The latest solar technology makes self-sufficient cruising much more achievable. Duncan Kent gives the lowdown on everything you need to get your boat sorted
SOLAR POWER ON BOARD
Solar power is fast becoming the most popular and economic method of keeping the batteries charged on a boat.
Particularly now that the efficiency of photovoltaic (PV) panels, charge controllers and batteries is improving every day.
Furthermore, the latest technology in regulators and charge controllers has brought about a noticeable increase in useable power output, so the problems of shading and non-alignment can be compensated for more easily.
Not only has PV equipment become more efficient and cost-effective, but many of the modern devices we want to use on a boat have become less power hungry.
This means it is now far easier to provide your entire yacht’s electrical needs, both 220Vac and 12/24Vdc, from natural energy resources – particularly solar power, even if you are planning on a fully electric boat.
WHAT DO YOU NEED?
For instance, a boat with two new, good quality, deep-cycle house batteries of 100Ah each would supply 100Ah of energy to consume between charges, if you only use the recommended 50% of available charge between each charge cycle to protect the batteries.
From this you could run:
- a modern 12Vdc fridge (approx. 1.5Ah, or 36Ah over 24hrs),
- all LED lighting (say 20Ah per day),
- various small device chargers (20Ah)
- and a number of other items such as water pumps, TVs and stereos (25Ah/day)
- Totalling around 100Ah.
- For this you’d need 400W of solar capacity.
Of course, if you like to run a lot of AC devices off-grid such as hair dryers, microwaves, toasters and the like, then you’re going to need a DC/ AC inverter, which will take you to another level in power consumption terms.
But even then, with careful planning, solar could provide a large portion of the power you need before resorting to engine charging or a generator.
THE AVAILABLE SPACE
In practical terms, a modern 40ft monohull would have the space for around 1,200W of PV panels (cockpit arch, sprayhood top, deck), maybe 1,500W with the addition of a few portable panels for use at anchor.
The 1,200W of fixed position solar array could produce around 360Ah on a sunny summer’s day (zero shading) or more likely 250Ah on the average UK summer’s day.
So that’s enough for your 100Ah general DC consumption plus another 150Ah of AC consumption via the inverter.
Of course, to do this you’ll most likely need to increase your battery capacity to around 400-500Ah for maximum flexibility (you’ll need to store as much as possible during daylight hours), a typical figure for a 40-50ft offshore cruising yacht these days.
Typical daily inverter loads for a cruising yacht off grid might be:
- induction cooking plate (20min) 60Ah
- microwave (15min) 30Ah
- coffee maker (20mins) 25Ah
- hair dryer (5min) 15Ah
- laptop charger (2h) 10Ah
- or around 140Ah in total.
The trick is to monitor the batteries’ state of charge (SOC) at all times and vary your use of the inverter to suit.
For example, you might want to cook supper mid-afternoon, when solar is in abundance, and then reheat it in the evening when you want to eat it.
In some cases, when you’re cruising in warm climates such as the Med, you might end up with excess charge from your solar panels.
In this situation, many long-term cruisers devise a method of ‘dumping’ the extra energy by heating water for showers.
Do bear in mind if you’re planning to live aboard full time, then it’ll be a whole different story on cloudy days and during the winter, when inverter use might need to be knocked on the head entirely.
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There’s often confusion as to how much power you can harvest from a solar installation.
A PV panel is nearly always advertised stating its theoretical peak output power (Pw).
But in reality, on a yacht where there are limited areas in which to mount them, they will more likely produce a maximum of 60% of their peak output if mounted horizontally, increasing to 80% if tilted towards the sun and regularly adjusted.
The latter is rarely achievable on a boat, however, as even at anchor it can swing through an arc of 180° in wind or tidal shifts.
Having trawled through hundreds of ‘deals’ to get the best price on the most efficient panels you can afford you now need to know how to install them to best fulfill your energy generation needs.
The output, even from the highest quality photo-voltaic array, will only be as good as the installation itself.
So following our guidelines should ensure you extract every last drop of energy from your investment.
Sailing boats are not the ideal structure on which to mount wide, flat PV panels.
So before you go ahead and purchase what looks like the biggest and best, take a few minutes to decide on exactly where you can mount them, as this will affect what size and type of panels you should buy.
In many cases the first choice would be on an arch, davits or gantry aft, especially if you already have, or plan to fit one.
These allow a solid metal framework to be constructed that will be strong enough to take the heavier, more productive rigid PV panels.
You can also build in some form of adjuster to the framework that will allow the panels to be orientated towards the sun for the best performance.
With luck (or careful planning) a gantry will also keep them aft of the boom, thereby eliminating loss of output caused by boom shading.
The next most popular position for mounting the panels is on a cockpit sprayhood or bimini, although this will often mean using the flexible or semi-flexible panels, which are generally less efficient than the rigid ones for the same area.
Alternatively, there are kits available for mounting panels onto lifelines, which can allow their elevation to be manually adjusted to a certain degree.
Finally, panels can be fitted directly onto the deck by either gluing them down using mastic or attaching them onto a rigid support frame.
Once again you will probably need to use semi-flexible panels – especially if the deck surface is curved.
Rigid, glass-coated panels will obviously not be suitable for deck mounting in an area that is frequently walked over.
Don’t be tempted to drill through the panels, even along the edges, as this will invalidate the warranty and possibly damage the panel.
It might seem obvious, but the key to an efficient system is to avoid shading wherever possible.
It’s no good fitting expensive, high-efficiency PVs right under the boom as they’ll perform little better than the cheaper types.
Saying that, in good quality panels each cell will be isolated from the next by a series of diodes (one-way electrical valves), so that if one cell is shaded at least it won’t drag down the other cells within the same panel.
Older panels often didn’t have these, so the slightest partial shading caused the output of the entire panel to cease.
Another important factor that is often ignored when installing the panels is that of overheating.
If a PV panel gets too hot, which is quite likely if mounted directly onto a flat surface without an air gap behind, its output will drop quite noticeably.
To allow for some air circulation behind the panels it’s best to apply mastic adhesive in numerous large dabs.
This is best achieved by placing wooden spacer strips between the dabs until the mastic has completely cured, after which the spacers can be removed.
You might need some form of trim around one or more of the outside edges, though, if they are positioned where sheets and other lines might get caught under them.
Raising the panels up will also help water to drain off and thereby helping to avoid possible delamination from sitting in water for too long.
A PV module cannot supply an electrical device directly due to the changeability of the sunlight, which in turns varies the current it can produce.
Therefore, it has to be connected to a battery, which stores and smooths its output.
Whatever the size of your solar array you will need to fit a regulator, or charge controller as they are now more commonly known, to the system in order to control the output and to help extract as much power from the panels as possible.
There are two types of PV charge controller.
The older designs, called Pulse Width Modulation (PWM) types, were fairly basic voltage regulators and simply output volts at just above battery level.
The latest controllers use Multi Power Point Tracking (MPPT) technology and can accept much higher input voltages (up to 240Vdc).
MPPT controllers can be up to 30% more efficient as they use the peak output of the panels to charge the batteries, even compensating for partial shading.
BEWARE FAKE GEAR
If you buy online do be careful to ensure you’re getting what you pay for.
There are a huge number of fake MPPTs out there, which are simply the much cheaper PWM dressed up with fake labels.
It’s hard to tell which is which, but the old adage of ‘if it looks too good to be true, it usually is’ makes good sense.
MPPT controllers are usually bigger and heavier than PWMs, but if in doubt call or email the supplier to discuss the pros and cons of their kit before buying.
If they’re not happy to chat and advise you then I would steer clear of their gear.
Some good MPPTs are made in China, but unless they have a UK supplier, I wouldn’t bother with them as you’ll have no follow-up advice.
To calculate what size controller you need simply divide the panel’s peak power in Watts (Wp) by the battery voltage, which will give you the maximum current (Amps) they could theoretically supply.
For example 240W/12V = 20A. Although it’s unlikely you’ll ever get near the peak output from any PV panel, it’s best to go for the maximum possible.
PV panels come with a short length of cable, usually around 1m long.
Some are supplied with MC4 connectors already attached but most only provide bare wires.
The latter can be easily extended using proper waterproof connections but thought must be given as to the current rating and voltage drop (usually max 3%) for the size of cable you intend to use.
If in doubt, bigger is better!
Panels can sometimes be ordered with the wiring on the back so that the cable can go straight below deck through a hole under the panel.
SERIES OR PARALLEL?
A commonly asked question is ‘should I wire my PV panels in series or in parallel?’
The simple answer is, if there’s any danger of frequent shading to one or more of the panels then install them in parallel.
If wired in series the shading of a single panel will drag down the output from all of the others in the same series.
PARALLEL IS PREFERRED
Most commonly, multiple panels are wired together in parallel to a single charge controller, with diodes protecting each panel from discharging the others should one become partially shaded.
With the advent of MPPT controllers, however, there can sometimes be a benefit to wiring two or more identical panels into a series bank, thereby presenting a higher voltage to the controller.
It’s worth noting that, like batteries, wiring PV panels in series increases the voltage only – the current capacity of the array remains the same as for a single panel.
‘Where’s the benefit of wiring them in series then?’ you might ask.
Well, the higher the voltage fed into the MPPT, the more consistent it will be with its output, which could, in some cases, prove more efficient than a parallel installation with PWM controllers.
It’s also likely to be necessary if you have a 24V domestic system.
Series wiring is usually only done when the cable runs are long, as it helps negate the voltage drop caused by the resistance of the cable.
While a decent controller will have no problem handling the output from four or even five panels wired in series, it is often inappropriate for sailing yachts as shading just one of the panels will reduce the output of the entire series array.
If you need to do so in order to reduce cable runs then it’s best to split the panels between each side of the boat – a series bank on each side.
If you do this, then you would ideally fit a separate controller to each series PV bank and then connect their outputs together in parallel to the battery bank.
Note, however, that panels wired in series must all be the same types with an equal number of cells per panel.
Furthermore, the charge controller needs to be sized for the total of all panel voltages added together and the current rating of one individual panel.
Differently rated panels can be connected together in parallel but only if each panel has its own controller.
The outputs of the individual controllers can then be joined together to go to the battery bank.
BATTERY BANK QUESTION
Another frequently asked question is ‘Can I connect another charging source to the battery bank while the solar array is charging?’
The answer is yes.
Any decent PV controller will be protected against feedback from other charging sources.
CABLE SIZE AND CONNECTORS
A frequent cause of reduced output from PV arrays is wiring that is too small.
The resistance of a wire conductor increases in direct proportion to its cross-sectional area, so go as big as is practicable for the least cable loss.
Each panel should be supplied with the correctly sized cables for its own maximum output.
But if you’re combining panels, either in parallel or in series, you will clearly need to rate the single feed cable to suit the maximum current available at theoretical peak solar output and to minimise voltage drop.
Likewise, the cable from the controller to the batteries should be sized to suit the controller’s maximum output current and protected with a fuse.
For outside it’s important to use exterior grade cable, which is double- insulated and UV-proof.
And wherever possible use compatible weatherproof connectors (usually MC4) to those found on the panels rather than cutting off the plugs and hard-wiring them.
Field- assembly MC4 plugs are available, so you don’t have to drill large holes in the decks or bulkheads when feeding the cables through.
When joining more than one panel together try to use the approved multiway connectors; not only do they keep the wiring neat and tidy, but they also offer a greater contact area than budget terminal blocks.
If you have to use screw-type connectors make sure to fit proper ferrules to the wire first to avoid any stray wires in the multistrand shorting across the terminals.
When feeding a cable from above to below deck, try to go through an upright bulkhead where possible to minimise ‘pooling’ of water around the access hole.
Also, use a proper watertight deck seal that matches the cable you’re using.
If drilling through a cored deck you need to drill a larger hole first, fill it with epoxy resin and then drill the required size hole through the epoxy to ensure no water gets into the deck core.
Ideally, the charge controller should be mounted no further than 2m from the battery bank.
If you need to go further, you’ll require larger cabling to reduce the voltage drop.
CONTROLLER LOAD TERMINALS
There is often confusion over the ‘load’ output of a charge controller (often depicted by a light bulb) and what can safely be connected to these terminals.
Rarely explained in the manual, the load terminals should be pretty much ignored in a marine installation as the output on these terminals is usually very limited (10A max).
Some attach an LED light to them to indicate the controller is operating, but all your usual electrical loads should remain connected to the batteries with the battery terminals on the controller connected directly to that battery bank via a fuse.
It is possible, though, to control a high-current switching relay in certain conditions.
Unlike most cheap PWMs, the majority of good quality MPPT charge controllers come with an alphanumeric LCD screen to let you know what is going on.
This can either be a remote display or simply one on the front of the box.
It’s obviously a lot better to have a proper numerical display than to rely on a few flashing LEDs to tell you when something’s not right.
So if your chosen controller doesn’t have one be sure to fit a battery monitor (the shunt type) into your solar circuit between the controller and the batteries.
It doesn’t have to be a very ‘smart’ monitor, just one that can display the voltage and current being supplied by the panels.
For smartphone addicts there are several wifi apps that will do the job remotely on your phone or tablet.
All good quality PV panels feature built-in diode protection between each cell to prevent a shaded cell from dragging down the productive ones.
In addition, there will be internal blocking diodes on the final output to protect the panel from polarity reversal and to ensure that the batteries can’t discharge back into the panel during the night.
The latter can be added externally, the former can’t, so check before you buy.
A fuse, rated just above the maximum current available, should be fitted between each panel and the charge controller.
Another fuse should then be installed between the charge controller’s output and the batteries.
In the case of multiple arrays, this second fuse will be rated higher than the individual panel fuses and should match the maximum current rating of the cable.
With this protection installed other charging devices can be connected in parallel at the battery, meaning the solar can be left connected even when you are hooked up to shore power and the battery charger is operating.
In some circumstances, however, this arrangement can affect the sensing of the battery by the charger, causing it to fall back into float mode.
If this becomes apparent it can be overcome by installing a manual/auto switch to disconnect the solar array when on shore power.
EXCESS POWER DUMPING
A solar charge controller works by disconnecting the supply from the PV panels when the batteries are fully charged.
But for some full-time liveaboards in sunny climates that can be considered a waste, when the excess power could be put to good use – heating water, say.
This is commonly done using an inverter to supply AC power to the heating element.
Alternatively, you can now buy a 12Vdc element for your calorifier (hot water tank) and supply this directly from your battery bank.
Both of these methods would require a voltage sensitive relay (VSR) to disconnect the element should the battery voltage drop below a pre-set level.
Don’t expect boiling hot water, as there will probably only be enough spare power to take the chill off it before your battery bank reaches its lower threshold voltage.
A 600W/12V element will draw some 50A, from the batteries, whereas a 1kW AC element run through an inverter will need close to 100A.
RIGID, FLEXIBLE, OR SEMI FLEXIBLE?
Despite massive recent improvements in semi-flexible panels in recent years, the solid glass panels still offer a higher power density.
That said, they are heavier, more awkward to mount and can’t be walked on, so unless you have a dedicated gantry aft, you’re better off choosing the more rugged semi-flexibles.
Modules incorporating monocrystalline cells also have a better output than those with polycrystalline cells (that’s cells made from a single slice of silicon as opposed to layers of smaller pieces).
Output voltage also depends on the number of cells on the panel.
In the past this has commonly been 32, but now some 36 and even 40 cell panels are available.
That said, they’re larger, of course, so an array of interconnected smaller panels might be a better solution.
Module efficiency is now more often around the 20% mark, as opposed to 12-15% for older models and semi- flexible (up to 20° bend) are usually better than flexible (up to 180° bend).
There are a huge number of panels on the market, but many use the same cells.
If the maker is offering a 25-year guarantee instead of a 3-5 year one, you can be pretty confident they’re good.
When it comes to charge controllers it’s definitely worth paying a little more for a decent MPPT.
A cheap PWM might be okay just to keep a small starter battery charged with a 30W panel, but the MPPT will give you much more when it comes to heavy service.
Victron are probably top of the range, while cheaper brands like MakeSkyBlue and EPever are also good value – but treat imports of unclear origin with care.
ABOUT THE AUTHOR
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