I have seen comments on this forum to the effect that LED tricolours are, for some reason, bad news because not compliant with the regulations. Mabe that is out of date. An initial look around for a replacement tricolour masthead LED fitting produced some rather eye watering figures (in the hundreds of pounds). Can that be right?

Thank you for your interest.

[newhaul]

The diferent numbers on your batteries refer to the 5 hour discharge rate and the 20 hour rate so together you have 100ah which actually means 50ah usa le ( not.good on batteries to.take below 50percent dod )
As far as the price.of the tricolor led lights afraid so that is about right .

[leftbrainstuff]

Simple math to start then you need to validate.

Some conservative assumptions:

1) useful battery capacity is 50 to 80% charged. Assume only 1 of your 2 batts is available for house loads.
2) it could take 2 or more days to recover charge
3) shading on a sailboat is problematic. Assume only half your panels are effective.

It appears that one of your batts is not deep cycle. The 5hr rating. Is that a UPS battery?

It's also prudent to reduce consumption first. If you repalce the mast head light you can reduce the consumption markedly.

You also did not add any budget for charging phones, operating radios, etc. Always good to be ultra conservative.

I'll assume you're not interested in crunching the numbers yourself. There are lots of online calculators that you can use.

Lets say your consumption is 3 Amp hours for 10 hours per night. That equates to 60 Amp hours for 2 days or 20 hours.

If your panel is half effective and you can only charge every second day then as per https://www.batterystuff.com/kb/tool...alculator.html 2 x 100W panels would meet your needs. This is a very conservative calculation.

Less conservatively you could have 1 x 100W panel if you charge everyday. I overestimated your continuous load too. You might even get away with a 50W panel.

[Copacabana]

I can't imagine replacing your tricolour bulbs with LED bulbs costing "hundreds of pounds". Perhaps that's the price for a new nav light unit? I made LED replacement bulbs for my Aquasignal nav lights (masthead tricolour and pulpit ones) that cost a fraction of the LED replacement bulbs and they are much brighter. Just remember that you have to use red, green and white LEDs for your nav lights (a white LED behind a red lense will put out almost no light, but a red LED behind the same lens does). From your description of your consumption and your boat, I would try to fit 2 cheap 100w panels on the boat and use a decent PWM controller (cheap and sufficient for your needs). This way you'll be able to comfortably keep up with your power needs.

[thomm225]

Originally Posted by newhaul

OK here goes if you were a customer of mine I would recommend changing the lights to led which will reduce yur power needs by 75% to 85% . Then i would recomend upgrading to a 50 watt panel and controller . The panel you have now only produces 11watts .
This is basicly what I would install .
Renogy 50 Watt 12 Volt Monocrystalline Solar Bundle Kit | eBay

Mine is similar to this but all bought separately.

100 watt solar panel of similar size, two $12.00 controllers (I bought a spare). I also have to 20 watt panels but have yet to install

This panel charges a starter battery (left over from when the boat had a diesel) and a deep cycle battery in parallel.

Power needs are autopilot, GPS, VHF, depth, lights (when necessary only), bilge pump, inverter for laptop, phone, fan, and one fluorescent light

Most everything is off during the day for charging. If sailing, I'll run autopilot only if batteries are low

[TeddyDiver]

Originally Posted by Sauntress

The batteries are identical bought at the same time from the same supplier. All I did was copy down exactly what is written on the side of the battery which I then incorporated in the post. I was puzzled by 12V-42.5AH/5R and then, below it the 12V-50 AH/20HR, by I am no electrician. What I do know is that they were very heavy for their size (which I believe is good) and not exactly cheap, for which read expensive. (If you go to the manufacturer's site you will see an illustration of the battery precisely as I have described it).

LED or not? A previous post said replace. I have seen comments on this forum to the effect that LED tricolours are, for some reason, bad news because not compliant with the regulations. Mabe that is out of date. An initial look around for a replacement tricolour masthead LED fitting produced some rather eye watering figures (in the hundreds of pounds). Can that be right?

Thank you for your interest.

The numbers mean how much capacity you have with different discharge rates. You got 5 and hour 20 hours rates with their respective capasities (42.5 and 50Ah). Your comsumption is during night time, maybe some 10 hours which is somewhere between those figures given on your batteries, so quessing 46Ah /10hr wont be too far from the thruth.

Replacement LED bulbs for tricolours designed for conventional bulbs are generally non compliant as you say so you have to figure out to either buy more solar or new LED and less solar..

BR Teddy

[newhaul]

Originally Posted by TeddyDiver

Replacement LED bulbs for tricolours designed for conventional bulbs are generally non compliant as you say so you have to figure out to either buy more solar or new LED and less solar..

BR Teddy

I agree with you on the tri color lights

Do what you have to to ensure you are legal / safe. ( rules like this are written in blood)
As far as the go led or go more solar I disagree . It is better to go with more solar and convert to led to reduce power usage. ( the op currently only has 11watts of solar.) .
I have never had a customer complain they had to much power available.

[Suijin]

Since you don't have an auxiliary for charging, I would really err on the side of conservatism when sizing your solar panel(s). I would definitely go with at least 100w total, regardless of whether you find ways of reducing your electrical needs.