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The Ultimate Guide To Narrowboat Solar Power – Part 1

Written by Tim Davis of Onboard Solar

Back in 2012, I wrote a series of articles on boat electrical systems, power management and an introduction to solar systems. At the time I had just completed the first full year as  Onboard Solar, at the time of writing I was on my vintage tug at Penkridge in December waiting for Park Gate lock to reopen after maintenance so I could get back to my winter mooring that year at Alvecote. This time I write from Woodend Lock, a beautiful spot on the Trent and Mersey in the midst of the January 2021 Coronavirus Lockdown. If you have not read the articles, they do make a good preface read to this article to save repetition. 

This new article updates on where we are with solar systems and also shares all the experiences with customers technical challenges over the intervening years, highlighting issues that have come up often many times.  

Since 2012 there have been some advances in technology, which I will go through in detail later in the article. Though not the major advances that are normally associated with modern technology. The main thrust of this piece is the learning of what makes successful off-grid living not only possible but also easy and simple. I have tried to focus on the things that have come up time and time again with customers over the years.  

There are 3 sections depending on your area of interest you may want to skip to the relevant section headings. 

Section 1 – BACKGROUND, TECHNICALITIES AND PREPARING the boat for off-grid living sharing my experiences. 

Section 2 –ONBOARD SOLAR HISTORY AND DEVELOPMENT Early history and development  of Onboard Solar Systems 

Section 3 – ONBOARD SOLAR – WHAT WE OFFER NOW and how to get the very best out of a solar setup, how to monitor it.


This is perhaps the biggest issue that I come across on a day to day basis. A very common call I get is from alarmed new boaters. Very often they have purchased a boat, more often than not a “used” boat but increasingly a new build. The fundamental problem discovered very soon after ownership is that power capacity drops very quickly after stopping the engine, often within a few hours. The concerned owner immediately believes the solution is a solar system. The problem, however, is always the same – the boat has been set up to be plugged in at a marina or other mooring that has access to shore power. Many new continuous cruiser folks are buying boats to live aboard “on the cut” that have been lived on often for many years in a marina. Other boaters have themselves lived in a marina for a  good while and are now wanting to venture out on the cut for extended periods. A third category – new build customers, most notably a problem in wide beam boats but also new narrowboats that these days are often configured by the boat builder to be primarily plugged in… So what are the problems? They split broadly into two issues, technology choice for the boats onboard equipment and appliances, and secondly a mindset that changes the way you live off-grid. 

The first and main technology issue is a too greater reliance on 240v mains power. While plugged in a boat can be configured almost exactly the same as a house with 240v fridge and freezer, 240v kettle, 240v TV, 240v washing machine and 240v sockets in which to plug the huge range of device chargers USB power blocks etc that we now all have. The power is coming straight from the shoreline plugged into the boat. 

Most boats have an onboard battery charger (often a combi system inverter/charger of which more below). This means the batteries are kept in a charged up “float state” while plugged in. These charging systems are also acting as a 12v power pack, this means when 12  v systems such as lights, pumps etc are used the charger system actually provides the power, not the batteries. This gives the end-user – the boater – a seamless power for 240v and 12v  systems – effectively everything on the boat is being driven from the incoming 240v supply.  The upshot of all this is the boater does not have to worry everything works and keeps working 24 hours a day 7 days a week to an abundant level. There are many many boats sitting in marinas configured like this right now.  

What happens when you unplug then – what is so different? 

Once unplugged the boat has two completely separate power systems. 12V power for things like lights and pumps and of course the 240v power for all the typical domestic appliances.  The problem is once unplugged where do the 240v appliances get their power from? The answer of course is an Inverter. An inverter on paper is a great idea. It takes power from your 12v battery bank and steps it up to 240v and converts it from direct current to alternating current. However, to do this there are great inefficiencies in multiplying the power 20 fold. Most modern inverters are around 95% efficient but it is the raw Amp conversion that causes the issue. Remember volts is your tank of power, Amps the flow though the cable and Watts the thing that is consumed by the equipment. We can work out  how many amps a consumer draws using the simple formula WATTS/VOLTS=AMPS 

Example – 240V typical mains fridge rated at 150W Running of mains directly 150W/240=0.6A 

The key thing here is only Amps drawn at the high 240V are very small – why is this important? Batteries store AMP HOURS, a fixed amount of current that can be delivered under a much lower “pressure” of 12 instead of 240 volts therefore at 12V many more amps are needed to run the same piece of mains powered equipment… Are you keeping up so far? So, therefore… 

Running the same 240V fridge rated at 150Woff mains through a 12V inverter 150W/12=12.5A 

That means over 24 hours your 240v mains fridge will consume 12.5 Amps per hour for a  total of 300AH. Now, of course, there are A+++ mains fridges might use say 80watts so you might well get the draw through an inverter down to 6.5A BUT the inverter will still have to be switched on 24 hours per day. Big inverters can consume 2A just sitting there switched on in ready state… 

Typical 12V Fridge 

Most 12V fridges use a 12V 40W Danfoss compressor which using the formula consumes under 4A of power. So big power saving in AMPS but also remember when the fridge thermostat switches off there is no inverter still running so in reality over an hour a 12V  fridge likely uses about 1.95A per hour. 

So the point of all this is? Using a 12V fridge is the MOST important thing that will dramatically reduce the amount of power that you draw – indeed the MORE you can do at  12V the better for lower power consumption. I would say this 12 or 240 fridge issue is the biggest one I come across on a regular basis, with many customers starting out with 240v  refrigeration but quickly biting the bullet and changing to 12v units.

Key things to keep in mind in preparing your boat for off-grid living. 

So here are the key things that I highly recommend you add to your tick list in order of priority of things to do if you are planning to live off-grid. You may not be able to achieve them all but the closer you can get to having each one ticked the easier off-grid living will be… Remember the key to living afloat is compromise – it will benefit you and your power  NOT to have a huge chest freezer and NOT to have a 56” TV for example. 

  1. Go 12v on refrigeration and try to have a combined fridge/freezer. Make sure you ONLY use compressor fridges not the cool boxes for use in cars as these run full time and consume a lot of power they are designed for use in a  car while travelling with the engine running. 
  1. Do have 12V sockets fitted. These can be used with low-cost adapters that present  USB ports for charging of phones and tech. If you use laptops then buy 12V car chargers for them and again use them with your 12v sockets to charge/power your laptop. 
  2. Make sure all light bulbs are changed to 12V LED – incredibly efficient compared to normal bulbs typically consuming 1 tenth of the power. Most 12V light fittings you have can simply have the bulbs changed. 

  3. If you use 240V table type lamps or floor lamps etc again these can be converted easily to 12v. Simply change the bulb and change the plug to a 12V one. 

  4. Use 12v or latest generation solar TV. 12V TVs these days are commonly available and great specification flat-screen LED type a far cry from a few years ago. A new development is the solar TV. I have one of these now it is a 32 inch LED and has a  built-in lithium battery pack. It comes with its own dedicated solar panel and charges during the day giving up t 10 hours of viewing on one charge, very impressive. 

  5. Look carefully at other electronic devices such as 4G internet routers most are capable of running at 12V – just ditch the 240V power block and connect to a 12V  socket. 

So you can see 12V and 12V sockets are a very good thing. 

These simple measures will dramatically reduce day to day power usage. Change your State of Mind! 

Firstly adopt a view if you really don’t need it – don’t have it! Coffee machines can be replaced with percolators or similar for example. 

Have two systems, one for use when plugged in, one for when unplugged. For example, I  keep a 240V kettle and toaster for when plugged in the winter but use a hob top “gas”  kettle and the gas grill while out cruising… 

If you need to run heavy-duty mains equipment such as a washing machine – through your inverter – then make sure you do this when the engine is running. The large output domestic alternator should be able to put back in a large number of amps that the heavy-duty equipment will need. 

OK so hopefully you have a good picture now of what is needed technology-wise to effectively live on battery power – now we need to focus on off-grid charging systems and talk about how things have developed over the years we have been installing systems. We’ll look at onboard solar systems and development in the next section.