SOLAR POWER FOR CRUISING YACHTS
(Most of this paper is also applicable to RVs)
Many articles I have read about this topic seem to focus on how many Watts of panels should be purchased compared to a detailed forensic examination of onboard power requirements invariably presented in neat tabular form. In reality there are many complex variables at play that are very difficult for the average cruising sailor to calculate. Let’s face it, we sail with shade on our panels, often in varying degrees of cloud cover and occasionally need to cool more beer on really hot days than the legislated 2 cans per man, per day, perhaps, suggests. Time for a bit of experienced based rule of thumb approximation and cold hard reality to be applied. Be warned, there are some ugly truths ahead, so parental guidance is recommended.
FIRST THE BATTERIES
I reckon that because batteries are the epicenter of the power supply system, we need to give some serious thought about them at the outset. Good batteries are expensive and need to be given loving care.
Boat batteries are often hidden away in a relatively inaccessible area and as a consequence are difficult to maintain. Wet cell batteries also give off significant amounts of hydrogen gas when being charged and there is a potential risk of explosion in poorly vented enclosures. On our yacht, the house batteries are located beneath the galley sink cupboard and are quite near the stove. Absorbed Glass Matt (AGM) batteries require very little maintenance and an occasional check on the terminals as well as a visual check of the case is about all that is necessary. Our ship is fitted with Concorde Lifeline AGM batteries that have very low hydrogen release figures to negate the risk of explosion, but you might find others that perform well also. I can see very little reason to choose anything other than an AGM battery, they will not spill acid even if your ship is knocked down or rolled. If you choose AGM make sure you pick a really good one and it will last around 6 to 8 years if you treat it right. Any serious manufacturer should be able to provide a detailed manual for their batteries so that you can make informed choices.
Even the best batteries lose some capacity over time (check the manuals), so it is worthwhile adding more battery capacity than bare minimum calculations with brand new batteries suggest.
If you hate your battery, routinely discharge it well below 12.2 Volts and leave it in a discharged state for long periods and you will be rewarded with early battery failure. If you value your battery make sure that you rarely if ever discharge it below 50% capacity which is determined by a Voltage reading of 12.2 Volts. Also ensure that you fully recharge it daily if at all possible. The battery Voltage should be measured with all charging sources disconnected and a light load (such as a cabin light) connected.
NOW THE PANELS
How many Watts do you need? On most yachts the answer is simple, buy as many solar panels as can be conveniently fitted. Multihulls cheat because they have large areas where panels can be mounted and consequently they have the luxury of being able to generate large amounts of solar power.
The adoption of domestic rooftop solar power supplies for houses has led to a significant fall in the price of solar panels so this means that there is not much sense in being miserly with panels on your yacht. If you already have some panels in good condition and wish to add more, there seems to be no significant problem with mixing panels of a different type, eg amorphous, polycrystalline, monocrystalline. Mixing panels of different Wattage ratings is quite OK as long as they are connected in parallel. You need to be careful that you only mix panels of a similar nominal system Voltage, eg 12 Volts with 12 Volts or 24 with 24 Volts, if you are connecting panels in parallel.
A useful quick test to see if one panel in an array is working is to place a broad brimmed hat over the working surface of the panel. This should cause the output current of the array to fall by almost the single panel's current rating. A more accurate test is to place a suitably rated Ammeter across the panel terminals and note the current. Check the panel's data sheet and compare your reading to the short circuit current rating (Isc). Obviously the panel should be fully illuminated by the Sun.
SOME UGLY FACTS
Solar power is produced by sunlight illuminating the panel. The amount of solar power produced depends on how much sunlight illuminates the panel. The amount of sunlight varies throughout the day and throughout the year. Shade reduces the amount of sunlight illuminating the panel and therefore the amount of power produced.
The Wattage (power) rating of the panels is a theoretical guide to the amount of power produced by near perfect solar radiation on the panel. Solar radiation on Earth is almost never perfect due to atmospheric effects. This means that the actual power rating is somewhat less than this.
The maximum power produced by panels only occurs at a particular Voltage and current but regrettably this virtually never occurs at the Voltage necessary for efficiently charging the battery. This can cause losses of around 30% when panels are not operated at the maximum power point (MPP). NASA employed a device known as a Maximum Power Point Tracker (MPPT) to significantly reduce this loss on solar powered spacecraft. An MPPT does not cause panels to follow the sun but it does match the electrical characteristics of the panel to the battery in order to achieve maximum charge efficiency. Suitable MPPTs are now readily available.
When sun shines on a solar panel, it raises the temperature of the panel. Raising panel temperature reduces panel power output by around 5% for every 10 degrees (C) above 26 degrees (C).
Panel power output of many panels degrades with age by about 1% per annum.
Most panels on yachts are mounted horizontal to the sea surface due to the fact that yachts swing around at anchor and sailors rarely attempt to track the sun. Horizontal mounting reduces daily power output to around 50% of a properly installed domestic solar array.
Solar panel regulators protect batteries from overcharging. Solar panel regulators consume power.
Batteries need more power to recharge them (perhaps 10% more) than they supplied to electrical loads such as fridges, lights etc. Batteries also lose some power internally when supplying power to loads.
In Brisbane (Australia), the average daily time (over a year) for useful solar power generation is quoted at around 5½ hours for panels mounted in a typical domestic installation
Undersize wiring causes Voltage drops and wastes power.
WHAT DOES ALL THIS MEAN?
Let’s consider a 12 Volt 100 Watt solar panel and see how much power is actually generated each day.
Information from domestic solar power installers shows that a 1000 Watt solar array produces a yearly average of 4200 Watts per day in Brisbane. Such an installation employs an MPPT and the panels face north and are tilted (around 15 degrees plus latitude).
For our 100 Watt panel a similar calculation reveals that the panel would produce an average of 420 Watts per day in Brisbane. On our yacht we need to reduce this figure to about half of this because our panels are horizontally mounted. This gives an average of around 210 Watts per (5 ½ hour) day. In truth we will probably do a bit better than this, however this gives a useful fudge factor for other approximations.
Because battery capacity is usually expressed in Ampere hour capacity, I prefer to work in Ampere hours rather than Watts. Dividing the Watts per (5 ½ hour) day figure by the battery Voltage gives the Ampere hours produced each day, ie 210 Watts/12.00 V or 17.5 Ampere hours per day.
If we think about a light that draws 1 Amp and is switched on for 8 hours per night it consumes 1 (Amp) x 8 (hours) or 8 Ampere hours. This means that we could run two such lights for 8 hours each night and still fully recharge the battery daily as long as we had reasonable weather.
A fridge is more complex because it does not run all the time. Some fridges draw 4 Amps and run for about ¼ hour each hour. Therefore it uses 4 (Amps) x ¼ (hour), per hour which gives 1 Ampere hour per hour or 24 Ampere hours per day. It is clear that our fridge would slowly flatten the battery over the next few days. For light entertainment, work out the number of Ampere hours per day that your electric toilet consumes.
If we had sufficiently more power being produced by the solar panel than was required to fully recharge the battery as a result of the fridge load, the fridge would be effectively running from the solar panel for a fair bit of the day because the solar regulator would otherwise be wasting this power if the fridge was turned off. In this circumstance the cunning sailor may choose to run the fridge at a colder temperature while excess power was available and so reduce how often the fridge would run of a night at normal temperature. Many sailors already do a similar thing when the engine is running. Anyone for beer iceblocks?
WHAT ABOUT THE REGULATOR?
Regulators are necessary to avoid overcharging the battery.
Early regulators were known as shunt regulators and had limited effectiveness in charging batteries correctly and have no place in a serious installation.
Modern series regulators should be at least 3 (or 4) stage Pulse Width Modulation (PWM) types. These will look after your battery nicely but do draw a small amount of current in order to power their operation.
The domestic power installations include an MPPT regulator (also employing PWM technology). Some fairly wild claims are made in terms of the increase in power that is available. MPPTs work best with low solar radiation, cold temperatures and low battery Voltage. Depending on conditions, expect between 10% to 30% improvement. Don’t forget that MPPTs draw current in order to operate. They may also generate annoying radio interference as do most digital devices.
Some series regulators and MPPTs have very useful metering capabilities and some even have data logging (history) of a range of parameters.
Another useful feature available on some regulators and MPPTs is night light switching. Just made for those of us who forget to turn on the anchor light before visiting another vessel at dusk, or forget to turn it off next morning.
Regulators (including MPPTs) must be matched to the maximum current that the panels can supply. As a guide, a 360 Watt panel array would require a regulator with at least 360 Watts/12 Volts = 3o Amps panel current capability. The manufacturers specifications are sometimes a little more conservative and should be followed. When buying a regulator you would be well advised to purchase one that will enable you to fit more panel capacity later if you choose.
Regulators normally have a load terminal which has a finite current capability. The load terminal is often metered by the regulator and may also have over current protection and various switching options. At times this can be inconvenient when using loads with currents greater than the load terminal can supply. The answer is to connect high current loads (eg inverters, pumps etc) directly to the battery terminals. The regulator will still charge the battery correctly. Add an ammeter in the battery line if you wish. The load terminal can still be used for other purposes such as night light switching.
With some regulators you may notice that the charging current falls for an instant on a regular basis. This can be quite normal because in order to make a valid reading of the battery Voltage, the charging source (solar panel) is automatically switched off for a brief time. This is so that the regulator is able to determine the true state of charge of the battery and adjust the charging Voltage to suit.
If you intend to add portable panels to your fixed installation at various times, be aware that most portable arrays have their own regulator. This should be removed or bypassed and the panels connected via the main installation regulator. This is necessary to avoid conflicts between the regulators that are likely to result in reduced charge currents but you do need to make sure that the main installation regulator can handle the additional current.
WHAT DO YOU NEED TO DO?
Work out all the things that run off your house battery and consult the manufacturer’s info on each device to discover how much current (in Amps) they draw and how often they run each day. This will enable you to work out the total daily Ampere hour usage. Use a neat tabular arrangement if you really need it. Don’t forget to make a generous allowance for things you may wish to add in future. You should not be surprised if all of this adds up to around 50 or more Ampere hours per day on a typical modern cruising boat.
Work how many nominal Watts of solar panels you can squeeze on your boat and use the 17.5 Ampere hour per day per 100 Watts nominal panel figure to see how many Ampere Hours per day you can reliably produce. Remember that this figure is a bit conservative and that you may get around 20 Ampere hours per day per 100 Watt panel on a yearly average. You will most certainly do much better than this under ideal conditions. Pessimism is nevertheless an essential feature of reliable design.
You should also consider the endurance of your system so that you can cope with heavy overcast conditions. Endurance is about battery capacity. If you have a normal average daily drain of 50 Ampere hours (Ah), you should have a 100 Ah nominal battery capacity per day to ensure that you only discharge to 50% capacity. Therefore 300 Ah of nominal battery capacity will provide 3 days endurance without going below 50% capacity.
Recovery will depend upon how many Ampere hours you can generate each day. Don’t forget that you will still need to generate 50 Ah per day to run all the gear on your boat as well as recharging the battery. Although your battery bank is 300Ah in this example, you have only used 150 Ah over the 3 overcast days in order to not go below 50% battery capacity. If you have fitted 4 x 100 Watt Panels you are probably going to be able to generate 4 x 20AH per day which is 80 Ah per day. So you are able to generate 30 Ah per day over and above your normal daily usage. The batteries will be fully charged in 150/30 days, which is 5 days if the weather is moderately kind.
BUGGER! I CANNOT FIT THAT MANY PANELS.
The solution is to use less power or generate more by other means.
Using less power may be possible by fitting more efficient electrical components such as LED lights or by turning off unnecessary equipment. Don’t forget the cunning sailor’s fridge trick.
A word about LED lights may be useful, particularly your anchor light which is usually mounted near your VHF antenna. Most large LED bulbs have an internal LED driver module that performs a number of functions. The driver module employs digital switching technology and generates radio interference. I have met numbers of sailors who complain about poor VHF reception while at anchor of a night. Turning the LED anchor light off usually solves the problem, but don’t blame me if you get T boned by another vessel because you left the light off too long.
When replacing incandescent lights with LED lights, a handy approximation for working out the power savings that can be made for similar light output (Lumens) is to divide the original (incandescent) light current by 6. For example if you have an incandescent light of 10 Watts it will draw 10 Watts/12 Volts or 833 milliamps. Therefore a LED light with an equivalent light output (Lumens) will draw 833/6 = 139 milliamps. It is worth remembering that LED lights do generate heat and while this is relatively little with low power LEDs, higher power LED lights can produce considerable heat and may require a heat sink.
Generating more power is usually annoying but easy enough to do. You can run the engine, run a generator, fit a wind generator or enter a marina and plug in your battery charger. None of these options are particularly cheap.
Running a diesel engine for long periods with a light load is not a good idea but you can reduce running time by fitting a smart regulator to the alternator to increase output. Conventional automotive (and boat) alternator regulators restrict maximum charge currents to ensure that conventional automotive batteries are not damaged and generally only charge batteries to about 70% of their capacity. Make sure that you install a good AGM starting battery as well because ordinary starting batteries cannot cope with high charging rates whereas good AGM batteries permit recharging at very high rates. You may also be able to complement this by fitting a high output alternator.
Portable generators often have a fairly miserable battery charging capability but this can usually be remedied by plugging a decent battery charger into the 240 Vac outlet.
Wind generators are useful because they work for 24 hours each day if the wind is blowing. Even if they are only putting out 1 Amp in light breezes you will generate 24 Ah in a day and night. In a good breeze you may be able to fully recharge your battery banks in a very short time. You also have to be dead unlucky to experience heavy overcast weather and no wind. Some wind generators employ an MPPT to ensure that output current is adjusted to ensure maintenance of the most efficient rotor speed. Most modern wind generators are a lot less noisy than their predecessors.
Your onboard battery charger should have an output of at least 20 Amps (more is better) and employ 3 (or 4) stage charging routines. If you regularly visit marinas you may consider using an appropriate 12 Volt mains operated regulated power supply and switch your 12 Volt supply line to this when in port thus saving wear and tear on your batteries and extending their life. Batteries should nevertheless be kept on trickle charge.
Care needs to be taken when attempting to use multiple charging sources on a battery because they are likely to interact and cause problems. This is where dual battery banks come in handy, for example one bank may be capable of being charged by the engine alternator maximum output (say 30Amps), while the other is charged via the solar panels at perhaps 15 Amps. This arrangement is likely to maximise the available charge current (30 + 15 = 45 Amps), whereas if the alternator and the solar panels were both connected to the the one battery bank, the alternator would perhaps be the dominant charging source and as a result the solar regulator would think that the battery was fully charged and go into trickle charge mode and total charge current would be 0nly 30 Amps or so. All of this is dependant on the state of charge of the battery and the sensing characteristics of both the alternator and solar regulators. A worse case would be if the solar regulator was dominant thus limiting charge current to 15 Amps. The easy answer is not to mix charging sources.
ANYTHING ELSE?
Connecting batteries in parallel for long periods of time is not a good idea. This is because batteries have slight differences in their internal resistance. Batteries with a higher resistance will accept less charge than their parallel companions over time which in turn may raise their internal resistance even more and further reduce their ability to accept charge. This means that the worst battery gets the least charge and the best battery gets the most. This process gradually continues and progressively reduces the battery bank capacity. At some point the worst battery becomes useless.
I vividly recall experiencing this when the original battery bank on my ship comprised 3 rather nice 100Ah AGM batteries connected in parallel. When I checked them some weeks after purchasing the boat, one was faulty but the others were fine. I disconnected the faulty battery. Over time another failed and was disconnected. The remaining battery was still in really good nick and happily soldiered on for a considerable period of time.
I eventually revamped the battery arrangement so that I did not need to parallel batteries on a permanent basis. I purchased 4 x 6 Volt 220Ah AGM batteries and arranged them as 2 separate banks, each consisting of 2 x 6 Volt batteries in series. I built up a battery switching panel that enabled me to switch between the banks when necessary. The bank supplying the house equipment always has the solar charging connected. I am able to change over the banks when necessary and I can switch the wind generator to either the unused bank or the bank in use as required. Each or both banks can be paralleled in the short term without disastrous effects. I can switch the engine battery in parallel with either or both banks and have used heavy wiring to permit the house battery to be used to start the engine if the starting battery fails. One modification that I found useful was to add a solenoid so that paralleling the house battery to the engine battery when running the engine and disconnecting it when the engine stops, became automatic. This arrangement can be turned off when necessary. I can also switch a battery charger to each or all batteries when in port. One unplanned benefit was that lifting individual 6 Volt 220Ah batteries aboard was far easier than attempting to lift 12 Volt 220 Ah batteries.
Good metering arrangements are a necessary part of ensuring that all is well. I have digital Voltmeters on each house battery bank as well as the engine battery. I am able to measure solar charging current, wind generator current and alternator charging current. Some battery meters also have a “fuel gauge” arrangement to indicate battery charge state but I am happy enough to manage my batteries with the aid of digital Voltmeters and Ammeters. As indicated previously, I regard a 12.2 Volt reading as an indication that a battery that needs recharging.
When we are at anchor and the solar panels are busy recharging the batteries, there are times when the boom shades a panel or panels. I just swing the boom to the other side of the boat and secure it there. Luckily I have arranged panels either side of the centerline of the boat and so there is nearly always a panel or panels working anyway but swinging the boom usually allows all panels to be fully illuminated. Panels mounted across the centerline of the boat are more often partly shaded and consequently produce reduced output.
Panel output can be increased by providing airflow beneath the panels thus reducing the temperature. Some solar installations cool the panels with a spray of water to increase output. A windscreen washer might do the trick here if you have plenty of fresh water available and don’t mind the occasional squirt of water (yuk!).
240Vac INVERTERS
A word or two on 240Vac inverters may be in order. Sensitive electronic equipment (ie anything with solid state components) can be damaged by Voltage spikes produced by MODIFIED sine wave inverters. If you wish to power these types of devices use a PURE sine wave inverter. Most power tools can be successfully run from (lower cost) modified sine wave inverters.
Inverters are quite efficient when the load matches their power output rating but their efficiency drops markedly when running much lighter loads. So if you have a 2000W inverter and use it to charge your mobile phone, you are wasting power. The trick is to use the high power inverter for heavy loads and buy a small PURE sine wave inverter to run light loads. A 300 Watt inverter works well for charging your phone or running your computer and other small loads while maintaining acceptable efficiency.
A 2000 Watt (12 Volt) inverter running at full load draws 2000 (Watts) /12 Volts = 167 Amps. This means that you can only run heavy loads for a short time before your battery is fully discharged. Depending on your battery capacity, this is likely to mean that running a microwave oven from your inverter is a bridge too far. The high inverter currents also mean that your inverter battery wiring must employ heavy wiring such as starter motor cable. You really need to switch off the battery supply to your inverter when it is not in use because there is a real risk of a major fire if the inverter fails. The on/off switch on the inverter does not completely disconnect the battery from the inverter. There have been a number of incidents where caravans have been destroyed as a consequence of inverter failures causing fires. You need to install a high current battery switch that matches your inverter battery current.
WIRING SIZES
All too often wiring used for solar installations is less than ideal. The wire used should ensure that Voltage drop is no greater than 3% for the particular circuit involved. When considering the length of cable runs, the return path must also be considered. A useful calculator for wire sizing can be found at www.12vplanet.co.uk/knowledge-centre together with other useful information. You may also find the table provided near the end of this article, helpful.
IN CLOSING
I hope that this will provide some kind of starting point for you to fit your ship with a reliable electrical system to suit long term cruising. Only you know your precise needs and the compromises that you need to make (warm beer tastes horrible!). If you just nip out of the marina of a weekend for a quick sail, you might regard all of the above as quite fanciful but if you wish to cruise long term with all the cute electrical gear and remain independent of shore power, some of the preceding may be useful to you.
As a rough guide, if your batteries are fully charged by around mid morning on a sunny day you have probably done a pretty good job and your system will still function well in moderately overcast conditions.
You also need to be aware that the numbers calculated here are far from surgically precise but nevertheless serve to provide a basis for further thought. Most solar power systems on boats tend to be a bit underdone. Don’t take my word for it, check for yourself.
A SUMMARY OF SOME SUMS YOU MAY SOMETIMES NEED
VOLTS = WATTS/AMPS AMPS = WATTS/VOLTS WATTS =VOLTS x AMPS AMPERE HOURS = AMPS x HOURS
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DATA FOR HORIZONTALLY MOUNTED SOLAR PANELS |
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|
LOCATION |
HOBART |
BRISBANE |
DARWIN |
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|
|
MONTH |
JANUARY |
JUNE |
JANUARY |
JUNE |
JANUARY |
JUNE |
|
1 |
Solar Day Times |
0600-1900 |
0800-1600 |
0600-1800 |
0700-1600 |
0700-1900 |
0800-1800 |
|
1 |
Hours |
13 |
8 |
12 |
9 |
12 |
10 |
|
1 |
Total Watts/sqM/day |
8839 |
2087 |
8523 |
4020 |
7562 |
5474 |
|
2 |
Max. Watts/sqM/day from a 17% efficient solar panel at MPP # |
1503 |
355 |
1449 |
683 |
1286 |
931 |
|
2 |
Max. Watts/sqM/day from a 17% efficient solar panel NOT at MPP (-30%) # |
1052 |
249 |
1014 |
478 |
900 |
651 |
|
3 |
Mean Sun hrs/day |
8.2 |
4.3 |
7.4 |
6.8 |
5.7 |
9.9 |
|
3 |
Clear Days/month |
4.9 |
4.1 |
4 |
12.1 |
.6 |
16.3 |
|
3 |
Cloudy Days/month |
13.4 |
14.3 |
14.3 |
8.1 |
24.2 |
3.8 |
|
|
Other Days/month |
12.7 |
11.6 |
12.7 |
9.8 |
6.2 |
9.9 |
1 Solar Tables, Spencer JW, CSIRO, 1979. Data for horizontally mounted solar panels under clear sky conditions.
2 MPP refers to the Maximum Power Point for a solar panel. Typically panels connected to a battery (or via a conventional regulator) are operating off the MPP and as a consequence may only operate at around 70% efficiency. Maximum Power Point Trackers (MPPTs) are used to offset most of the loss in ideal conditions.
3 Bureau of Meteorology Climate Statistics. The visible sky is divided into 8 parts known as Oktas. Zero Oktas represents an unblemished blue sky while cloudy days have 6 to 8 Oktas of cloud cover.
# NOTE: A typical 150 Watt solar panel has a surface area of around 1 square metre and converts the energy of the Sun to Solar electric power with an efficiency of around 17% or so.
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APPROXIMATE MINIMUM WIRE SIZE FOR 10 METRES OF TWIN CABLE (ie + & -) FOR LESS THAN 3% VOLTAGE DROP (12 Volt Systems)
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Nominal Panel Watts |
Max. Panel Current (Amps) |
Wire Cross Sectional Area (sq mm) |
Wire Diameter (mm) |
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50 |
3 |
3 |
2 |
|
100 |
6 |
6 |
3 |
|
150 |
9 |
10 |
4 |
|
200 |
12 |
16 |
5 |
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The table can also be used to approximate other values. (If you wish to be more accurate, use the suggested calculator). For example, to approximate wire diameters for a 75 Watt panel go up to the next wire diameter (ie 3mm). Similarly, for a 300 Watt panel array, add 100 Watt and 200 Watt wire diameters (ie 3mm + 5mm = 8mm). Cable length variations can also be approximated proportionally (ie to double the cable length, double the wire diameter). To halve the length, halve the wire diameter (for a 200 Watt panel and 5 metre cable length this gives 2.5mm wire diameter, but to be on the safe side go up to the next whole mm diameter (ie 3mm). |
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