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  • Writer's pictureYannick Slock



The advancement in solar technology has significantly enhanced the feasibility of self-sufficient cruising. Eu Yachtbroker offers comprehensive insights into the essentials for optimizing your boat's capabilities.

Solar power has emerged as the foremost and cost-effective means of maintaining battery charge on a boat. This trend is reinforced by the continuous improvement in the efficiency of photovoltaic (PV) panels, charge controllers, and batteries. Additionally, the latest advancements in regulators and charge controllers have notably increased usable power output, enabling easier compensation for issues such as shading and misalignment.

Notably, the efficiency and affordability of PV equipment have seen remarkable improvements, while many modern devices used on boats have become less power-intensive. Consequently, harnessing natural energy resources, especially solar power, to meet the complete electrical requirements of your yacht (both 220Vac and 12/24Vdc) has become considerably more feasible—even for those planning a fully electric boat.


Consider, for instance, a boat equipped with two new, high-quality deep-cycle house batteries, each with a capacity of 100Ah. Under the recommended practice of utilizing only 50% of available charge between each cycle to preserve battery life, these batteries would supply 100Ah of energy for consumption between charges.


Based on the provided details, you could effectively power:

· A modern 12Vdc fridge (approximately 1.5Ah, totaling 36Ah over 24 hours),

· All LED lighting (around 20Ah per day),

· Various small device chargers (20Ah),

· Additionally, items like water pumps, TVs, and stereos (25Ah/day),

· Summing up to approximately 100Ah in total.

To meet these requirements, you'd need a solar capacity of 400W. However, if you intend to run AC devices off-grid, like hair dryers, microwaves, or toasters, a DC/AC inverter would be necessary, leading to increased power consumption.

Even in such scenarios, with careful planning, solar power could fulfill a significant portion of your power needs before resorting to engine charging or a generator.


Practically, a modern 40ft monohull could accommodate approximately 1,200W of PV panels (cockpit arch, sprayhood top, deck), and possibly 1,500W if additional portable panels are used while anchored.

The fixed position 1,200W solar array could generate roughly 360Ah on a sunny summer’s day (without shading), or around 250Ah on an average UK summer’s day. This would cover your general 100Ah DC consumption along with an additional 150Ah AC consumption via the inverter.

To achieve this, you'll likely need to expand your battery capacity to around 400-500Ah for maximum flexibility, a typical range for a 40-50ft offshore cruising yacht in current times.


Typical daily inverter loads for a cruising yacht off-grid might include:

· Induction cooking plate (20 minutes): 62Ah

· Microwave (15 minutes): 28Ah

· Coffee maker (20 minutes): 24Ah

· Hair dryer (5 minutes): 14Ah

· Laptop charger (2 hours): 11Ah

Totaling around 145Ah, it's crucial to monitor the batteries’ state of charge (SOC) continuously and adjust inverter use accordingly.

For example, scheduling cooking during abundant solar hours, like mid-afternoon, and reheating meals later when desired. Additionally, in warm climates like the Mediterranean, excess energy from solar panels could be utilized for heating water for showers.

However, it's essential to consider that living aboard full-time might present challenges on cloudy days or during winter when inverter usage might need to be significantly reduced or even suspended.


In a scenario of off-grid cruising on a yacht, typical daily inverter loads might encompass:

· Induction cooking plate (20 minutes): 60Ah

· Microwave (15 minutes): 30Ah

· Coffee maker (20 minutes): 25Ah

· Hair dryer (5 minutes): 15Ah

· Laptop charger (2 hours): 10Ah

This accumulates to approximately 140Ah in total. It's essential to consistently monitor the batteries’ state of charge (SOC) and adapt inverter usage accordingly.

A strategic approach involves scheduling energy-intensive activities during abundant solar hours, like cooking in mid-afternoon and reheating meals in the evening when solar power is more accessible. Moreover, in warmer climates such as the Mediterranean, surplus energy from solar panels might occur. Long-term cruisers often devise methods to utilize this surplus, such as heating water for showers.

However, living aboard full-time presents challenges on cloudy days or during winter when inverter usage might need significant adjustments or even suspension.


There’s often a disparity between the advertised theoretical peak output power (Pw) of PV panels and their practical output on a yacht. Typically, due to limited mounting areas, PV panels tend to produce around 60% of their peak output when horizontally positioned, increasing to 80% when tilted towards the sun and regularly adjusted, a practice rarely achievable on a boat due to its movement.

Proper installation is crucial for optimal performance. Even the highest quality photo-voltaic arrays deliver results only as good as the installation itself. Following guidelines can ensure maximum energy extraction from your investment.


Sailing boats pose challenges for mounting wide, flat PV panels. Careful consideration is necessary before purchasing panels to determine suitable mounting locations, impacting the size and type of panels you should invest in.

Aft placement on an arch, davits, or gantry is often a preferred choice, particularly if already existing or planned for installation.


Building a solid metal framework facilitates the installation of sturdy and robust rigid PV panels. Incorporating an adjustable feature in the framework allows for optimal orientation of panels towards the sun, thereby enhancing performance. Strategically positioning panels aft of the boom via a gantry prevents output loss caused by boom shading.

Another prevalent mounting position for panels is atop a cockpit sprayhood or bimini. However, this often involves utilizing flexible or semi-flexible panels, which generally exhibit lower efficiency compared to their rigid counterparts covering the same area.


Alternatively, specialized kits are available for mounting panels onto lifelines, enabling manual adjustment to a certain degree. Another option involves directly affixing panels onto the deck, either by adhering them with mastic or attaching them onto a rigid support frame. This method typically requires the use of semi-flexible panels, especially on curved deck surfaces, as rigid glass-coated panels are unsuitable for areas frequently traversed.

It's crucial to avoid drilling through the panels, even along the edges, as this could void the warranty and potentially damage the panel.


Efficient system performance relies on minimizing shading whenever possible. Placing high-efficiency PVs directly under the boom diminishes their effectiveness, rendering them little better than cheaper alternatives. However, quality panels often incorporate diodes to isolate shaded cells, preventing a drop in output from affecting other cells within the same panel.

Overheating is another critical concern during panel installation. Panels mounted directly onto flat surfaces without an air gap behind them are prone to temperature-induced output reduction. To mitigate overheating, allowing for adequate air circulation behind the panels is crucial. Applying mastic adhesive in several large dabs and utilizing wooden spacer strips during the curing process aids in creating the necessary airflow. Elevated panel mounting also facilitates water drainage, preventing potential delamination due to prolonged exposure to water.


A PV module cannot directly power an electrical device due to sunlight's variability, affecting the current it generates. Therefore, connecting it to a battery becomes necessary to store and stabilize its output. Irrespective of the solar array's size, fitting a charge controller to regulate the output and optimize power extraction from the panels is crucial.

There are two types of PV charge controllers:

· Older designs: Pulse Width Modulation (PWM) types were basic voltage regulators, providing volts slightly above battery level.

· Latest controllers: Utilize Multi Power Point Tracking (MPPT) technology, capable of accepting higher input voltages (up to 240Vdc). MPPT controllers, being up to 30% more efficient, leverage the peak panel output to charge batteries, even compensating for partial shading.


When purchasing online, exercise caution to ensure authenticity. Numerous fake MPPTs flood the market, resembling cheaper PWMs with counterfeit labels. Authenticating the product can be challenging, but the adage 'if it looks too good to be true, it usually is' holds wisdom. Engage with suppliers, discussing the advantages and drawbacks of their equipment before purchasing. Be cautious of suppliers unwilling to provide information or advice.


Calculating the required controller size involves dividing the panel’s peak power in Watts (Wp) by the battery voltage, determining the maximum potential current (Amps) they could supply. For example, 240W/12V = 20A. It's advisable to opt for the maximum possible controller size, although reaching the peak output from any PV panel is unlikely.

PV panels typically come with a short cable, usually around 1m long. While some feature MC4 connectors, most offer bare wires. Extending bare wires with waterproof connections requires consideration of current ratings and voltage drops (usually max 3%) for the cable size intended for use. If uncertain, using larger cables is recommended.

When dealing with potential shading issues, installing panels in parallel is preferable to wiring them in series. Shading of a single panel in a series affects the output of all panels in that series.



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.


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.


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.


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.


· Good quality PV panels include built-in diode protection between cells to prevent shaded cells from affecting productive ones.

· Internal blocking diodes are present on the final output to safeguard against polarity reversal and prevent batteries from discharging back into the panel at night. Ensure these features are included before purchasing.

· Install a fuse, slightly above the maximum available current, between each panel and the charge controller. Another fuse, matching the maximum current rating of the cable, should be placed between the charge controller's output and the batteries.

· When multiple arrays are used, the second fuse rating will be higher than individual panel fuses. This setup allows other charging devices to connect parallelly to the battery, enabling solar connection even while utilizing shore power with a battery charger.

· In certain cases, this setup might affect the charger's battery sensing, causing it to revert to float mode. This can be addressed by installing a manual/auto switch to disconnect the solar array when on shore power.


· Solar charge controllers halt PV panel supply when batteries reach full charge. In sunny climates, excess power can be utilized for other purposes, like water heating, using an inverter to supply AC power to the heating element.

· Alternatively, a 12Vdc element for the calorifier can be directly powered from the battery bank. Both methods require a voltage-sensitive relay (VSR) to disconnect the element if battery voltage drops below a set level.

· Expect limited heating due to spare power, for instance, a 600W/12V element drawing around 50A or a 1kW AC element via an inverter needing close to 100A.


· Despite significant improvements in semi-flexible panels, solid glass panels offer higher power density. However, they are heavier, more challenging to mount, and can't withstand walking. Semi-flexible panels are a better choice without a dedicated gantry.

· Monocrystalline cell modules have better output than polycrystalline ones. The number of cells on a panel influences output voltage, with larger panels available, necessitating interconnected smaller panels.

· Module efficiency has increased to around 20%, contrasting with older models at 12-15%. Semi-flexible panels (up to 20° bend) generally outperform flexible ones (up to 180° bend).

· While numerous panels are available, many use similar cells.





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