Solar Power on Boats

Posted on Monday 2 February 2009

Recently, I wrote a post on a sailing forum about the 25 Watt solar panel I added to the s/v Pretty Gee to act as a maintenance charger for the boat, while it is stored on the hard for the winter. On one of the forums, I was asked if a charge controller was needed. I thought I’d write a post that includes a modified copy of my response to the question here, as an expanded article on solar power on boats in general.

Solar Power on Boats

On many cruising sailboats there is a need for passive electrical generation of some sort. The need for passive electrical generation is obvious—there are power requirements for any boat that is in use—refrigeration, radio, lighting, etc. While these electrical needs could be met using a generator or running the engine, it really doesn’t make much sense to do so, as the long-term costs of doing so, in fuel and maintenance, are much higher than if passive generation methods are used. Using a generator or the boat’s engine will require the boat to re-fuel more often, which is not always desirable or even possible on longer cruises. A good passive electrical generation system will make the boat far more self-sufficient.

There are three forms that passive electrical generation on a small cruising sailboat can take: wind, water and solar generation—and of these, only solar and wind are of use when at a dock, mooring or at anchor. In this article, I’m only going to be discussing solar generation.

Types of Solar Panels

There are basically three types of solar panels, which I generally group in to two distinct categories.

The first category are the rigid panels—which include both the mono-crystalline and poly-crystalline types of solar panels. These are characterized by having individual cells that are linked in series and mounted in a rigid frame with a glass protective cover. Mono-crystalline and poly-crystalline panels are generally much more efficient than the second category of panels—semi-rigid or flexible panels.

The second type are the semi-rigid or flexible solar panels—which are also known as amorphous silicon solar panels. These types of panels are often called thin-film panels, since the panels are made by depositing a thin film of silicon onto conductive substrate and creating the solar panel from that. There is no crystalline structure to the silicon used in making these panels. These panels are usually semi-rigid—often made using a stainless steel sheet as the base layer—but there have been flexible versions of them as well. They are often durable enough that they can be walked on, with some care. They do not have individual cells, a rigid frame or a glass cover and are much less efficient than their rigid cousins. However, in low-light and shaded conditions, they do tend to perform better.

What Kind of Panels Should I Use

On most smaller sailboat, the rigid panels are generally used. This is often due to a lack of space, and needing to get the greatest amount of power generation for the space used. These panels are often mounted above dinghy davits on the stern of the boat, or on top of the bimini or dodger.

Larger boats, especially some of the big racing boats, often use the semi-rigid panels, as they are more durable, and the boats are large enough to use the less efficient panels. These semi-rigid panels are often mounted right on the cabin top, as they are durable to walk on to a limited degree.

Some boats use a mix of semi-rigid and rigid panels. However, if you do this, the panels should be grouped by type, with separate charge controllers for the two different types, due to the differences in the panel sensitivity under different lighting conditions.

Do I Need a Charge Controller

Chances are more than likely that you’ll need a charge controller, unless you’re using a very small 5-10 Watt maintenance type panel.

The basic rule of thumb is that if the panels approximate daily output is more than 1/60 of the bank’s rated amp-hour capacity, then a charge controller is probably necessary so that the panel doesn’t boil off the batteries.

For example, a 25 Watt panel puts out about 1.39 amps. The daily output is about 6.95 amps. So, unless your battery bank is larger than 417 amp-hours in size…you would need a charge controller. This is especially the case if you’re using AGM or Gel type batteries. However, using a charge controller makes a lot of sense for two major reasons.

First, they will protect the batteries from discharging back into the solar panels at night. Solar panels, unless connected using a blocking diode—which drops their effective voltage output about a volt—will discharge the batteries at night. Using a charge controller basically prevents this from happening, and not needing to use a blocking diode increases the effective output of the panel.

Second, a charge controller will also help protect and condition the batteries as they charge them. A battery goes through three phases of charging—bulk, absorption and float—as the charge level of the battery changes. An intelligent charge controller—like the FlexCharge NC25a, will manage the panels output, which can be as high as 20 volts on a shade-resistant panel, and drop it down to the voltage levels required by the battery—depending on what the charge state the battery is at. Many of these can also help condition the batteries. Some charge controllers even have the ability to “equalize” the batteries. Some are pulse-width modulation based. Others, like the NC25a, are not.

Types of Charge Controllers

First, there are dumb charge controllers. Don’t waste your money on these—they’re just not worth it. These usually use just a relay or shunt transistors to vary the voltage levels, and most only can do one or two-stage charging. They basically short the panel out when the batteries are “fully charged”.

Second, there are three-or-four stage smart charge controllers, like the NC25A I mentioned above. It is a non-PWM (pulse width modulation)-type three-stage intelligent charge controller and it looks like this.

Photo of a Flexcharge NC25A charge controller courtesy of FlexCharge

Photo of a Flexcharge NC25A charge controller, courtesy of FlexCharge

Companies, like Morningstar, make PWM-based three-stage charge controllers that are comparably priced. There are debates as to which charging method is better. I have used and like the NC25a, and keep the one I own on the boat as a backup if my MPPT charge controller fails.

Third, are the MPPT-type charge controllers. These are the best of the breed. Morningstar, BlueSky, and Outback make versions that are suitable for marine use. The major difference between an MPPT-type charge controller and a PWM-type three-stage charge controller is intelligence, as most of the MPPT-type charge controllers are based on a three-stage PWM-type controller core. MPPT charge controllers do this by trading voltage for amperage, or amperage for voltage. Using an MPPT charge controller will often yield a 15-30% increase in efficiency for your solar panel charging system. The FlexCharge NC25A was replaced by a BlueSky SB2000E MPPT-type charge controller. You’ll see why in a second.

Why Use an MPPT-type Charge Controller?

Very simply put, an MPPT-type charge controller will increase the efficiency of your solar panel system. This may allow you to be more self-sufficient, and reduce your need to run the engine or a generator. It may allow you to get by with a smaller solar panel array than you could otherwise.

It does this by making more efficient use of the electricity generated by the solar panels and minimizing the amount that is wasted as heat. What follows are two examples of how an MPPT-type charge controller can increase the effective output of a solar panel.

An example*: Normally a 25 Watt panel will be outputting 1.39 amps at 18 volts. 18 volts is too high for the batteries to charge with, since bulk charging requires only 14.4 volts. With a normal three-stage controller, the excess voltage is shed as heat—that’s why the charge controllers heat up so much. 18-14.4=3.6 volts—3.6 volts * 1.39 amp = 5 watts. So you’re losing about 5 watts out of 25 watts to heat.

With a MPPT-type controller, it uses a high-frequency DC-to-DC converter to drop the voltage down to 14.4 volts, but increases the amperage to 1.74 amps at the same time. So, instead of getting 6.95 amp-hours from the panel for the day, assuming about five hours of full output, you get 8.70 amp-hours, or a 25% effective increase in amp-hours to the batteries. This makes a lot of sense, since you’re basically recovering the 5 watts that was being lost to heating the three-stage, non-MPPT, charge controller.

Another thing most MPPT-type controllers will do is trade amps for volts, if necessary.

An example*: If the panel is partially shaded, the output voltage on it will drop. Say the output drops to only 12 volts, at 1.39 amps, because a third of the panel is shaded. Normally, this would just be lost as the three-stage controller can’t use it to charge the battery, which requires 14.4 volts to charge. Some MPPT-type controller will take the 12 volts at 1.39 amps and convert it to 14.4 volts at 1.16 amps. Now, instead of getting ZERO amp-hours, due to insufficient voltage to charge the batteries, you’re now getting 5.8 amp-hours for the day. Not too bad, is it??

How Large an Array Do You Need?

The size of the array you need really depends on what your purpose in having solar panels is. The three typical setups are maintenance, charging and self-sufficiency.


Some people just want a small maintenance charging system to keep the batteries topped off when they’re away from the boat—usually so they don’t need to leave a shorepower system connected and running. This type of usage is going require the panels be sized according to the basic losses of the unoccupied boat.

Typically, the loads that this type of panel has to make up for are the bilge pump, the internal self-discharge of the batteries, the memory feature on the electronics, etc. The bilge pump and the self-discharge are probably the biggest considerations. If the boat is a well-kept, dry boat, then the bilge pump usage should be minimal. The self-discharge losses can be as high as 1% of the battery bank capacity per day in warmer climates for wet-cell batteries.

In general, the maintenance panels are 30 Watts and under in size, unless the boat has an extremely large battery bank. Going with AGM or gel batteries can reduce this significantly, as they self-discharge far less than wet-cell batteries.

For instance, say the boat has a stereo that draws .01 amp per hour for the memory feature, and a small maintenance bilge pump that draws 6 amps per hour, which typically runs for about 30 minutes per day. The house battery bank is a 450 amp-hour bank consisting of four T105 six-volt golf cart batteries. The daily losses would be .24 amp-hours for the stereo, 3 amp-hours for the bilge pump and 4.5 amp-hours for the batteries. or 7.74 amp-hours total. The average solar panel can give you approximately five hours at its full rated amperage, so we need 7.75 amp-hours/5-hours or 1.55 amps per hour from the panel. A 25 watt solar panel can generate about 1.7 amps per hour, which gives us a bit of extra capacity to account for the less than perfect days, and should handle the maintenance requirements for the boat under most conditions.


Often, similar to the maintenance charging setups, people who mainly sail on the weekends and keep the boat on a unpowered dock or at a mooring, will want a solar panel setup that can top off the batteries over the course of the week so that the boat is ready to go for the weekend. This setup requires a bit more in the way of calculations, since you need to estimate what your average weekend usage is, and add that to the basic losses covered by a maintenance type setup.

What you’ll really need to do to start with is figure out what your realistic daily electrical budget is. The daily electrical usage can be expressed in amp-hours, since the voltage for the system is usually fixed at 12 or 24 VDC. So, you take the amperage a piece of equipment uses, and multiply it by the average amount of time you’re going to use it per day. Add all the amp-hours to get a rough daily usage.

Generally, the biggest users of electricity are electronics–like laptops, radios, TV—lights, and refrigeration. Underway, your instruments and running lights become factors. I recommend doing separate budgets for underway and at anchor/mooring/docked. Use whichever of the two numbers is higher as the base number for your calculations.

If you’re planning on being out for a weekend. you’d want to have the solar panel daily output be roughly 1/3 your daily usage, so that the panels can recharge the batteries over the course of the week. Say your weekend usage is as follows:

  • Anchor light two nights—2 amps x 20 hours,
  • vhf radio in receive mode for 12 hours—12 hours x 1 amp,
  • Engel MT27 refrigerator for two days, 25% cycle time at 2.3 amps or 12 hours x 2 amps,
  • two cabin lights for four hours each night or 4 hours x 4 amps,
  • Stereo for nine hours or 9 hours x 2 amps
  • GPS for 12 hours or 12 hours x .5 amps.
  • ST1000 autopilot for six hours or 6 hours x 1 amp, and
  • running lights for three hours or 3 hours x 4 amps.

This assumes you’re sailing about six hours each day, using running lights for about quarter of the sailng time, using the autopilot for about half the sailing time, and staying up about four hours past sunset each night. This also assumes you’re not motoring for any significant period of time.

This gives you a total of 134 amp-hours for both days or 67 amp-hours per day. This means your panels need to generate about 25 amp-hours per day or roughly five amps per hour. Five amps x 15 volts is 75 watts. That means you need about an 80 watt solar panel. I’ve rounded up to give the system a bit extra charging capacity and because 80 watt panels are very common, and to help account for the maintenance losses seen above.

Self Sufficiency

If you’re planning on cruising long-term, you’ll probably want to be as close to completely self-sufficient as possible when it comes to electrical usage. Unless you’re very lucky, you’ll probably need a combination of wind and solar to do this. Given the daily usage from above, which will tend to be a bit high, as most cruising sailors are at anchor a majority of the time, and the running lights and autopilot aren’t in use at anchor. This is offset slightly by the fact that when they are on a passage, they are sailing 24×7, and will be using the autopilot or a wind vane a good deal more than a weekend coastal cruiser.

They would need to generate about 70 amp-hours per day, based on the 67 amp-hours calculated above. This number is probably a bit high for most smaller sailboats, as LED-navigation and cabin lighting can really help reduce the electrical demands. 70 amp-hours works out to about 14 amps per hour, given the five hour average full output. 14 amps x 15 volts = 210 watts. Two 130 watt panels will provide a good cushion, since each is capable of providing about 9 amps per hour, especially if the system is built around an MPPT-type charge controller. Adding a wind generator to this setup would give the boat charging capability for overcast or stormy days.

*Please note: these are simplified examples that assume a few things, like no charging or MPPT-controller losses, but will give you a good idea of what is going on.

Where to Mount the Solar Panels?

Solar panels really need to be mounted where they will have a minimum of shading.  Shading drastically cuts the power output of monocrystalline and polycrystalline panels, but does not affect amorphous silicon thin-film panels as much.  Ideally, the panels will be mounted somewhere they will be out of the way of the boat’s lines and equipment during sailing—so that the panels do not have to be de-mounted when you want to sail, and located in an area that is relatively free of shading.

The main cabin top is generally a lousy location for solar panels.  The rigging, the mast, the boom and the sails all create shadows over the cabintop to some degree.  The foredeck has similar issues and the panels are generally in the way of the genoa sheets.  On center cockpit boats, the aft cabin top is often a good location for solar panels, unless there is a mizzen mast.

Another good location is on top of the cockpit bimini—which is often a good out of the way location that allows a fairly sizeable solar array to be mounted.  However, this requires that the boom be fairly high above the bimini, or the boom has a good chance of seriously damaging the solar panels. On my boat, the boom just barely clears the bimini—so it would not be a feasible location for me.

A third location that often works is on the stern rail or arch, or dinghy davits.  This is usually unshaded and out of the way. Again, if the boat has a mizzen mast, this is likely not going to be ideal.  On larger boats, the davits or arch can support several large panels with ease.  However, some boats use the stern arch for the mainsheet traveler, which would exclude them from having a solar panel array located there.

On multihulls, you will often have a few more options that aren’t available on a monohull.  On my boat, the solar panels mount to rails that run between the amas and the cockpit pushpit railings.  This keeps them out of the way for the most part, and fairly clear of shading. Currently, I have one 130 Watt panel mounted to the starboard side of the boat, and it can keep up with the refrigerator and other normal use.  I also have a smaller 25 Watt panel mounted to the cockpit railing which was used as a maintenance panel over the winter.  I will be adding a second 130 Watt panel to port later this year.

25 Comments for 'Solar Power on Boats'

    February 24, 2009 | 9:08 am

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    April 3, 2009 | 11:00 pm

    […] SB2000 and the Outback Flexcharge MX60 are MPPT type controllers. Would highly recommend you read this primer on solar panels on boats that I wrote. __________________ Sailingdog Telstar 28 New England You know what the first […]

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    […] the primer on Solar Power on Boats I wrote… the panel is probably shot if it is discolored and has permanent condensation inside the […]

    November 5, 2009 | 12:33 am

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    March 13, 2010 | 8:56 pm

    […] recommend you read the primer on solar power on boats I wrote. I would recommend you get Kyocera solar panels. The 130 watt panels are on sale for a very good […]

    April 12, 2010 | 1:09 pm

    […] I'd highly recommend you read the primer I wrote about solar power on boats on my blog. I'd point out that if you're going to be depleting the batteries significantly, as you would […]

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    June 12, 2010 | 12:49 pm

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    November 26, 2010 | 8:47 pm

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    January 26, 2011 | 3:58 pm

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    February 1, 2011 | 9:39 pm

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    July 19, 2011 | 3:33 pm

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    June 26, 2012 | 8:37 pm

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