Wednesday, March 14, 2012

Choosing a solar system for your boat

My favorite bits of equipment on my yacht is by far our solar panel array and battery banks. It's great to have the sun silently making power for us every day. Lots of cruisers opt for a wind generator and solar setup, but on Echo I decided to go completely solar. There are no moving parts, nothing to service, and the system is completely silent unlike wind generators. There is more than enough power to operate the vessel's systems.

Since I love the panels so much, I'd like to help others that are trying to install a solar system. This is a how-to describing choosing the panels and size of the system, what kind of charge controller to get, and what batteries to choose. In a separate post I've discussed how to wire up your system.

How much power will you draw?
This is always the big question. If you make the system too big it looks bulky and is too expensive, and if it is too small you'll end up having to conserve amps and constantly monitor your ammeter, praying for sunny days. The first thing to do is make a list of what will be using power, including how many amp hours each item uses. Products list how many amps they draw per hour, and this figure is expressed in amp hours. For example, my autopilot draws on average 4 amps an hour, if I use it for two hours then it uses 8 amps. Your power usage list should include how many hours a day you plan to use each piece of equipment as well. As an example, this is what Echo's power usage list looks like:
Manufacturers generally list the amp hour draw of their products somewhere in the instructions, on the packaging, or online. However, the above list is just an estimate. Items that I have aboard often draw more or less than their average. For example, I listed the amperage draw at 0.2 for the marine VHF. When it is merely on and standing by it is drawing 0.2 or less, however when it is receiving it is drawing 0.5 and when it is broadcasting it is drawing as much as 4 amp hours. The same point is relevant with items like the stereo system and the autopilot. In heavier seas or with weather helm the autopilot has to work much harder, drawing more amp hours. The stereo's draw is directly related to the volume it is played at. 

With all of that in mind, Echo can draw far more or far less on any given day, but it is still important to have a rough idea of what you'll be drawing.

How big of a solar system do I need?
Now that you have an estimate on your daily usage, you can appropriately size your system. To me, it was important that every day my batteries could fully recharge after a long night of drawing power and not producing any. However, solar panels are rated in watts, not amps! Deep cycle batteries are then sold with a rating on how many amps they hold, so it can get a bit confusing but it's really quite easy using this simple formula:

                      Watts = Volts x Amps

When using this formula to judge the amperage output of a solar cell, it is important to keep in mind that 12v solar panels operate at a maximum power voltage of 17.5. So when using the formula for solar panels, use 17.5 for volts. Here is a good example for a 100 watt solar panel:

                      100 = 17.5 x Amps
                      Amps = 100/17.5
                      Amps =  5.7

So in solving that formula, I just found that a 100 watt panel can produce 5.7 amps an hour at maximum capacity, since panels are rated at their maximum output capacity. Now you may think, "GREAT! my 100 watt panel will put out 5.7 amps for 12 hours of sunlight a day." That's simply not so. Generally you can rely upon 3-5 hours of maximum output, with the rest of the light hours of the day producing significantly less. If I had to estimate the daily output of a 100 watt panel I would do it like this:

                     4 hours @ 5.7 amps = 22.8
                     2 hours @ 4.0 amps = 8      
                     2 hours @ 2.0 amps = 4
                     3 hours @ 1.0 amps = 3

                     Total Daily Output  = 36.8 amps  

The above is of course an estimate, since it's impossible to predict cloud cover and other contributing factors, but I think it's about right for a 100 watt panel. Now that you know how many amps one panel will produce, it's time to choose the size of your battery bank.

Sizing the Battery Bank
Deep cycle batteries are rated on how many amps they can store. A medium size, group 31 lead acid battery like the ones I have aboard Echo for example can store 130 amps each at 12 volts. I have four, wired into 2 separate banks giving me 260 amp hours per bank with a total storage capacity of 520 amp hours. Now, not all of those 520 amps are usable. A battery can be drained to roughly half it's capacity before it stops putting out the 12 volts necessary to run the equipment aboard. So while I have batteries that are rated capable of storing 520 amps, only 260 of those amps are usable before the battery needs a recharge. Since I figured that I will draw roughly 130 amps a day, I can operate all the vessel's equipment for 2 days without any charge.

As a guide, battery banks should be rated (by the manufacturer) for 4 times the amount of amperage you plan to use every day. Some may say that this is overkill, and too many batteries, but I take comfort in knowing that I have plenty of power for whatever I desire and even if the solar panels were to malfunction I could reduce consumption and run the vessel for several days.

You're also not always drawing power from the batteries. Once they are fully charged during the early part of the day, all of my equipment runs directly off the panels, keeping the full charge in the batteries. My system generally fully recharges my batteries by 11am after a long night of running lights, autopilot, and stereo. After 11am the autopilot runs directly off of the power produced by the panels, as well as all my other equipment. This amperage allocation is handled by a smart charge controller, which is what we'll talk about next.

The Charge Controller
A charge controller will do the job of regulating amperage flow to your 12 volt system. Every solar system needs a charge controller of some kind. Mine measures the battery bank's charge and charges it appropriately, switching between trickle, acceptance, and bulk charges. When the battery banks are full, it will also put amperage directly to the equipment that is being used. It does this because it is a MPPT controller. There are different types of charge controllers available on the market today, but when choosing a charge controller spend a few extra dollars and get an MPPT controller. MPPT stands for maximum power point tracking, and is a technology that intelligently manages the power put out by your panels and can equal a claimed 20-30% higher charge rate than other types of controllers. Most modern MPPT controllers are 95% or more efficient, and will do a much better job than others when charging and maintaining the quality of your battery banks. It is very important to get a charge controller that is rated high enough for your application. If your charge controller is only rated for 20a output and you figured your solar panels can put out 30a on a sunny day then you'll inevitably fry it.

What panels?
Now that you know the size of the system you need, it's time to choose your panels, batteries, and controller for your system. Solar panels are not all created equal. There are different technologies behind them, different panels convert sunlight in different ways and are made of different materials. The two most common types of rigid panels used for 12v systems like the one on your boat are monocrystalline panels and polycrystalline panels. The difference in technology comes from the number of crystals a solar cell is cut from, but this is not a science course. If you want to learn more about the different types of technologies then this is a simple and easy to understand guide. All that we need to know is that monocrystalline panels are more efficient but also more expensive. On Echo I sprung for the more efficient monocrystalline panels, but both types will work just fine and it is completely up to you to choose. No matter what the technology, a panel rated at 100 watts is still a 100 watt panel.

It's also important to consider the quality of the panels themselves. Do they come from a reputable company? Is the frame made of a high or low grade aluminum? Are they well sealed for a marine environment? There are hundreds of different solar panel manufacturers out there, so do a bit of research and read some reviews to get an idea of what other people like. I chose 95w panels made by a company called ET Solar and they are of a very high quality and are working flawlessly.

What kinds of Batteries?
Battery technologies also vary widely. For this application a deep cycle battery is what will be needed, so we will focus on the different kinds of deep cycle batteries. There are two main kinds deep cycle batteries used in a marine environment, lead-acid batteries and absorbed glass matt batteries. Each will do the job well, but each is also a trade off in price, reliability, maintenance, and ease of use.

Lead Acid: inexpensive, easily available around the world, require maintenance and monitoring

AGM: will charge faster, are sealed and require no maintenance, are 3-5 times the cost of lead acid, generally larger for the same amount of storage capacity

It's up to you to decide between a lead-acid and AGM battery, but for my application I went with lead-acid batteries. The cost of an AGM was quite prohibitive, as well as the fact that I have limited space for installation and they were a bit bulky. I may choose to convert one of my battery banks to AGM batteries later on, and will comment on the performance.

The next thing to consider is what voltage of batteries to get. Lots of cruisers swear by 6 volt golf cart batteries, citing their deep charge capacity as well as limited amount of required maintenance. When choosing a 6 volt battery it is very important to take the amount of rated amps and halve it. A 6 volt battery will be rated to hold many more amps, but it is holding those amps at 6 volts. At 12 volts, the amount of amperage is halved. Since we're talking about 12v systems here, you'll need to figure the rated storage capacity for 12 volts. Basically, if that 6v battery you are holding has a big sticker on the top stating it's storage capacity is 200 amps, that's 100a @ 6v, at double the voltage you'll have 100a of storage capacity on that particular battery. Also, if choosing 6v batteries know that you must wire them for 12v. Two 6v batteries wired in a series will put out 12v, and act as a single 12v battery. But we're just choosing the system here, more on the wiring in this post. I simply went with 12v batteries wired parallel for my system, it was what was available at the time and they are working really well.

I hope this guide has shed a little light on the different choices that are to be made when designing a solar system for your vessel, any questions can be left in the comment form and I'll be happy to answer them to the best of my ability.


  1. Thanks for this post.

    Could you do a run down of which LED cabin and navigation lights you use and what amperage they each pull?

    Secondly, could you describe the sound system you use? From my research, the speakers and receiver I'm looking at don't list their amp hours for some reason.

  2. Rob

    You are mistaken on the amp hours of a 6 volt battery vs 12 volt. When you wire 2 6 volt batteries in series to make 12 volts the amperage stays the same and is not halved. Two 225 AH golf carts in series gives you 225 AH at 12 volts.
    In parallel it doubles - two 100 AH 12 volts in parallel gives you 200 AH

    The big advantage of golf cart batteries is their ultimate durability. Compare a 6 volt that weighs 66 lbs with a 12 volt at about the same weight. The 12 volt has 6 cells vs the 6 volt's 3 cells - comparable weight because the plates are much thicker. Plate thickness is the biggest factor in battery life.
    As an example Trojan T-105 will handle between 2 and 5 times as many cycles as any Trojan 12 volt with the exception of 2 that weigh 4 times as much - but still 150 more cycles than these 2.