Get the Most from the Sun and Wind


A quick guide to maximizing your energy independence  (published June 2014)

For decades now, solar and wind power have reliably provided inexpensive energy to cruisers. It’s not simply that the concept of harnessing power from the elements is inherent to the concept of cruising on a sailboat, it’s that these energy sources are effectively always present; require very little in the manner of mechanical devices to harness; offer a degree of peace and quiet; reduce maintenance and operation duties; and have low operating costs that are unmatched by petroleum fueled power sources.

Of course, energy in always equals energy out, and this means renewable energy sources won’t be powering air conditioners, hair dryers or electric ovens anytime soon. But for all of the common onboard electronics, from lighting to refrigeration and laptops to radios, cruisers are already finding current technology sufficient.

So why do we need yet another column on these fairly well known devices? Simply, there is still a lot of misinformation out there and many installations just aren’t getting anywhere near the maximum benefit from these sources that they could be. By covering just a few details you will know how to realistically get the best from your system. Add to that a few tips and tricks to ensure you’re actually getting that, and you’ll probably find yourself more independent from that genset than you thought possible.

Regardless of whether you’re looking to get the most out of the equipment you have, or you are interested in adding renewable energy to a vessel without any, it’s essential that you start with an understanding of how much energy you need. Hang in there, there is only a little bit of math and it’s all simple addition, subtraction, multiplication and division.

There are many good articles on how to do thorough energy audits aboard your boat that will give very accurate results. But if you’re interested in a simple back-of-the-napkin calculation you can do that right now,  just take the size of your battery bank and multiply it by the average amount you drain it in a given day (if you don’t run the engine or generator). If you have a 400 AH battery bank, and you drain it down to 65 percent in a given 24 hour period before charging it back to 100 percent, you’re using 35 percent of your bank capacity. Thirty five percent of 400 AH is 140AH, so you’ll be looking to put a bit more than that 140 AH back into your batteries in that same 24 hours using renewable sources to break even on an average day.

You’ll notice that solar panels and wind turbines are not rated in amp hours, but in watts. It is a simple conversion: multiply your amp hours by the main voltage of your battery bank (which is usually 12V), so for our example that would yield 1680 watt hours per day. But wait, you’ve no doubt noticed that watt hours is not the same thing as watts. Here is where the idea of time factors in. Your solar panel only produces useful output in full sun and when the sun is higher in the sky. At all other times you get a diminished amount of power. It is the same thing for wind generators—they produce rated power usually somewhere around 25 knots of relative wind speed, which means all other times you don’t get as much power. And of course, you can’t expect the devices to produce full power even under fairly good conditions, because there are always some losses. So we need to multiply the power produced by the duration of time the devices produce that power in order to compare against the power we remove from the batteries over the course of the same day.

So let’s say you have a 100W solar panel. In the tropics with a properly installed panel you should get within 20 percent of full rated power during the usable hours of the day, say 9 a.m. to 4 p.m. averaged across the seasons. We will ignore the extra hours of waning daylight and count them as a bonus to be conservative and to keep the math simple. So you can realistically expect 7 hours × 80% × 100W = 560 watt hours per day. Now we’re getting somewhere! You can directly compare these 560 WH to the 1680 WH you estimated and see pretty quickly that if you tried to meet your power needs entirely with solar, you would need about three panels in the tropics. Solar energy drops off dramatically the further outside the tropics you are, and by both theory and in my own experience, you can expect a third or less of this figure by the time you are at about 45 degrees north or south. There is also a dramatic seasonal variation the higher your latitude, so be careful to factor for that as well. If you want to get truly scientific about it, you can use insulation charts for the areas you are interested in to get fairly accurate data by season and geographic location.

This photo of a wind generator clearly shows the onboard heatsink fins which are used to cool a built-in dummy load.
This photo of a wind generator clearly shows the onboard heatsink fins which are used to cool a built-in dummy load.

A wind generator is similar, but slightly different. If the generator is rated at the same 100W in 20 knots of wind, you would estimate the power output based on the wind speed and how frequently it blows at that speed. Of course, this is a very rough estimation so being pessimistic in your analysis would be a good idea. But for regions where the wind is very steady and significant, it might be easier to predict, for example, that 30 percent of the time it will blow at 15 knots. There’s one more little trick to wind turbines: the difference in power between 20 knots of wind to 15 knots of wind is approximately half. And you lose half again at 11 knots. So your turbine that is rated at 100W in 20 knots of wind is good for roughly 50W at 15 knots and only 25W at 11 knots. It’s important to keep this in mind when rating your wind turbine. If you expect the wind to blow around 11 knots steady, with 20 percent gusts to 15 knots on average, this is 25W x 80% x 24 hours + 50W x 20% x 24 hours = 720 watt hours per day, for this theoretical turbine. I would personally take half this figure, as the wind will inevitably not be what you had hoped for.

These are rough figures, admittedly, but they are close enough to help you get a realistic grasp of what you can expect from your system when it is operating at its best. This should give you enough information to decide how much solar or wind you should buy and whether one makes more sense than the other in your cruising region. Now that you have a general idea of what to expect, let’s start getting that power into your batteries.

The first thing your newly produced energy encounters after being captured by the wind generator or the solar panels should be a circuit breaker that is located as close as practical to the source of power. A wind generator or solar array running in optimal conditions can produce a very significant amount of energy, and should a wiring fault occur, could easily start a fire or cause other damage. After the circuit breaker should be a charge controller. Charge controllers serve many purposes, but the main one is to convert the power from the energy source to the proper voltage to efficiently charge your batteries.

As you are no doubt aware, batteries last the longest and charge the fastest when they are charged in three or four stages, each with a specific voltage or current cutoff, which varies by battery type and occasionally by brand as well. You may also know about Maximum Power Point Tracker (MPPT) solar chargers. These optimally match the solar panel’s best power production with the level of charge your battery needs. In less optimum conditions, such as the few hours of dawn and sunset or on an overcast day, they can provide a tidy boost to the power your panels would produce without this type of controller. If you expect to be in areas with marginal solar production, these can significantly boost your average power from the panels, but you won’t see a dramatic improvement if you expect to stay in the sunny, cloud-free tropics your entire cruising trip, so weigh the cost against your needs.

Wind generators need a charge controller for another very important reason. When a wind generator is producing more power than the battery bank needs—such as when your batteries are close to full but the wind is still blowing strong—the generator can often produce much higher voltages than are safe for your batteries. Simply disconnecting the generator is an option, but it requires someone to be there watching the batteries and the wind. Often this is an unreasonable expectation on the crew, especially in foul weather or when the whole crew is ashore, and the risk of damaging the batteries is very real. Simply letting the wind generator “freewheel” or spin in high winds without resistance on the blades can also damage the generator itself and shorten its life by wearing out the bearings prematurely. So a wind generator charge controller should be connected to and capable of controlling a device called a “dump load” or “dummy load,” which is basically a big resistor with a large heatsink on it. It turns the excess energy into heat and helps slow the wind generator down while at the same time reducing the voltage to the batteries. These should be mounted somewhere with excellent ventilation and far from any flammable materials, as they can get quite hot in operation. Between a properly sized solar or wind generator and a good charge controller, you’ll be ready to extract maximum power from the elements around you.

These sort of charging devices are very smart and will begin to throttle back their charging to the batteries as they approach fully charged. They detect this by the voltage on the batteries compared to the current going into the batteries. The trick is that your wind and solar charge controllers are usually not set to the exact same voltages. If you have good sun and wind together, either the wind generator or the solar panels will put out the slightly higher voltage and this can cause the other device to start to throttle back the power it provides. This is usually fine, as it often happens when the batteries can be amply charged by only one of the two sources anyway. But if you are using your batteries at the same time, and the voltage setting between the two devices is different enough, it’s possible that you can be pulling the maximum power from the active device but not dropping the voltage enough to trigger the inactive device to throttle up its power as well. In this case, your batteries will be undercharged and the energy you could be getting is effectively wasted.

As an example, this is fairly common with wind generators that have built-in voltage regulators with simple charge controllers. These only see the current they are outputting and don’t sense whether or not the battery bank is being utilized. If the voltage they “see” on the batteries is higher than their cutoff voltage, they stop providing power. If your wind generator cuts off at, say, 14 volts and your solar array is set to 14.6 volts, when your batteries are at 14.3 volts the solar array will be putting power to the batteries but the wind generator will not. This is fine, up to the point that the solar array cannot provide any more power. Let’s say your refrigeration kicks on and you are also charging your laptop battery in addition to trying to charge your main bank, or the sun goes behind a cloud for a while: you may actually be draining your batteries in this situation but the wind generator will not come online until the voltage drops to 14 volts, even though the wind may be blowing strongly.

Because this happens most often during the last 20 percent of the charging cycle, when batteries are in absorption stage, this can cause battery banks to be chronically undercharged. Many chargers are capable of handling this on their own, so the issue typically only arises when there are two chargers that have voltage setpoints relatively different from each other. So, the moral of the story is to set your charge controller setpoint voltages as close as possible to both each other, and the optimum setting for your battery type.

To get the most out of your solar and wind setups, it’s important to set your expectations appropriately, which means understanding how much power you typically need and how much power your solar and wind units are realistically going to produce. Install your power sources with proper safety equipment and quality chargers. And, importantly, match the voltage cutoffs for your charge controllers with each other, so that they all share as much of the load as possible. If you do this, you’ll be sitting at anchor enjoying the true serenity of that beautiful place with nary the sound of a generator to mar the soft lapping of the waves on the pristine sand. And while you soak up the sun and enjoy the breeze, you’ll get double the satisfaction knowing your boat is doing the same.

Daniel Collins, an ASA certified sailing and navigation instructor, amateur extra class radio operator and small boat racer, enjoys experimenting with marine electronics. He is also actively involved in community-driven social change. Email him at He owns Aletheia, an Allied Princess.

Author: Daniel Collins