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The Ultimate Guide To Narrowboat Solar Power – Part 2


Written by Tim Davis of Onboard Solar


Right from the onset I decided rather than supplying a myriad of options, panels,  controllers, cables, brackets etc, I decided to spec up complete fully operational systems sourcing all the best available, right type equipment and then fabricating bespoke brackets and cabling looms allowing Onboard Solar to supply And FIT a complete working system designed for the job on a narrowboat. This also involved a process-driven installation method where we effectively arrive with a pre-configured kit which enables me to accurately cost the installation offering fixed prices to all wherever your boat is located – this has proved to be a very popular approach. The process we use on the day means that we can retrofit solar to a boat in a tidy manner critically without seeing the boat first.  

Solar technology 2011 – 2021 

So let’s look at how technology has changed… The key issue in the UK is we do not have guaranteed super sunny weather very often – far from it! Solar panels do generate power in any light conditions and there are ways of elevating the power output even on dull days  (coming up below) but generally, we need to over-spec the power capacity (watts) to reflect the fact that on a dull day a let’s say 500W solar system may only generate 150W. One good thing that has happened is the cost per WATT of power has dropped somewhat over the years meaning that one of the main differences in 2021 is we can to a degree over-spec the solar giving great output on the poor days and an abundance on the sunny days. 

The solar system is simply a battery charger. Your onboard consumers will be taking power from the batteries all day long and therefore the batteries need to be recharged. This is generally achievable in 3 ways… 

  1. Running the engine – spins an alternator which charges the bank – most newish boats have very large output alternators for the domestic batteries and a smaller output separate alternator for the starter battery. 

The important thing to note about alternator charging is it pretty much a “dumb” charge – technically called a “taper” charge it means in practice that when a battery is low on charge the alternator will output a high current but quite quickly the high current will fall away and within only a few minutes assuming batteries are not too low may well back off to a few amps. This is the reason why it is key to run the engine while running a mains powered washing machine. The washer will likely draw over 100A from the battery bank. While the engine is running the alternator will “see” this as a low battery situation and up its load accordingly to suit the demands of the unit. Say you have a 100A alternator and your washing machine draws 125A while heating up then 100A will come from the alternator and  25 from the batteries, as soon as the washing machine cuts out the alternator will switch back to providing current to the batteries to make up the shortfall and very quickly drop back to only a few amps…

  1. Built-in battery charger 

Most boats have a built-in battery charger, either a separate unit or part of a combi system – where inverter and charger units are combined in one box. In this instance, the system acts as a charger when plugged in and switches to providing 240v from the batteries as an inverter once unplugged. Combis can cause a big problem in marinas as if the marina power trips (which it often does) the system switches from battery charging to inverter automatically without you being aware – until your batteries are quickly flattened as while sitting on shore power you will likely not be paying much attention to what may be switched on! Most combis have a charger only setting to prevent this problem. Either way, these shore chargers are much cleverer than an alternator and charge batteries through a multi-stage process where empty battery receives a bulk charge as it is known, then an absorption charge, then switch to a float state when fully charged. In float state the batteries are monitored and as soon as a consumer such as your fridge or a pump, for example, cut it the charger ups its output to reflect that so effectively once charged your batteries stay in a fully charged idle state. It literally becomes as if you are running your 12V equipment directly from the mains “hidden” by the clever charger. 

  1. Solar charging. This is the solar systems we are talking about here. What the solar imitates is the built-in shoreline based charger described above. It is very important though to understand the complete solar system is more than just the panels. In the same way, you require a battery charger system between the mains and the batteries, in the case of solar a device called a charge “controller” is required. 

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Join me on a bespoke day for aspiring narrowboat owners on a narrowboat fully equipped for off-grid living (including one of Tim's solar arrays). Learn how to handle a 62' narrowboat on an all day cruise through rural Warwickshire

Solar Systems 

So a solar system consists of a panel or panels and a charge controller that sits between the panels and the batteries.  


Panels are rated in WATTS and have an output voltage that generally gets bigger as the panel gets bigger. In the early days of solar, the controllers we used were simply voltage droppers that lowered the voltage to a level that was safe to charge the batteries.

In those days the problem we had was if we used fairly high voltage large panels say 250W then they would have a high voltage output – usually around 48V. As most were being used for domestic 240V use this was fine however our early 12V controllers simply dropped the voltage to a safe level. Dropping a panel’s voltage from say 48V to 14.4 – the safe maximum charge voltage meant a huge loss in current, as the difference between that peak output voltage and the safe charge rate was simply wasted.

The second issue was domestic spec panels were very large and not best suited to the roof of a narrowboat. For this reason in the early days, we used 100W panels that were rated at 18V output – quite close to the  14.4V needed for charging so a suitable early PWM type controller was only shaving a few volts off the panel voltage so little was wasted.

In those early days, we offered a 2, 4, 4, 6 or 8-panel setup offering from 200 to 800 watts. With a 25% loss of power with the voltage drop and other inefficiencies, the 200W system would offer an output of about 10A rising to  40A on the vast 8-panel system (in reality 6 x 100W panels being the most that would fit on a narrowboat but not on a wide beam). What size systems did people need?

This, of course, would depend on how the boat was set up – good 12V orientated boats as described above would typically go for 200W 2 panel, 300W 3 panel or 400W 4 panel system. The bigger systems giving between 10 and 20amps output on dull days. The larger systems more typically used on wide beam boats which in my experience tend to be more 240V orientated as they are designed to sit in a marina plugged in.

So in the early days 6 8 or even 10 panel systems were specified for wide beams with separate 240V fridge and freezer onboard. These days I  much more strongly recommend changing to 12V refrigeration prior to going out and living without shore power. There is however an unwritten rule that you can never have too much solar – the more you have the better the charge rate in Amps even on dull days. Those who live without shore power ALL year-round also benefit hugely from a bigger system as light levels in the winter are much poorer – les of an issue if you overwinter in a marina plugged in of course.

The 100W panels, of course, had the added advantage of being small in size so they didn’t take up much room on the roof – later I will talk about our mounting systems that mean these small panels can be mounted in such a way that you can easily walk past them on the roof – very important consideration for single-handing owners who tend to use the roof a lot. 

Charge controllers 

  1. PWM Type – these early controllers we used to use in the beginning are essentially just voltage dropper units that make sure the output of the panels is latterly “controlled” down to the safe charging maximum of 14.4V. 
  2. MPPT Type – These came along in a mainstream way around 2014 and are a  computer-controlled system that effectively has two sides. One side controls the panels themselves and establishes an output voltage at the panels that “maximise”  the current flow in amps and thus regardless of a panels voltage output, will generate the best output. The other side of the MPPT faces the batteries and to the batteries looks like a 3 step battery charger as would a mains powered one described above. So effectively there are several game changes with the MPPT controller. 

MPPT advantages 

Panels can run at their maximum voltage with no loss of power in regulating down to safe  charging levels 

They operate at low light levels far better than old-style PWM controllers The 3 step battery charging is far superior to earlier taper charge from PWM controllers

MPPT controllers allowed us to overcome a real limitation with the low voltage panels. In  2016 we moved from 100W panels to 165W panels. These were slightly larger but allowed us immediately to offer larger capacity systems with the 2 panels moving from 200W to  330W ad the 3 panels from 300W to 495W. However, they were still 18V output.

With the earlier PWM dumb controllers, this had the advantage of not wasting too much power as the max of 18V was not much greater than the 14.4 required for bulk charging. However, this had a huge disadvantage in low light levels such as early morning, evening or just dull days.  The voltage only has to drop by 20% to hit the 14.4 level, once it drops below this the ability to properly charge the batteries is lost. So a huge disadvantage of PWM controllers was that we had the conundrum of needing low voltage panels so as not to waste power but the low voltage characteristic meant poor charging in low light levels.

MPPT controllers however not only allow higher voltage, they actually thrive on it. The higher the voltage entering the controller the more it can maximise output to the batteries in low light levels. However, by  2015 compact panels suitable for boats including the new 165W versions we started using were still 18v output – what we really needed was higher voltage small panels but at that time they did not exist. The solution was to use a series and parallel connection into the  MPPT controller.

So a two-panel system would be 2 panels daisy-chained together in series giving 36v  to the MPPT. A three-panel system would be 3 in series giving 52V at the controller and a 4  panel 2 in series plus 2 in series connected together in parallel into the controller (effectively 2 x36V solar arrays). 6 panel systems would, of course, be 2 arrays of 3 in series.  This method of connection worked extremely well especially on the 4 and 6-panel system as it meant each array connected in series were independent of the other, so any shading of one array would not affect the other.

The ONLY downside is if part of an array of 2 or 3  panels were shaded, then that whole array would lose output and in the case of the popular 495W 3 panel system all were connected as one serial array and any shading anywhere would bring the output of the whole 495w system down. After lots of testing though we took the view that the enhanced low light performance hugely outweighed the shading issue. It was not until 2020 when finally the ultimate solution appeared in the form of high current, medium voltage but compact panels had arrived giving us the ability to offer even better output over a multi-panel array with ALL panels connected in parallel, each panel outputting at 37V s with 215 Watt so lots of headroom above the 14.4 optimal charge rate and high current so good low light performance and all configured in parallel so at last no shading issues.  

Shading Issues 

Shading a panel is a big issue. The cells are connected in series – 36 of them normally. Most panels have two separate arrays in them of 18 cells each. If one cell is shaded all the others will drop their output. So trees and other obstacles are to be avoided!

You can read the third and final part of this article here.