Solar power on a boat: easy, right?
So you found this page because, 1 – you are starting your journey into installing solar on your boat or, 2 – you installed your solar and now you realise that eBay is not a good source for quality components that last. Okay, now ask me how I know…
So let’s chat about solar, the pitfalls, common mistakes, good practice and quality gear.
What do you need?
In order to purchase the correct solar equipment, you need to know what equipment you need. This is a simple but very important step that can easily be overlooked or ignored because finding out what you need is a bit fiddly. In essence, you need to add up the power consumption on your boat. You need to do this first so you can then determine the number and/or size solar panels you need as well as determining the size of your battery bank.
So how do you measure how power-hungry your boat is. Simple: all equipment on your boat is rated in terms of power consumption expressed in Watts. For example, think of your regular household 25W lightbulb or your 2,000W (2KW) toaster.
Look at your main electricity chompers on your boat like your fridges or freezers, lighting, instruments, wifi router, TV etc… Give this some serious attention in order to ensure you do not under power your boat or overwhelm your wallet.
In the end, you have a specific number of Watts. Naturally, you do not run your lights during the day, your TV may only be on for 4 hours per night and your fan may only run in your cabin for 8 hours during the night.
So when you identify your appliances and equipment in terms of wattage, you also need to know how many hours your equipment is on each day. You can do this in a simple spreadsheet to determine the total watts used in 24 hours expressed in watt-hours (Wh).
Usage (hours per day)
Daily Wattage (watt-hours)
So there you have it. Your watts and your watt-hours in a nutshell. However, there are two more main players in your solar system.
Voltage usually is 12 or 24 volts on boats but this can vary. The last player is Amps which only shows itself if your batteries are actually running your equipment. Let’s have a look at these 2 other players, what they do and their relationship with watts…
Here are the 3 players:
The simplicity of the mathematics is quite beautiful… Meet Watt, Volt and Amp together. If you know 2, you know the third. Why is this important? You need all these to calculate your power consumption and the subsequent solar system you need to design for your boat.
Confused about electrical stuff? Look at it from a different perspective and use water as an analogy…
Imagine electricity as water flowing through a hose. Disclaimer: never mix your analogies as water and electricity don’t go together so well. In any case, I digress… Here we have watts, volts and amps. Look at the voltage as the water pressure. There may be no water flowing but the pressure is there which becomes clear when you turn on the tap. Amps, on the other hand, is the water flowing through your hose. It can be a lot or a little depending on what happens at the end of the hose. Watts can be compared with the power the water produces at the end of your hose against the old water wheel.
But for our purposes…
- Amp – an ampere is the unit for measuring electricity flow.
- Volt – the basic unit of electromotive force often referred to as potential difference that causes a current to flow.
- Watt – the basic unit of electric, mechanical, or thermal power, equal to one volt-ampere.
So let’s put this together in some calculations. Looking at the triangle above you can simply substitute your known factors to find the missing one. For example, if your battery is 12v and your fridge is 5A you simply multiply 12 x 5 to get to 60W. Conversely, if your 12V spotlight reads 150W, you simply divide 150 / 12 to get 12.5A.
Okay, so what about Ah?
An ampere-hour (abbreviated amp hour, or mostly just Ah) is the amount of energy charge in a battery that will allow one ampere of current to flow for one hour…
So what does this mean?
It means that for example, a 5 watt light running on a 12-volt battery will use 5W / 12V = 0.4167A. Therefore a 120Ah (amp-hour) battery will last for 288 hours until the light goes out. (120Ah / 0.4167A = 288 hours. There are other factors that reduce this duration but we will shelve them for now. Later, we will introduce the proverbial “fudge factor” to account for these discrepancies and others.
Let's have a look at your boat...
When not connected to shore power, a 40-foot yacht may consume about 120 amp-hours every 24 hours. So is a 120Ah battery all you need? No: if you design your battery bank from scratch it is always a good idea to at least double your usage. This would make your bank a total of 240Ah. But wait, there is more… You should also add 25% fudge factor to account for deterioration in your batteries and other factors that reduce your capacity. In this case, we add another 60Ah to the bank which makes your total battery bank 300Ah.
Naturally, you may use much more energy than 120Ah. Your habits, boat, battery bank size and equipment all play a role in determining this 120Ah educated guess. But for a prudent energy consumer, 120Ah will just be enough. Ie, look at it as a starting point…
Okay, stuff this adding up of watts. Is there an alternative?
Mais oui! If you are already fully set up with a battery bank then you can use a shortcut. Get shunted… Ie, install a good shunt. You know- shunt it. What is it? Simply a low resistance path through which ALL your amps flow. An amp meter if you like for your total system. With it, you can measure your amp usage over time. Particularly if you use the VictronSmart Shunt. This sexy beast measures what you use and when you use it and sends this data to your smart devices such as your tablet or phone. It records and stores historical data so you do not have to add up your equipment’s wattage after all…
The Victron App
If you go with Victron equipment then you are going to use the Victron App (VictronConnect). You can get real-time and historical data of any Victron product via Bluetooth including your shunt but also your solar controllers. VictronConnect works on your iOS and Android phone as well as on your laptop on Windows and Mac, all through an intuitive and clean interface.
VictronConnect is the perfect tool for not only monitoring your power and solar but also helps in the initial configuration of your system.
Okay, now you know what power you are using and now you also know a bit more about your battery bank. Next to consider are your solar panels.
How many watts of solar do I need to maintain my battery bank?
The short answer is simple. As many as you can fit on your boat. However, I can see you twitch as this will neither look good for the boat nor the wallet. So let’s have a look at this with some example calculations based on our 40-foot boat with a 300Ah battery bank.
Solar Panel ratings are commonly stated in watts. For example, 200W. In order to calculate the energy the solar panel can supply the battery with, we simply divide the 200W by the voltage of the solar panel. For example, 200W / 18V = 11.1A (remember our triangle from before).
Please note that solar panels are often advertised as 12 volts. This is in almost all cases not true. The voltage is derived from the panel’s spec sheet and is usually denoted as “Max Power Voltage”. The 12 volt merely means that they can, with the appropriate solar controller, be used in a 12-volt system.
So how many watts do I need?
Again, you can not have enough panels. Just think about less sunny days, mist or hazy/foggy periods or any other external issues you may encounter. Also, think about the timing in which your batteries are recharged: the more wattage, the quicker the charge. Any excess wattage can be used to run your ice block machine, your hot water system or any other extravagant power-consuming gadget you may deploy on your vessel.
But, really, how many panels do you need for your 300Ah battery bank with a 120Ah daily consumption?
There are many factors that determine this. As a rule of thumb, you can double your battery bank Ah which would give you as a minimum 600 watts. This will in most cases maintain your batteries with modest use of power.
Breaking this down to some rough calculations, we can say that our 120Ah consumption x 12-volt system = 1,440Wh. But as you know, the stated performance in the panel’s spec sheet is seldom achieved due to dozens of external factors and/or some plain misrepresentations from some foreign manufacturers (let’s not name any specific countries). So your 200-watt solar panel may only produce 150 watts because the sun is not at right angles, there is a little dust in the air or on your panels and the odd cloud is drifting past…
The 1,440Wh divided by 150W = 9.6 hours of sun to keep your batteries charged.
There may however only be 2.5 hours of sun per day so we can calculate back and say we need 1440Wh / 2.5 = 576 watts.
In our scenario with a 300Ah battery bank and a 120Ah daily consumption, we may therefore be looking at 4 x 200w solar panels giving us 600 (actual) watts.
Polycrystalline vs Monocrystalline solar panels
So let’s complicate things a little. Solar panels aren’t solar panels. There are 3 main types.
Monocrystalline solar panels are more modern, better and a little more expensive compared to Polycrystalline solar panels. Go with a reputable brand otherwise, you may find that the closest your panel came to being monocrystalline is the sticker that says: “Monocrystalline solar panel“.
There are also thin-film solar panels that may suit your circumstances. These are often flexible and can be shaped on top of your bimini. However, they do not last as long. Flexible solar panels will still require a solid backing. The can not just lay on top of your bimini as they will deteriorate at a rapid rate that way.
Grading solar panel
Naturally, that is not the last of it. Solar panels are also rated in terms of their quality.
“A” Grade solar cells are prime flawless solar cells. Again, look for a good brand otherwise your A-Grade consists of just a sticker saying “A-Grade“.
“B” Grade solar cells are solar cells that contain a visual flaw that does not affect the power, their price is a little lower than A Grade cells.
“C” Grade solar cells are those with a flaw that affects the power output, so the output power is somehow lower than A and B grade cells, and the price is subsequently a little lower.
Then there is efficiency...
Efficiency in terms of solar panels measures the amount of solar energy that falls on a solar panel’s surface and is subsequently made into electricity. Due to the latest advancement in solar cell technology, the average panel conversion efficiency has increased from 15% to 20%. This substantial jump in efficiency resulted in an average size panel’s output to jump from 250W to 340W. So same size, more power. Or more importantly for us yachties. Same power, smaller size…
So if you buy a cheap panel you may see an efficiency rating of “more than 17%” which could be a blatant porky. Conversely, on a better quality solar panel you may find something like “20% – 22%“.
Let's talk about controllers
Controllers come in many different configurations. The most important distinction is the difference between PWM and MPPT controllers.
In essence, and oversimplified, the difference between the two is this:
- Pulse Width Modulation
A PWM controller draws the current out of the solar panel at just above the battery voltage.
- Maximum Power Point Tracking
With an MPPT controller, the current is drawn out of the solar panel at the panel’s “maximum power voltage”.
Therefore an MPPT controller is better but also a fair bit more expensive. Be very aware of eBay offers where MPPT is no more than a 2 cent sticker on a PWM controller. Again, I will not mention any specific countries of origin here…
Moreover, solar controllers are rated for both voltage and amperage…
Using multiple solar controllers
It is not uncommon to see multiple solar controllers feeding into a single battery bank. There can be several reasons but the main reasons are:
- The solar panels have different voltages. It is NOT recommended to connect solar panels with different voltage outputs. This may damage the controller.
- The cost of larger capacity solar controllers goes up exponentially. It may be more cost-effective to use several smaller controllers to get to your required amperage.
- Redundancy is also one of the reasons you may install multiple solar controllers. If one array goes down, you still have the other.
Connecting in parallel or series?
PARALLEL – pros and cons?
Exclusively wiring your panels in parallel is used for smaller, more simplistic systems, and is often used with PWM controllers.
When applying the maths in the triangle above you can see that connecting your panels in parallel increase the amps and keep the voltage the same.
The problem with parallel systems is amperage. Lots of Amps need a thick high gauge wire. Consequently “transporting” high Amps also becomes problematic if your panels are more than 3 to 4 meters away from your controller.
Look at your controller and make sure your connectors can take the heavy gauged wire required.
Series – pros and cons?
Exclusively (as in without combining series and parallel) connecting your solar panels in series also suits smaller setups specifically where an MPPT controller is used. As opposed to parallel, a series connection increases voltage and leaves the amperage the same.
High voltage lends itself better to be “transported” over long distances. Consequently, your wires can be of a much smaller gauge which on a yacht can make all the difference.
Shading problems can occur with older systems when panels are wired in series. It is a bit like the old Christmas tree lights: when one fails, all the lights went off. Parallel connections are not affected by this. Saying this, newer panels may have bypass diodes. This simply means that when one panel is “blocked” perhaps due to shading, the current can flow parallel to (bypass) the panel, so the flow is not blocked.
Wiring up your solar controller
From panels to the controller
The wiring from the panels is relatively simple. Use the appropriate gauge wire and simply feed these into the controller using the correct polarity. Most controllers have these inputs clearly marked. Consider if you run multiple panels in series or parallel. In series (positive of one to the negative of the next) your voltage increases and amps stay the same. Whereas in parallel, the voltage stays the same and the amps increase.
So your controller needs to be configured to fit in with your wiring choice.
From your controller to the batteries
Run the output positive to your battery bank via a 50A circuit breaker. Run the negative through your AC charger such as the MasterVolt 12/40. If wired up correctly, it ensures that the amps are counted on your shunt. More often than not, your solar controller has a temperature input. Connect the temperature lead directly to the negative terminal of your battery. The lead may have to be purchased as an accessory…
Us busbars to keep control of the wiring. Again, use the appropriate gauge wire with quality connectors. Remember, your system is only as good as your weakest component.
Unlike many other brands, the Victron solar controllers have generously sized input so connecting large gauge wire is no issue.
Solar cables are designed to be UV resistant and weather resistant. 10 gauge wire is under normal circumstances sufficient but for larger arrays, special calculations may be required to determine the gauge.
The wire that runs from the solar controller to your battery may have a larger gauge to reduce loss and should have a mechanical breaker incorporated.
Accessing your data
Now we are getting to the heart of things: gadgets… But not just toys. These Smart Displays will help you understand your system and can also be used for configuration. ie. two-way communications. They warn you when your batteries are running close to their minimum percentage.
Displaying your data
With Bluetooth built-in, the BMV Smart is ready for the Internet of Things (IoT) era. With Bluetooth being implemented in most other Victron Energy products, wireless communication between products will simplify system installation and enhance performance.
GX Touch 50
If you want to go a bit more fancy, the GX Touch 50 is the display accessory for the Cerbo GX. The five-inch touch screen display gives an instant overview of your system and allows you to adjust settings in the blink of an eye. Simply connect to the Cerbo GX with one cable. It’s super slim waterproof design, top-mountable setup and simple installation brings a lot of flexibility when creating a crisp and clean dashboard.
Fancier still, this all-new communication-centre allows you to always have perfect control over your system from wherever you are and maximises its performance. Simply connect through our Victron Remote Management (VRM) portal, or access directly, using the separate GX Touch 50, a MFD or the VictronConnect app thanks to its added Bluetooth capability. This latest addition to the GX-range combines all the best of connectivity, redefining smart power solutions in every way.
Putting it together
I have no affiliation with Victron but found their products consistently better than the competition. The service is also extremely helpful and friendly. I recommend Victron because it will save you money. Unfortunately, I found out the hard way.
“Leave a comment. I would love to hear about your setup, what you did differently or even if you totally disagree with my analysis and opinion…“
5 thoughts on “Solar panels on a boat: is Victron worth it?”
Thank you for a great write-up on solar!
Thanks Charles, I appreciate your comment…
For better efficiency you should wire the panels in series IF the panels have bypass diodes.
Most moderns panels do have bypass diodes.
Many modern panels have several diodes to bypass sections of the panel if it will be a shadow.
In series the voltage will be higher and that give less loss in cables (it is more easy to transport voltage over amps) and you can have longer cables and less amount of cables are needed in the setup. less point of failures as well.
You get can harvest sunpower for a longer duration when panels are in series because the voltage will go high much easier then in a parallel wired system, better effect in cloudy days.
If you look at the MPPT victron chargers they don’t handle very big cables and I say it because today series is the way to go with panels.
That is solid advice Håkan. Thanks for taking the time to point this out…
Thank you for a well written,(clear concise with a splash of humour 👍)
I came across your blog while looking to understand the heat exchange system in my Volvo D2-55B
Of course l then had to read what other gems of information l could learn from.
My very well maintained Ovni 385 came from a Dutch owner, with a Victron Solar controller.
This l have added to, new solar controller with blue tooth, BMV712 x2 and a Cerbo GX to pull it all together for remote monitoring.
Could l ask you to check your final ‘putting it all together diagram’
It seems that your solar panels are connected to the batt connections and the PV connections wired to the alternator and boats batteries, on your Solar controller.
Many thanks for sharing your experiences.
May l wish you a Very Happy and Healthy New Year.