Sizing my solar panel

[GHA]

As noted above, you probably don't need anything at all if you use the engine regularly but it is nice to have.

Bear in mind it takes a *loooong* time to actually get back to fully charged, 4 hours or more after voltage has reached absorption of high 14teens for most open lead acid. Which is where solar is a very good addition, plenty time with a low current source to actually get all the way back up. Engine alone no way can you guarantee getting back to full charge.

Spending time not fully charged must be the mostly likely cause of premature battery failure.

 

[[3889]]

FWIW, I've just removed the 100w panel I had fixed to the coachouse roof as it was a very inefficient location. For the last month I've used it only at anchor when it roams around the boat according to sun angle, and this has been far more effective at battery charging despite the reduced charging hours. It also precludes the need to route cabling through the deck.
On my last boat a 20w panel left on the cockpit sole kept 160Ah domestic bank fully charged over a Brittany winter. My boat-but-last with 100Ah battery recovered to full charge after a week in UK summer, using similar kit to the OP (minus AIS), even when I sailed onto my mooring with no engine use. That also had a 20w panel.
IMO, for a fixed panel to be effective they need to be on a bimini, sprayhood or arch. Anywhere on deck is too often in the shade.

 

[ghostlymoron]

"IMO, for a fixed panel to be effective they need to be on a bimini, sprayhood or arch. Anywhere on deck is too often in the shade."
I disagree, my 20w panel was mounted on the coach roof forward of the hatch garage and so subject to shading from mast and boom. The boat was on a swinging mooring so no control over horizontal orientation. Despite this my batteries were always fully charged by next weekend and stayed charged for the weekend sailing.

 

[Buck Turgidson]

I had a single semi flex 20w panel for similar sailing profile and it kept the battery well charged and returned to full charged in between visits to boat. However, times change and prices have fallen so I would now install 2 x 50w panels which would allow limited use of my cool box. Get a good pmw controller too.

Not sure why anyone would recommend a PMW controller when MPPT are so cheap these days.

 

[lw395]

Not sure why anyone would recommend a PMW controller when MPPT are so cheap these days.

It makes surprisingly little difference, particularly for a maintenance charger. But there seems to be very little available which I would call a 'good PWM controller'.

 

[Mistroma]

FWIW, I've just removed the 100w panel I had fixed to the coachouse roof as it was a very inefficient location. For the last month I've used it only at anchor when it roams around the boat according to sun angle, and this has been far more effective at battery charging despite the reduced charging hours. It also precludes the need to route cabling through the deck.
On my last boat a 20w panel left on the cockpit sole kept 160Ah domestic bank fully charged over a Brittany winter. My boat-but-last with 100Ah battery recovered to full charge after a week in UK summer, using similar kit to the OP (minus AIS), even when I sailed onto my mooring with no engine use. That also had a 20w panel.
IMO, for a fixed panel to be effective they need to be on a bimini, sprayhood or arch. Anywhere on deck is too often in the shade.

I have historical data for last 7 years and output from my 3 panels under each the boom has always been very good. I'm comparing actual and predicted (based on EU funded model). I can move panels if required but that only happens in odd situations. e.g. Anchored in a tidal river resulted in panels almost always shaded.

I had 2x40W and 1x65W in parallel, using a PWM controller (Marlec HRDi).

A single 100W panel wouldn't be the best option. I can believe output might not be great. Did you collect any data? It might have been worth trying 2-3 smaller panels.

 

[GHA]

Not sure why anyone would recommend a PMW controller when MPPT are so cheap these days.

Especially as you'll want something with float, which must blow out the dead cheap PWMs.

 

[ghostlymoron]

https://pwr.ie/products/victron-bluesolar-pwm
I say pwm because i believe mppt are only better for large installations.QUOTE=lw395;6574239]It makes surprisingly little difference, particularly for a maintenance charger. But there seems to be very little available which I would call a 'good PWM controller'.[/QUOTE]

 

[FairweatherDave]

Well I have gone for a 50W panel. It would have been easy to go up to 100W but my current amount of sailing does not justify it. I aspire to need a bigger panel but I would be deluding myself. For a change I have decided to be sensible and have my feet on the ground. My wife will be pleased I have actually stuck to my budget.

 

[yerffoeg]

I have a similar battery capacity and have been playing around with a solid 20w panel this year. Over the summer, when away from the boat, the voltage on the domestic got up to 12.7 and 13.0 on the start. But since I put in a dual MPPT controller instead of the PWM controller, the domestic is fully charged as well. However, I am doubtful that the 20 W panel will be sufficient over winter. I suspect that small draws on the battery overnight eg from the controller itself, VSR, automatic bilge pump etc need to be considered.

 

[ghostlymoron]

50w should produce 4a under ideal conditions. If that's not enough, you can fit another.

Well I have gone for a 50W panel. It would have been easy to go up to 100W but my current amount of sailing does not justify it. I aspire to need a bigger panel but I would be deluding myself. For a change I have decided to be sensible and have my feet on the ground. My wife will be pleased I have actually stuck to my budget.

 

[ashtead]

FWD are you able say which panel maker and controller you went for as I have been researching acquiring a small panel as a winter project but not really certain about the controllers and in particular if you start with a controller for 50w it can handle more panels? What's the wife's idea of the budget ?

 

[GHA]

https://pwr.ie/products/victron-bluesolar-pwm

A lot for the money, but looks like fixed 2 hour timer before switching to float. Maybe OK for a day onboard then a week away, no way will you get fully charged if you are onboard using the boat every day though, not enough time at absorption. Unless you can switch the regulator off and on again to reset the timer.

 

[Buck Turgidson]

https://pwr.ie/products/victron-bluesolar-pwm
I say pwm because i believe mppt are only better for large installations.QUOTE=lw395;6574239]It makes surprisingly little difference, particularly for a maintenance charger. But there seems to be very little available which I would call a 'good PWM controller'.

[/QUOTE]
From the same source
https://www.victronenergy.com/upload/documents/White-paper-Which-solar-charge-controller-PWM-or-MPPT.pdf

When I went through this process I ended up with 2 50W panels in series to a victron 75/15 MPPT.

High voltage low current from the cells which solves shading issues and keep area relatively low and an MPPT which is smart and capable of expansion should I wish to add more later.

 

[ghostlymoron]

Also MPP are much more expensive than PWM which was the clincher for me.

 

[GHA]

Also MPP are much more expensive than PWM which was the clincher for me.

Not having control over time to float is a major downside if you are using the boat much, the victron ones seem to be stuck at 2 hours - can yours be adjusted?
Just aboard for a day or 2 at a time then probably not much of an issue but longer than that regularly then the greater control over the output starts to make MPPT look more attractive.

 

[GHA]

So lots of "keeps my bank full' reply's - how do you know?

Not an easy thing to be accurate about.

 

[FairweatherDave]

I went to Photonic Universe and bought a kit for £160 ish. No doubt I could have got something cheaper but I have read many good reviews and met them at the London boats show a couple of years ago. I persuaded my wife on the basis it was just over a hundred.....
I'm no boat electrician and don't get the more technical stuff so the gist of forum wisdom really helps my decision making.

For Ashtead

https://www.photonicuniverse.com/en...roller--regulator-for-12V--24V-batteries.html

With that I am told I could add 2 further 50W panels

 

[lw395]

Not having control over time to float is a major downside if you are using the boat much, the victron ones seem to be stuck at 2 hours - can yours be adjusted?
Just aboard for a day or 2 at a time then probably not much of an issue but longer than that regularly then the greater control over the output starts to make MPPT look more attractive.

I thought the Victron things switched to float after 2 hours of the current being stable at 14.5V or whatever?
So the 2 hours is a minimum?

Once the power requirement is low enough that a PWM controller is getting the battery to voltage regulation, then the advantage in MPPT is lost. When you have excess power and you have to regulate it, then extra efficiency doesn't buy you anything.

I think there is confusion because there are many different Victron products around, and a whole bunch of other makers. Then there are cheap 'unbranded' versions. Some of which are good, some of which are not.

There's no shortage of ways to spend money to get a 'better' electrical system, that's for sure.

 

[lw395]

So lots of "keeps my bank full' reply's - how do you know?

Not an easy thing to be accurate about.

12.7 Volts at dawn is as good as it gets really.
Or seeing the float current drop to a low stable value. But that varies from one battery to another.
In my workshop, a battery is fully charged when Mr CTEK says so.

But most people are not so worried about whther their batteries are 100% full, they're happy if the starter spins strongly and their lights, fridge etc run for a good long time before the volts fall away.
People can tell soon enough when their batteries were no sufficiently charged.