Seven o’clock in the morning, moored alone in a stunning bay, but suddenly the idyll is interrupted by the shrill sound of the battery alarm. The anchor light must have used up the remaining charge overnight – the charge that was still in the battery banks after two days and nights of sailing without a shore connection. Moments like these make one think about a self-sufficient energy supply.
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Power supply has always been a challenge on sailing yachts. Whilst more energy-efficient appliances, such as LED lights, are now available, an increasing number of ever-more-power-hungry devices have also found their way on board, including large multifunction displays, AIS, mobile internet routers, laptops, tablets and smartphones.
A no-fuss solution for every situation
Of course, in the aforementioned scenario, one could simply start the disengaged boat’s diesel engine to generate electricity via the alternator. However, this is neither efficient nor convenient. Diesel generators offer a more favourable cost-benefit ratio, but they also consume fuel and thus reduce the range. Fuel cell generators offer an alternative. They operate efficiently, quietly and with virtually no emissions, but are quite expensive. Furthermore, the fuel – methanol – is not readily available everywhere.
Wind and hydro generators supply the much-needed electricity without using any fuel at all. The former, however, require moving air, although there must not be too much wind. Furthermore, one has to put up with a certain amount of noise and, at times, vibrations as well. Towed generators, on the other hand, only work whilst the vessel is underway; they are not a solution for extended periods at anchor without shore power.
Solar panels are considerably less demanding, as they require neither wind nor propulsion. What’s more, there are no moving parts, meaning minimal wear and tear and little maintenance. As soon as the sun rises in the morning, you can expect a corresponding yield. The extent of this yield depends on various factors that should be taken into account when designing an on-board solar system.
How much electricity is required?
The first step is to carry out a thorough assessment of requirements: how much electricity is needed, and for how long should the yacht be able to remain self-sufficient? There are at least as many answers to this question as there are ideas of what constitutes the perfect sailing trip. Someone who connects to shore power every other day will need fewer reserves than someone who prefers to anchor or is even planning to sail round the world.
And there are also differing preferences when it comes to the electrical appliances required: some need nothing more than the anchor light at night, whilst a paraffin lantern hangs below deck. Others value a certain level of comfort, with a running fridge, internet access and entertainment. Then there’s the equipment: is there an inverter on board to power a laptop, for example? Is an induction hob or perhaps even an air-conditioning system in use? Not to mention a yacht with an electric propulsion system.
Everyone has to work this out for themselves. A table listing all appliances and their average daily operating times is helpful here. The power consumption of each appliance in watts (W), multiplied by the number of daily operating hours (h), gives the energy requirement in watt-hours (Wh/day). Adding all these figures together gives the total daily requirement.
How much electricity a person and a boat use in 24 hours
| Consumers (example details) | Number | Power per appliance (W) | Total power (W) | Duration (hours per day) | Consumption (Wh/day) |
| LED anchor light (overnight) | 1 | 2 | 2 | 8 | 16 |
| LED position lights (at twilight) | 3 | 2 | 6 | 2 | 12 |
| Fridge (25 per cent of the rated power of 40 W) | 1 | 10 | 10 | 24 | 240 |
| Interior lighting (in the evening) | 4 | 2 | 8 | 4 | 23 |
| Plotter (on the move) | 1 | 10 | 10 | 5 | 50 |
| Autopilot (interim use) | 1 | 24 | 24 | 3 | 72 |
| Logge/Lot (on the move) | 1 | 5 | 5 | 5 | 25 |
| Instruments (whilst on the move) | 2 | 5 | 10 | 5 | 50 |
| VHF radio (used briefly, otherwise on standby) | 1 | 10 | 10 | 0.5 | 5 |
| Pressure water pump (short-term use) | 1 | 40 | 40 | 0.2 | 8 |
| USB charging for mobile phones/tablets (quick top-up) | 2 | 20 | 40 | 2 | 80 |
| Mobile 4G/LTE router (USB) | 1 | 7.5 | 7.5 | 4 | 30 |
| Entertainment (radio/television) | 1 | 25 | 25 | 2 | 50 |
| Miscellaneous/Reserve | 50 | ||||
| Total (Wh/day) | 720 |
The biggest power guzzler is the fridge, followed by USB charging and autopilots. To convert watt-hours to ampere-hours: Wh / V (nominal voltage of the batteries). 720 Wh therefore corresponds to 60 Ah in a 12 V system. With conventional batteries, a battery with a capacity of around 120 Ah is required to meet this demand. If LiFe batteries are used, 67 Ah is, in principle, sufficient.
The more detailed the breakdown, the more reliable the result. Whilst a pressurised water pump has a higher power consumption than a chart plotter, it rarely runs, whereas the navigation electronics are in operation for several hours. In the case of a fridge that runs continuously, the average power consumption is actually below the rated power because the compressor switches off intermittently; in this case, an estimated value is used.
What type of load is required?
Next, we turn our attention to the on-board battery capacity, which is usually measured in ampere-hours (Ah). A 120 Ah on-board battery can, in theory, supply 120 amperes (A) for one hour or deliver 5 amperes over 24 hours. However, this depends on the type of battery: whilst lithium iron phosphate batteries can be discharged almost completely, thereby providing virtually their entire capacity, 90 to 95 per cent, lead-acid batteries – including AGM and gel batteries – can only be partially discharged without compromising their service life.
As a guideline, 50 per cent of the rated capacity can be assumed. A lead-acid battery with a capacity of 120 ampere-hours therefore realistically provides only 60 ampere-hours. Multiplying this by a nominal voltage of approximately 12 volts (V) gives 720 watt-hours (60 Ah × 12 V = 720 Wh). This capacity is available from a fully charged lead-acid battery without recharging.
Now the available capacity is compared with daily consumption. This tells us the maximum length of time we can manage without recharging. Let’s assume that, so far, the 720 watt-hours have been enough to last us one day – 24 hours. In future, we want to spend two additional days under sail and at anchor. This would mean an additional requirement of 1,440 watt-hours (2 × 720 Wh). These would need to be generated over the three days in total during which we wish to be self-sufficient, i.e. 480 watt-hours per day (3 × 480 Wh = 1,440 Wh).
The actual requirement also depends on the circumstances. If we set off during the day with a fully charged battery, it naturally cannot take on any further charge initially. Furthermore, engine running times are not taken into account. The example calculation is intended merely as a guide.
Theory and reality
If we assume 8 hours of sunshine per day, the solar panel would need to deliver an average of 60 watts to provide the required electricity (8 h × 60 W = 480 Wh). At first glance, this does not seem like much – a 60-watt panel is neither large nor expensive – but there is a significant gap between theory and practice. The output of solar panels is usually specified in watts-peak (Wp). This refers to the peak value under ideal conditions. A 60-watt-peak module could certainly deliver 60 watts, but only under optimal sunlight conditions. However, the intensity of sunlight varies with the sun’s position in the sky and also throughout the year. Geographical latitude is another factor.
The website of the solar panel manufacturer Sunware (www.sunware.solar) provides comparative figures for various regions. According to these figures, a 100-watt-peak module in Split, Croatia, generates 545 watt-hours per day in July, whilst in Kiel the figure is only 370 watt-hours. In September, the daily yield in Kiel drops to 207 watt-hours, whilst in Split it is still expected to be 348 watt-hours. These figures represent monthly averages, with days of poor weather already factored in.
The average yield in Kiel from May to September is around 325 watt-hours per day. This corresponds to 3.25 peak sun hours (PSH) per day; this unit refers to the hours with equivalent solar irradiance of 1 kilowatt per square metre (0.325 kWh : 0.1 kW = 3.25 h). For the 480 watt-hours we require, a 100-watt-peak module would therefore not be sufficient on the Baltic Sea coast here. We would need approximately 1.5 times the output (325 Wh × 1.5 = 487.5 Wh), i.e. a 150-watt-peak module. Provided, of course, that there are no power losses due to unfavourable orientation or shading. Losses can also arise from cabling and the controller. If you want to know the exact figures, you can calculate these line losses as well.
Cell types and space required
The amount of surface area required to achieve the desired output depends on the efficiency of the cells. Solar modules are usually made up of several cells connected in series, consisting of monocrystalline or polycrystalline silicon. Monocrystalline cells can achieve an efficiency of around 20 per cent. Further developments, such as PERC (Passivated Emitter and Rear Cell), IBC (Interdigitated Back Contact) and HJT (Heterojunction Technology), increase efficiency to up to 25 per cent.
Polycrystalline cells have a lower efficiency, ranging from 15 to 18 per cent. Thin-film cells – amorphous cells – are slightly less efficient, but are quite flexible and are also suitable for partially shaded areas. Today, silicon-free CIS and CIGS cells are also used in flexible modules, delivering impressive performance. So, if you’re prepared to invest in efficient modules, you’ll need less space. For a 150-watt-peak solar module, you should allow between 0.7 and 1.2 square metres, depending on the design. Using multiple modules increases the space required.
The right place
As a general rule, the less shade there is and the more directly the panels face the sun, the higher the yield. This is why many blue-water yachts are fitted with a sturdy mounting frame at the stern, on which the solar panels can operate unhindered; in some cases, the angle of the panels can even be adjusted for optimum alignment. For those who do not wish to install such a structure, there are a wide range of alternatives. The market now offers a practical solution for almost every type of boat and requirement, from fold-up modules for the gunwale to walk-on variants for the cabin superstructure, right through to flexible designs for the sprayhood and bimini. There are even pre-heated modules available for the mast. In addition, there are special designs for the boom and sails.
If the only requirement is a power supply whilst at anchor, mobile options that can be set up on deck, or modules that can be hung from the railings, would also be feasible. If the system is also to work whilst underway, a mounting bracket would be the best choice, or you could look for suitable solutions for permanent installation on deck or on the cabin roof.
Based on the example given, with a power requirement of around 150 watts peak, the average price for corresponding individual modules is around 350 to 450 euros. Simple entry-level models are sometimes available at lower prices, depending on the design and what’s included in the package; however, kits featuring mounting systems, charge controllers, etc., can cost up to 1,000 euros or more, for example for high-performance cells as well as foldable and flexible versions. If space on deck is limited, it might also be worth considering investing in increased battery capacity at the same time, to reduce the need for additional solar power. Either way, investing in solar energy pays off in the form of greater freedom, as it frees up more time to spend on the water in a relaxed and emission-free manner.
The right controller
To charge the on-board batteries efficiently, a suitable charge controller is required, which is connected between the solar panel and the on-board battery. It ensures an optimum charging current and protects against overcharging. Furthermore, the charge controller should be matched to the specific panel and battery type.
Technologically, two types dominate: PWM (Pulse Width Modulation) controllers and MPPT (Maximum Power Point Tracking) controllers. The former are considered simple, inexpensive and robust. MPPT controllers are more complex in design and more expensive – but also significantly more efficient. They feed almost all the energy supplied by the solar module into the battery bank.
Some models have multiple outputs, for example to charge the starter battery at the same time. If several modules are connected in series, the charge controller must be able to handle the higher voltage. Parallel connections, on the other hand, increase the current. High charging currents require suitably sized cables and particular care during installation.
Please note: Solar charge controllers are specifically designed for photovoltaic systems and must not be used with other power sources!
Practical factors that reduce solar yield
- Shading: Even small shadows – caused, for example, by a swinging boom, the rigging, the shrouds, a block or the spreaders – can significantly reduce the output of a solar panel. In practice, a loss of output of between 10 and 30 per cent is not uncommon.
- Alignment with the sun: A solar panel can only achieve its maximum output when it is exposed to optimal, as vertical as possible, sunlight. Solar panels that are consistently laid flat or tilted at an unfavourable angle to the sun reduce the average daily yield. The extent of this reduction depends on the season.
- Temperature: High module temperatures reduce efficiency. In the Mediterranean or in tropical regions, this can result in a further loss of 5 to 10 per cent.
- Cabling and charging technology: Losses caused by line resistance and substandard controllers can add up to a further 5 to 10 per cent.
Book recommendations
Seamanship covers key areas of yacht equipment, boat care, maintenance and electronic navigation. Readers are provided with guidance on day-to-day technical matters on board and are better equipped to understand how to select and operate their equipment.
Safe Sailing at Sea focuses on the safe use of a sailing yacht. Among other things, the guide covers the risks associated with on-board electrical systems and digital aids such as AIS and GRIB files.
How much technology does a yacht really need to be self-sufficient for several days? Share your experiences of using solar power on board in the comments.
Sven M. Rutter
Freier Mitarbeiter