New solar installation

[Star-Lord]

2 x 100w solar panels a Victron MPPT controller and two 100ah batteries (non AGM) should suit you nicely. But 3 house batteries would be even better! Solar panels on the side are sometimes referred to as batwings. I started with solar panels on an arch but found when I added the two batwings I was now totally self sufficient being able to spend months at anchor. I think batwings are the more efficient than arch but having both is obv best. A BEP battery monitor may also be an idea. These monitors are great but when they say 100% full it does not mean your batteries are 100% full! Research is needed If you want a nice system to attach your batwings you could attach a ss tube between two rear stanchions and then use NOA clamps (Swedish online company) to mount the 100w panels (they do not support more than 100w) and they are very robust and easy to tilt.

As for running the fridge 24/7 maybe you will have enough power maybe you will not. I turn off my fridge at the thermostat (i leave the compressor on all the time) before bedtime always and I still have cold milk in the morning.

Also it is advisable to hook up to shower power every so often to boost the batteries for 24 hours - they will last longer. On my system it is recommended once a month but in the summer it can be very 2 or three months before I boost.

 

[Star-Lord]

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

Yes putting the panels on like batwings can cause problems when rafting alongside or med mooring! I just about get away with it by never rafting alongside any more and when med mooring in a tight spot (which I usually don't) I can lift the panels all the way up vertical on the NOA clamps. The advantage of NOA clamps (and possibly other types as well) is if you plug in your panels via a screw plug you can put a cap over the plug and just take the panels off and stow them below if needed.

 

[Alan S]

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.

Here's another daft question...

A 1 square metre 15% efficient solar panel in full sun receives about 1000W from sun and delivers 150W electrical power so presumably the other 850W is partly reflected and the rest heats up the panel.
Does this mean that if the panel is disconnected either manually or by solar controller the panel will get hotter as that 150W has to go somewhere as the power received and reflected remains the same?

Puzzled.

 

[Baggywrinkle]

Here's another daft question...

A 1 square metre 15% efficient solar panel in full sun receives about 1000W from sun and delivers 150W electrical power so presumably the other 850W is partly reflected and the rest heats up the panel.
Does this mean that if the panel is disconnected either manually or by solar controller the panel will get hotter as that 150W has to go somewhere as the power received and reflected remains the same?

Puzzled.

No question is daft, but I don't happen to know the answer to this one .... at a guess I'd say a mixture of reflection or transmission, but it's the infra-red part of the spectrum that causes heating and the visible spectrum that generates electricity.....

In a solar cell, solar radiation with wavelengths of 380 nm to 750 nm (violet to red) strike the material with enough energy to knock electrons from their weak bonds and create an electric current.

If there is no load, then I assume the electrons return to their previous state.

Perhaps someone more knowledgeable can answer.

 

[duncan99210]

A panel not actually producing electricity will, in terms of heating up, behave like any other material in direct sunlight. It will reach temperature equilibrium where heat absorbed from the sun equals heat lost by convention, conduction and radiation. That temperature will be marginally higher than that of an panel producing electricity, which is dumping some of the energy it receives as current.
Panels in the Med get very hot even when producing as many watts as possible: like REALLY uncomfortable to walk on.

 

[Star-Lord]

Solar panels are more efficient in northern Europe than the Med. I find dousing panels with cold water at about midday works wonders.

 

[Mistroma]

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?

Sorry for the delay in replying. All my system details are on my profile, just look in the "About" tab. The deck and arch panels are rigid and I have a spare semi-flex. one for use at either end of the season. The latter makes a big difference as I can angle it to the sun late in the day and keep batteries at 100% for another couple of hours when days are shorter.

No chance of walking on the rigid panels. They should survive hail but putting a foot on one would be an instant disaster. I don't have any reason to walk near them and the 3 deck panels are now about 8-9 years old. Fit rigid panels if you have a space you never walk over as they are so much cheaper and should last many years. Mine are just tied down and easily removed if required.

Your earlier comment about Scottish weather and fridge power was correct. My power use tended to remain fairly constant as I'd run the central heating on cold evenings. You will probably find that a non-insulated bag in the cockpit overnight will often be colder than the fridge for a fair bit of the season. You could probably get away with putting stuff outside for quite a bit of the Scottish sailing season. That would certainly counter a lot of the reduction in solar output if you could be bothered.

I did do that many years ago on previous boats and it did work quite well in reducing power usage. No solar panels in those days and I was interested in reducing engine run times and depth of battery discharge..

 

[Molteni]

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​

Great to have some real world figures Does anyone have any figures taken for other parts of the UK/World?

 

[Mistroma]

Great to have some real world figures Does anyone have any figures taken for other parts of the UK/World?

These aren't real world figures, I used the PVGIS model to generate the figures. You can enter any location in Europe to get a prediction based on historical data interpolated from nearest known datapoints. I've been using it for many years now and have compared the forecast with my own data in various location throughout Europe (none for Scotland though). It has always been very close to my actual figures.

PVGIS is Photovoltaic Geographical Information System

 

[davidmh]

Mistroma gave a good table in #7 for a flat 100W panel, it looks like its about 3 times the maximum hourly rate of the panel per day. Many sources suggest about 4 times the maximum rate with the panel at the best elevation. You say you can achieve better with a well angled panel would this be about 4 times the max output. Perhaps being in Oban you do not get as much sun as on the South coast.
would be interested to hear what you think you would get with the best panel angle.
David MH

 

[Mistroma]

Mistroma gave a good table in #7 for a flat 100W panel, it looks like its about 3 times the maximum hourly rate of the panel per day. Many sources suggest about 4 times the maximum rate with the panel at the best elevation. You say you can achieve better with a well angled panel would this be about 4 times the max output. Perhaps being in Oban you do not get as much sun as on the South coast.
would be interested to hear what you think you would get with the best panel angle.
David MH

I usually quote output for panels lying flat on deck as that's the most useful orientation in the majority of cases. Optimising the orientation works on land but boats move around a lot. It can be worth tilting panels to improve early morning or late evening performance. However, you can end up with zero output if the boat swings away from the sun. It tends to be more useful in predictable conditions (wind, tide, tied up alongside etc.).

If you track the sun with a 2-axis system I expect you'd get a 50%-60% increase on land in summer. The gain would be much smaller on a boat because of movement as power expenditure for tracking would increase and you would still get some shading from mast etc.

Simply fixing the panels at about 35 degrees wouldn't gain much as that's optimised over the whole year and probably only 2%-5% better in summer. Half that angle might be better but that still assumes your boat doesn't rotate.

The simplest tracking solution for most people is to manually tilt panels for early morning and late afternoon use. You get a significant increase in Amps. but only for a few hours and mostly just offsets usage from fridge for a few extra hours. It works if the boat isn't going to move or if you are on board and able to manually re-adjust if required. You'd probably get an extra 10%-20% each day with manual adjustment.