Narrowboat Solar Power

By Tim Davis

Before we start exploring all the ins and outs of solar power, an introductory paragraph to “set the scene“ as it were.

Firstly a bit about me and my involvement in solar energy; I have been a live aboard boater for 12 years now and during that time have worked exclusively in the boat building and maintenance industry.

My role has always been all the technical side of boat building; things like engine fitting, heating and plumbing, gas and of course one of the biggest jobs on a boat, wiring and electrics, in fact anything that’s not wood work. My wood working colleagues often joked than no one knew what Tim did as all his work was covered up with lovely wood!

Narrowboat Solar Panels

It was around 4 years ago I first got involved in fitting of solar panels for a customer. It was a very impressive system but also hugely expensive which largely made it prohibitive for most boaters at the time, as although it undoubtedly worked very well the payback time would have been many years with too greater upfront cost to make it practical.

Two years later and I was planning an extended cruise around the system and decided to re visit solar and see if I could build a system for my boat as I was getting fed up with running either the engine or generator for a couple of hours each day when I wanted to sit somewhere for a week or two and the pain of having to turn the fridge off if I wanted to leave the boat for a day or two. After a lot of research I came up with the system I have now and found many boater friends became interested it how good it was.

As my main objective was to come up with a system that gave me the MOST output for the LEAST cost I had “accidentally” created a product that other boaters might want. Within a short space of time it turned into my current business “Onboard Solar” a complete service supplying and fitting of complete solar packages AND helping people reduce their energy draw on the boat!

So what do solar panels actually do?

Well, it’s really quite simple, they act as a battery charger – and that’s it! Many of you will have built in chargers that you switch on when plugged into a marina shore supply (Luxury!). The panels effectively do the same thing, but the power comes from the sun. They don’t have anything like the same output as a mains charger though – not even on a relatively large solar installation so one myth to quickly dispel is you can fit solar and then expect an abundant supply of power as you would when plugged in. A solar system effectively trickle charges your domestic batteries during the day. Most boats have one piece of equipment that is on all the time and they want to keep going every day – can you guess what it is? The TV? No! it’s the fridge of course! Most boats sensibly have a 12V fridge and this can be kept running comfortably through the spring and summer/early autumn by a modest solar system – indeed I left my boat tied up for a week in the summer and left the fridge ON oh yes! Remember though the solar system charges the batteries which the fridge then runs from.

So a solar system is designed to keep your basic 12V needs up and running while you sit on the cut in a lovely place somewhere for a few days (or perhaps as many of my customers do have a linear mooring with no power). They won’t run heavy duty mains appliances like washing machines (you still need to run the engine and use the inverter for that) and you WILL still have to be as careful as ever with your power but you WONT have to run your engine anywhere near as often – AND you can leave the boat for a day or two and not worry.

Solar Power ControllerI also encourage boaters to adopt power saving technologies such as LED lighting (fantastic these days and amazing – you can have around 10 LEDs on for the power of one halogen bulb – oh and we sell them too!). The thing is you can spend a relatively large amount on solar and it will work very well IF you think about minimising your draw and continuing (as we all do) to be careful with your power.

There are other tricks too like running laptops through power converters rather than using the big inverter – please feel free to contact me for further details on all of these “asides”

How do they work?

Good news folks – you don’t need to know! It’s all quite boring talking about silicon atoms, photons and electrons! (email me if you do want to know)  In short, you shine the sun on a panel and it outputs low voltage DC electricity (just what we need on a boat eh?). That it! However what you DO need to know is that as the sun changes brightness the voltage varies enormously and often way above the charge rate a battery likes to have so you have to use a regulator between the panels and the batteries. These have a side advantage that there is a digital display on them giving you info such as battery voltage, charge rate in amps and amount of charge in amp hours – all quite handy to know! Surprisingly, many boats don’t have any way of telling what’s in the battery. There are actually different types of regulator as well which I will come back to later.

I’m thinking of buying a solar package – what size? How many panels?

Well this is tricky to answer as it’s the old “how long is a piece of string thing“. Be very careful here though as you could spend a couple of hundred pounds on say an 80W system that only gives you 2 or 3 amps which is just not quite enough and you will still need to run the engine often.

However from experience I can tell you that for most boats that are what I like to call “12V” based. That is to say you have a 12V fridge and have done your best to minimise your power draw so a 200W system is a good start.

In my packages (remember the mantra of MAXIMUM power for MINIMUM outlay?) I discovered that 100W panels offer the best price per watt, whilst still being of a manageable size. Therefore to achieve bigger output one simply has multiple 100W panels – thus a 200W system would have 2 panels etc.

It seems smaller panels and larger panels do not have this economy on cost. This gives the system a nice modular approach and means with forward planning you can add another panel at a later date easily. A 200W system will give a charge rate, using the standard controller, of around 12 amps on a good midsummer day. That’s 12 amps continuously going into the batteries all day long while the sun shines. The fridge draws about 4 amps when the compressor is running so there is plenty left over to charge the batteries as well ready for the evening when you want to use the power for lighting and the TV.

On a dull winter’s day like this late November (more rain!) I was seeing 1.7 amps off mine, OK, not much but it is still a charge. Over last weekend we had some of that lovely unbroken winter sun all day on Sunday and I tilted my panels into the low sun and saw 5 amps. So the motto here is you need to have enough watts to make it worthwhile and you can never have too much. It’s then down to your budget. And, yes, sadly there WILL be days especially in the winter where you will need to run that engine still.

So what types of panels are best? Are stick on ones any good?

Another very often asked question. There are different types of panel available; the two common types seen are the rigid panel type, usually glass coated and in an aluminium frame, and the flexible type which sit flush on the roof and look great!

However beware – I did a lot of investigation in my research and came up with this: Do I want a panel that looks great and really enhances the look of the boat, or do I want one that works better AND is much lower cost?

Joking aside, the issues are as follows.

1. Panels work best when aimed directly at the sun – there is no scope for tilting the stick on panels into the sun.
2. The light transfer capability of the gel coating on the sticky panels as opposed to glass on the rigid panels doesn’t work as well.
3. Panels like to stay a cool as possible, when hot they output less – we all know how hot the roof gets in the summer, imagine how hot a stick on panel would get!

So the system I chose for my boat is as follows: I use 100W panels. I, as many of my customers do, have 2 of them. They are the rigid frame type with aluminium surround. They are mounted on tilting brackets (see photo). These are A shaped brackets 8” high mounted at each end of the panel. These brackets have several great advantages.

1. You can easily tilt the panels into the sun – this can make a difference of plus 60%
2. As they are above the roof on the brackets they keep really cool.
3. Having them on the brackets mean they can be installed on the centre line of the boat as they ride beautifully over the top of any mushroom vents, are easy to get round and actually look quite smart.
4. They are very easy to clean under with no rust or dirt traps and of course easy when you want to repaint the roof.
5. The final advantage is without doubt the MOST important. The stick on panels are about twice the price of the system I have just described.

As an aside, I was worried. I have a lovely vintage style BCN tug all in proper livery and I didn’t want to spoil it with solar panels, but I did want the power so I was very nervous, but you know what? A few weeks in I was used to them and people who always say admiring things about the boat when I’m in a lock still do and often actually complement the panels so it was not a big deal in the end! “Fusing technology with tradition” is what the marketing department would call it!

So what’s all this watts/amps/volts stuff?

Again it’s not critical to know any of this but it helps to understand what’s going on. Voltage is your store of power – like a water tank. Solar Panel BracketsFully charged batteries sit at 12.6 volts. Amps is the flow of current to a consumer such as the fridge or lights – like the flow of water in a pipe – it is critical as the more amps you draw the quicker the batteries go flat, the more you put in the quicker they get charged. Your shore power charger might be 50 amps or more so you can see that 200W of solar with an average of say 8A output is quite small. Watts is the actual power that a consumer uses so your 12V telly say might be 120W. From this you can easily work out how many amps it uses by dividing it by the volts (12) – that example tells me it’s drawing 10amps.

Solar panels are also all sized in watts because they are a power source. So 200W divided by 12 = around 16 amps right? So hang on, why don’t you get 16A out of a 200W system then? It’s all down to the regulator/controller. The standard one works by actually lowering the voltage of the panels down to an acceptable charge rate (maximum of 14.4 volts) in doing so it introduces loss as the extra volts are simply wasted. (My panels generate 21 volts in full sun for example). To overcome this I have a new regulator called an MPPT controller – it stands for Maximum Power Point Tracking and means just that – it allows the panels to run at their full voltage with no wastage and uses that to drive a more sophisticated charger (like a mains powered charger) within the controller. I have seen the full 16amps off of a 200W system using the new controller so it definitely works!

So what’s the deal with Onboard Solar?

The first thing I like to do is get an idea of what you are running on your boat and the size of the battery bank, together we look at any scope for trimming down power then I can recommend the right size system – my take on this is if you are going to spend a few hundred pounds on a solar system – you want to get the very best return. Though generally 12V based narrow boats start with 200W. Some wide beams with 230V fridge and freezer run through an always on large inverter require 400W or more just to keep up with the load.

In both cases I now encourage use of the new MPPT controller as it does make a difference. I then supply and fit everything – keeping cables and fittings as tidy as possible, then of course show you what‘s what – though you will be pleased to hear it all self manages!.  It takes a couple of hours and removes the headache and drama of drilling holes in your own boat because you have to get that right first time!

You will have a little controller that gives you useful info about the charge rate and more usefully the battery voltage so this time of the year you can look and see if your batteries are not getting enough charge so you know when it’s time to run the engine! Optionally I also supply power management panels. These show you the amps that you are drawing as well as the amps going in and keep a count up of amp hours used/charged.

So say at night you are drawing 6 amps for the television and some lights, every 10 minutes you will use 1 amp hour. In the morning once the solar kicks in these amp hours get counted back in again (or when the engine or charger is on) so it’s a fuel gauge for your batteries.

What sort of budget do I need?

Cost is currently £625 all inclusive for supply/fit of the 200W and £1,050 for the 400W system with an extra £150 for the MPPT upgrade. (300w or greater than 400W also available)

So in summary then, solar panels act as a charger to keep your basic 12V  needs up and running allowing you to sit anywhere for days at a time without having to worry. It is a great thrill to use power of an evening, get up on a bright spring or summer morning and see your batteries under charge at 13.5 volts instead of way down and thinking “ok better start the engine up”

What about payback?

This is a tricky one, because it depends how you measure it, financially most customers agree about a year IF you spend a lot of time out and about or especially if you are a live aboard on a mooring with no power – this is a real win situation. If you are in a marina then there is not a huge benefit as the main charger “takes over”. I do have a couple of customers in marinas who have opted for 400W systems (to increase the charge rate) and they no longer plug in and are thus saving the huge cost of marina electricity standing charges etc. However the payback of NOT running that engine every morning just to keep the batteries charged is huge – real convenience.

My final word is a quote from a customer and good friend of mine. We were sitting by his boat and having rather nice Gin and Tonics crammed full of ice from his 12V fridge/freezer on a lovely summer evening on the North Oxford canal. As he passed me the drink he gestured towards his solar panels on the roof and said – “there you go, Tim – ice cubes made from the sun!“  Brilliant!

I hope this brief article has helped to explain something of what the solar energy applied to boats is all about.  There are many pictures and lots of info on my web site www.onboardsolar.co.uk or please feel free to email me tim@onboardsolar.co.uk or call me on 07810 885734.

Useful Information
Paul Smith

After six and a half years living on a narrowboat on England's inland waterways, Paul and his wife Cynthia wandered Europe by motorhome during the winter, and on the Dutch and French waterways in the warmer months on their 35' Dutch motor cruiser. However, the pull of England's muddy ditches proved too much for them. Now they're back where they belong, constantly stuck in mud in a beautiful traditional narrowboat.

songbird - Monday,3 December, 2012

Great article, very clear and simple to understand. Lets just hope the sun shines a bit more!


Paul Smith - Monday,3 December, 2012

songbird said
Great article, very clear and simple to understand. Lets just hope the sun shines a bit more!

After a poor start, it’s wall to wall sunshine in the depths of rural Warwickshire. What a shame I don’t have solar panels.


GM - Thursday,6 December, 2012

Just a couple of thoughts particularly for the boaters who do not liveaboard and moor with no shoreline.

There is enough sun in the winter for a 200/300 watt system with a mppt controller to keep your batteries full charged throughout the winter.

I run a 270 watt setup charging my 24v system an the average in the winter is about 5Ah a day, on a 12v system that would be 10Ah a day.

If you self install using an mppt do connect panel in series, the mppt can then operate and do its job of giving the best power output even on the darker winter days. Just make sure that the total peak voltage for the cells does not exceed the maximum input voltage for the mppt. With and average cell output of 17.7v and an average max mppt input of 150v to exceed the max voltage would need 8 100 watt cells. Big system, such as would be fitted to an electrically drive boat.

I know mppts are more expensive than the cheaper solar controllers but they are really worth the extra.

Oh just a thought, do use the right size cables. When sizing cables calculate the sizes allowing no more than 1% voltage drop. That applies for the cable from the cells to the mppt and from the mppt to the batteries.

From my phone


pearley - Thursday,6 December, 2012

I have 4 semi-flexible panels, two 45W and two 80W, all having an open circuit voltage of 20.5. Each pair are wired in series with those pairs then being wiring in parallel into a Tracer 30A MPPT controller. When checking with the manufacturer their only concern was that in very cold weather the voltage could rise too high for the controller.

Given that winter temperature doesn’t drop much below -10C and the total wattage is only 250 they calculated that it would never happen but, of course, if the solar array is larger you could approach a level that could damage the controller.





GM - Thursday,6 December, 2012

Hi Pete, you have me puzzled. If i have understood your post correctly the way your panels are wired they have a combined output of 41 volts. According to a couple of websites the Tracer has a max input of 150 volts over 3.5 times that of the cells output voltage. Can’t see how they got to that being a problem. Puzzled


pearley - Friday,7 December, 2012

Having spent the last 45 years as a vehicle electrician, wiring up the panels is childs play but, I know nothing about the technical side of solar so rely on what others, more qualified, tell me. So this is what I was told when I asked about connecting my 2 different sizes of panel:


“Yes, you can. The two panels have maximum output voltages within 10% of each other (in this case close to 5%). The end result will be that both panels will be putting out slightly less than their optimum power, at a voltage which is somewhere in between the individual panel values.

The only concern that you need to have is that the ~45 volts Voc of the first string does not rise above the maximum input voltage of your new MPPT controller when the panels are cold.

I assume from spec below that max input of controller is 150 volt so should always be below that given that max UK temperature is unlikely to get above 30C.

Yes, the maximum input voltage is 150 DC, so even at the lowest credible temperature you will be OK with two panels in series.

But note that the temperature coefficient of the panel voltage (not listed in the spec you copied) is negative, so the highest voltage will be at the lowest panel temperature.

How cold does it get in your part of the UK?

The coldest panel temp will usually be in the early morning, just as the sun comes up. Until they are heated by the sun, the panel temp will be the surrounding air temperature or even a bit lower if the sky is clear and the panels can radiate heat into the night sky

Lowest temperature this week was -1C at about 6am. Average coldest of the year is in February when the coldest I’ve measured is -10C.

A worst case temperature coefficient for Vmp, Pmax and Voc for monocrystaline cells is .5%/degree C. So if Voc is 22.5 at 25C, then it would be 17.5% higher at -10C. That would be ~26.5 Volts, 53V for two in series. Iffy for a CC with max input voltage of 50V, no problem at all with a max of 150V.”


Hence my comment about large arrays. It is not uncommon to see six 100W panels on a boat which in series would be around 120 volts. If the voltage rises when cold then it presumably could go over the limit.





GM - Friday,7 December, 2012

I see where your going Pete.

If with a temp of -10 you also had max sunlight, can’t remember the full definition on the boat no reference data, then I would agree. With the angle of the sun in the UK through the winter it is not going to happen. I run 13 panels at home,2.79kW, min temp measured -16C, with no where near enough light to achieve 50%of max output.

Most mppts have an overvoltage alarm and cut out. My boat setup has never even approached 50% of the max ip of the mppt.

From my phone


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