New solar installation

[scruff]

I have a couple of jobs for the winter, one of which is to install solar power to our boat which is based near to the Oban area and has 2x. 110ah batteries - one engine, one domestic

I would prefer not to install a stern gantry / arch for this and we do not have davits to place the solar panels across. I am currently swaying towards mounting them on the guardrails where dodgers would be. These would be left in the vertical when boat is left on a swinging mooring, but has the potential to placed in the horizontal when we are on board to increase output.

Solar panels are currently very cheap but the question I'm pondering is how many watts do we want installed? The common question is to work out the loads needed, but that leads to the second job which is the upgrade of the cool box to a fridge and the third job of installing an electric autopilot so there is quite a big unknown in power demand.

Boat usage is being left on a swinging mooring and used every second weekend with a couple of fortnight holidays...

I gather to ensure batteries are brimming on arrival at the boat, a comparatively small panel will be needed. However I want to be able to run the fridge full time when we are aboard and eat fresh food and less tins. Would 100w each side of the guardrail combined to a quality (mppt?) controller ensure I have fully charged batteries, cope with my power usage and don't burn the boat to the waterline? Then of course do I look to install a second domestic battery (so 2x110ah domestic, 1x110 engine)

Cheers,

 

[lw395]

What fridge are you looking at?
That is the big user, how big a box, how well insulated, how cold? it makes a huge impact on daily demand.

 

[scruff]

Looking at Isotherm classic kit from force 4 so 70-80w. As to how well insulated it is at present, pass - where it is located is very well built in. I'll be looking at adding some more insulation but I'm currently a 3hr drive away from the boat. I would prefer to buy the panels / mounting kit & fridge kit before I go to the boat next...

 

[Graham376]

Our fridge (Isotherm BD50F) draws 7 amps and is on 24/7. We just managed with 200w in Algarve sun, living aboard.

 

[scruff]

And am I correct in thinking that you cant have "too much" solar and that once batteries are full, the panels will stop producing current? if so where does / how does the controller dump the excess power?

 

[Graham376]

And am I correct in thinking that you cant have "too much" solar and that once batteries are full, the panels will stop producing current? if so where does / how does the controller dump the excess power?

Regulator doesn't dump power, it restricts it. Can't have too much solar and if your two 110AH are leisure, yes it's worth paralleling them but fit a car battery for starting, cheaper and better suited for the job.

 

[Mistroma]

And am I correct in thinking that you cant have "too much" solar and that once batteries are full, the panels will stop producing current? if so where does / how does the controller dump the excess power?

Yes, the controller will simply stop collecting the excess solar energy.

You can expect to get the following Ah/day from a 100W lying flat on deck in the Oban area with no shading. The figures are for an average year and can be improved if you adjust the panels to maintain an optimum angle to the sun.

	Ah/Day
April	18​
May	23​
June	24​
July	22​
August	17​
September	12​
October	6.5​

I'd say that you are light on domestic battery capacity to run a fridge constantly. You don't really want to run a 100Ah leisure battery down too much and you don't want to take out more than 50Ah.




Many fridges use around 5-6A and the duty cycle could be 30% -40% on with a standard system in a Scottish summer. That puts you in the region of 40-60Ah each day just for the fridge and is close to the maximum you'd want to take. On the bright side, solar will counter this usage during the day and that means you might only be taking out 24-40Ah/day outwith charging hours. But this only works if you start with fully a charged battery.

The table above shows you can expect 24Ah/day from a 100W panel in June and 200W would therefore produce around 48Ah/day. It isn't as simple as saying "I use 48Ah/day and therefore need to fit 200W panels to keep the battery fully charged".

1) Flooded lead acid batteries need a lot more Ah in than Ah out as charging efficiency isn't great above 85% State of Charge and gets very much worse as you approach full charge. Not helpful.

2) Solar panels will put Amps. directly to the fridge (or other loads) during the day. Efficiency is high and only drops for any excess going into the battery. This is a good thing.

A second domestic battery would be a very good idea. I'd guess 200W would be a reasonable target and 100W-150W would still let you run the fridge as long as you have the engine for a bit on each day. Things become trickier if you intend to anchor in one place for a week and get a long spell of cloudy weather. The figures I've quoted assume the normal mix of weather for the Oban area (i.e. cloud and rain, perhaps some snow ).

 

[NormanS]

We are also based in the Oban area. We tend to sail for half of the summer in periods of two to three weeks at a time. We have 140w of panels (1 x 100 + 1 x 40) approximately horizontal on the coach roof. We have a fridge on 24/7 which is well insulated, and with a top opening, so it doesn't lose all its coolth when opened. I don't know what current it takes, but it is definitely the main drain on our batteries. Lights are LEDs, and in the summer are hardly used.

That, and obviously some charging from the engine, gives us all the leccy that we need, in the summer months. BUT, we have a theoretical domestic battery capacity of 510 ah. So I would encourage you to increase your battery capacity.

April, May, June, July, August, are fine, but September and particularly October, when we are aboard and using power, we have to be careful, or else choose to use the engine to boost the charge. Maybe at that time of year we should think about getting shore power occasionally.

 

[scruff]

Thanks for the input chaps. I'll look at increasing battery bank too - there is secure holding for two batteries in the engine compartment but I'm not sure where else I can locate additional batteries. Unless I double capacity by going lithium....

As always "small" projects sprout branches!

 

[Ceejay]

I have a couple of jobs for the winter, one of which is to install solar power to our boat which is based near to the Oban area and has 2x. 110ah batteries - one engine, one domestic

I would prefer not to install a stern gantry / arch for this and we do not have davits to place the solar panels across. I am currently swaying towards mounting them on the guardrails where dodgers would be. These would be left in the vertical when boat is left on a swinging mooring, but has the potential to placed in the horizontal when we are on board to increase output.

Solar panels are currently very cheap but the question I'm pondering is how many watts do we want installed? The common question is to work out the loads needed, but that leads to the second job which is the upgrade of the cool box to a fridge and the third job of installing an electric autopilot so there is quite a big unknown in power demand.

Boat usage is being left on a swinging mooring and used every second weekend with a couple of fortnight holidays...

I gather to ensure batteries are brimming on arrival at the boat, a comparatively small panel will be needed. However I want to be able to run the fridge full time when we are aboard and eat fresh food and less tins. Would 100w each side of the guardrail combined to a quality (mppt?) controller ensure I have fully charged batteries, cope with my power usage and don't burn the boat to the waterline? Then of course do I look to install a second domestic battery (so 2x110ah domestic, 1x110 engine)

Cheers,

I would suggest that 110 amp for a starter battery is unnecessarily large. I’ve just installed two 50 watt solar panels on the side rails of my pushpit, charging two domestic 135 amp batteries and 65 amp starting battery through a Votronic MPPT controller. I’m at Ardoran if you would like to look at my installation.
CJ

 

[William_H]

The question re how much solar power is coming from the wrong direction. The priority question is just how much panel or what size can you accommodate on the rails. Just go for the largest panels you can find room for. Solar power is relatively cheap, you can't have too much. Especially so in those less than ideal days for sun. In any case running a fridge you will need to monitor battery voltage and if necessary use an alternative charging source. ol'will

 

[Mistroma]

We are also based in the Oban area. We tend to sail for half of the summer in periods of two to three weeks at a time. We have 140w of panels (1 x 100 + 1 x 40) approximately horizontal on the coach roof. We have a fridge on 24/7 which is well insulated, and with a top opening, so it doesn't lose all its coolth when opened. I don't know what current it takes, but it is definitely the main drain on our batteries. Lights are LEDs, and in the summer are hardly used.

That, and obviously some charging from the engine, gives us all the leccy that we need, in the summer months. BUT, we have a theoretical domestic battery capacity of 510 ah. So I would encourage you to increase your battery capacity.

April, May, June, July, August, are fine, but September and particularly October, when we are aboard and using power, we have to be careful, or else choose to use the engine to boost the charge. Maybe at that time of year we should think about getting shore power occasionally.

Your findings for 140W seem in line with the figures I calculated for #7. The limiting factor is usually space and most people run out of space to fit panels before getting to the stage of having too much for their needs.

Panels on the coach roof are worthwhile, especially if they are cheap rigid panels as cost makes it a no-brainer. You will lose quite a bit due to shading but can usually pull the boom to one side at anchor. Shading actually becomes less of a problem on cloudy days as the light is diffuse. I get sharp shadows under the boom on sunny days and get a huge hit on output if I don't move the boom. No sharp shadows on cloudy days, output is obviously lower but I rarely need to move the boom (not that I see too many clouds in summer).

I have 200W on an arch but 145W under the boom makes a very considerable contribution. I'm lucky to have unused space there and don't need to worry about expensive semi-flexible panels. Cheap rigid ones have been fine.

 

[Laminar Flow]

Many smaller panels are better than one or two large to deal with shading issues. We have 300w in 50w increments and a MPPT controller. Also stepping on one or damaging it - at 35 euros a pop you don't have to slit your wrists. Cruising in Northern Europe, well into September and October even, we never had to run the engine to charge. Panels are installed on wheelhouse roof; I used to bother noving the boom aside - I don't anymore. Fridge, 5A, runs 24/7. Make sure the panels can back vent for cooling - don't know if that's a problem in Scotland, but excessive heating zips the output.

 

[scruff]

I would suggest that 110 amp for a starter battery is unnecessarily large. I’ve just installed two 50 watt solar panels on the side rails of my pushpit, charging two domestic 135 amp batteries and 65 amp starting battery through a Votronic MPPT controller. I’m at Ardoran if you would like to look at my installation.
CJ

That's very kind offer. I'll mull my install over and may PM at a later date if that's ok?

 

[scruff]

Your findings for 140W seem in line with the figures I calculated for #7. The limiting factor is usually space and most people run out of space to fit panels before getting to the stage of having too much for their needs.

Panels on the coach roof are worthwhile, especially if they are cheap rigid panels as cost makes it a no-brainer. You will lose quite a bit due to shading but can usually pull the boom to one side at anchor. Shading actually becomes less of a problem on cloudy days as the light is diffuse. I get sharp shadows under the boom on sunny days and get a huge hit on output if I don't move the boom. No sharp shadows on cloudy days, output is obviously lower but I rarely need to move the boom (not that I see too many clouds in summer).

I have 200W on an arch but 145W under the boom makes a very considerable contribution. I'm lucky to have unused space there and don't need to worry about expensive semi-flexible panels. Cheap rigid ones have been fine.

Roof mounted options are interesting. We do have space on rear cabin roof, but worry about them breaking from being stood on & broken on first sail.

What panels do you use and can they cope with being trod on occasionally?

 

[scruff]

The question re how much solar power is coming from the wrong direction. The priority question is just how much panel or what size can you accommodate on the rails. Just go for the largest panels you can find room for. Solar power is relatively cheap, you can't have too much. Especially so in those less than ideal days for sun. In any case running a fridge you will need to monitor battery voltage and if necessary use an alternative charging source. ol'will

My perhaps naive expectation is that there will be a correlation between intensity of sunlight and how hard the fridge needs to work - with a cabin ambient temp of 17°, the fridge will have an easier time than at 27°.

We also have nice Loooong summer days which will offset the lower intensity of the sun...

 

[Stemar]

Big +1 for putting both batteries in parallel and adding a starter battery. This can be a lot smaller - my VP2003 starter is rated at 1.4KW, so will draw 140 amps at 10v, less of all the connections are good and voltage drop is lower. On that basis, any 40AH or so battery has a CCA of 300+ amps, so more than enough.
Set up a split charging system with a voltage sensitive relay and include a switch that allows you to use the domestics to start the engine in an emergency.

As for where to put the panels, I wonder if putting them on the guard wires won't leave them vulnerable to mishaps coming alongside, especially alongside another vessel. My cheap as chips rigid panels are over the hatch garage on a frame made up of aluminium angle "iron", which lifts them far enough above the cabin roof that they're unlikely to be trodden on. Seven years good service so far. I don't know what really clever (=expensive) stuff there may be about to control two battery banks from solar power, but I have one of these: EP Solar Duo-Battery Solar Charge Controller 12/24v 10A. £33.30, though with 100w of panels, you'd probably want its big brother, EP Solar Duo-Battery Solar Charge Controller 12/24v 20A. Fast Delivery

 

[Malabarista]

Regulator doesn't dump power, it restricts it. Can't have too much solar and if your two 110AH are leisure, yes it's worth paralleling them but fit a car battery for starting, cheaper and better suited for the job.

Please excuse me ‘butting in’ but this is also a question that has been on my mind. When the batteries are full and there is no draw on the system where do the amps go coming from the solar panel?
Does the regulator ( mppt) just turn them into heat or is there something more clever going on?

 

[Baggywrinkle]

Please excuse me ‘butting in’ but this is also a question that has been on my mind. When the batteries are full and there is no draw on the system where do the amps go coming from the solar panel?
Does the regulator ( mppt) just turn them into heat or is there something more clever going on?

It just disconnects the battery charger part and no amps flow (a simplification), except those required to run the MPPT... the energy from the solar panels doesn't need to "go" anywhere .... a bit like switching a plug socket off, there are still 240V 16A available, but nothing switched on to use it.

PS: The detail of what the MPPT does when the batteries are fully charged and the MPPT charger goes into float mode is a bit more complex than stated above, but the effect is as stated.

 

[Malabarista]

It just disconnects the battery charger part and no amps flow (a simplification), except those required to run the MPPT... the energy from the solar panels doesn't need to "go" anywhere .... a bit like switching a plug socket off, there are still 240V 16A available, but nothing switched on to use it.

PS: The detail of what the MPPT does when the batteries are fully charged and the MPPT charger goes into float mode is a bit more complex than stated above, but the effect is as stated.

Ah thanks. Apologies for the daft question.