Electrical Inverters for Sailboats

It seems sailors these days just can’t get along without some source of alternating-current electricity on board, either from shore power, which requires staying at the dock all the time, or by running an AC generator or possibly an inverter, which draws power from the boat’s direct-current battery bank.

I love comparing today's boats to what my wife and I used on board when we were cruising back in the 1980s. Our boat didn't have any AC circuitry! We read at night using gimbaled oil lamps; we used a sun shower for hot water. Coffee was brewed using a French press heated on the propane-fueled stove-top, music came from a battery-powered AM/FM cassette player and we dried our hair in the breeze as we sailed. Oh, and our refrigeration system was powered by a strange thing known as block ice. My, have times changed!

Last month in Hands on Sailor, we discussed selecting a generator for your boat. Now, let's take a look at a closely related device, the inverter/charger. We'll consider models from Mastervolt, Magnum Energy, ProMariner, Victron and Xantrex, all well-known players in the marine marketplace, and compare their advantages and disadvantages, with an eye toward adding just a little bit more convenience to your sailing.

Once you’ve decided that blow-drying your hair in the wind is not the approach you want for your cruising lifestyle, you need to carefully figure out how much power you need. Modern inverters are available with 5,000 watts or even more power potential, and many models can be connected in parallel, effectively doubling — and then some — their output in kilowatts.

Sailboats equipped with an AC generator will usually have one in the 4 to 10 kW size range, depending on the gear sailors intend to use regularly. When installing an inverter, you may have to decide to eliminate some of the items you would power either from shore or your AC generator. It all depends on how much battery power you have on board.

RELATED: Choosing a Generator for Your Sailboat

The pluses of inverter power drawn from the battery bank are silent operation and not having to smell diesel exhaust. But, there are trade-offs. The batteries needed to power an inverter are heavy and take up a lot of onboard real estate, especially if your boat has older-style lead-acid batteries. Think of battery power as the fuel for an inverter. The greater your electrical appetite, the more storage capacity you’ll need. In some installations I’ve seen, the lack of space for batteries has been the limiting factor on the electrical system.

Generally, inverter power is not used for items that run continuously, such as refrigeration and air conditioning, which are instead run either directly from the battery bank, in the case of refrigeration, or run from shore power or generators. That said, it can be done, but you need to understand that the amount of battery power you’ll need to make this happen will be considerable and might bring you to a point of diminishing returns. The capability will depend upon whether you already have upgraded to new technology, such as thin-plate pure-lead absorbent glass mat or perhaps even lithium technology.

Several important technical limitations play into the whole inverter-battery relationship. One is the fact that modern inverters are approximately 90 percent efficient, so there is a 10 percent loss of power right from the start. Second, battery amp-hour capacity will be a limiting factor. The term I use to describe this is “current density.” Simply, this means how much battery capacity you can squeeze into the space available aboard your boat. This is an area where significant improvement has occurred over the past decade. Historically, we have used a 50 percent level of discharge as a maximum for flooded-cell lead-acid batteries. So, a battery with a 100-amp-hour rating would only have 50 amp-hours available if you wanted to maximize its cycle life.

Many of the new AGM battery vendors are now claiming an 80 percent level of discharge as acceptable and not damaging to cycle life expectancy. So, that is a 30 percent improvement in available amp-hour capacity, if you are willing to spend the money for the initial purchase of these new batteries. Moreover, you can theoretically get more available power into the space you have available compared to using less expensive flooded-cell batteries. This could be the difference between being able to run an air conditioner all night on inverter power and enduring a hot, sweaty night on board.

The take-away: You are going to have to perform an honest evaluation of how much power you intend to use in a 24-hour period. Next, you must determine whether you will have battery-recharging capabilities. Your options include shore power, an onboard AC generator supplying power to a permanently installed battery charger and running your engine-driven alternator long enough to get your batteries back up to charge.

This brings up another point regarding modern batteries: They can be recharged more quickly than older flooded-cell batteries. Battery acceptance rate is the term here. Flooded-cell batteries have a recharge acceptance rate of around 25 to 30 percent of their amp-hour rating as a maximum. This means that no matter how many amps you try to shove back into them, they will only accept power at this fixed rate. AGM-technology batteries have a slightly higher acceptance rate, in the 30 to 40 percent range, saving engine run time if your recharge method is either with an AC generator or propulsion-engine alternator.

With these factors in mind, you can see that selection of an inverter and its rating is going to require a bit of calculation to determine your expected loads, the amount of time you expect to use them and what your real-world battery capacity is going to be.

Also, if you are going to be powering air conditioning or refrigeration equipment using a compressor motor, you need to take into account motor start-up current. All inverters have a peak output rating in their specifications. Just make sure your peak amperage demands for motor start-ups don’t exceed your inverter’s peak output rating. Most, but not all, have a peak rating that is twice the nominal rating. (One of the Magnum 3,000-watt units we looked at has a maximum rating of only 3,900 watts, an exception to the rule.)

Ten years ago, the matter of whether you needed a true sine wave or modified sine wave inverter was a decision with a significant price differential. You were going to pay big-time for the true or “pure” sine wave inverter compared to a modified square-wave model. Today, that price differential has shrunk considerably, and depending on whose inverter you choose and its rated output, you will only be looking at a $200 to $300 price increase.

The essential difference between the two types is the AC waveform that gets created as part of the ­conversion from battery-supplied direct current to alternating current. The pure sine wave AC is nearly indistinguishable from utility-delivered power, and it is able to run the most sensitive of equipment. The modified square-wave option introduces what is described in the world of electronics as “noise” into your electrical system. This conducted electronic noise can affect the performance of some onboard appliances that are becoming quite popular on modern cruising boats. Audio ­amplifiers might turn the electrical noise into an audible hum, for instance, and noise can cause television screens to flicker. As more and more electrical devices acquire digital controls as a part of their designs, the potential for issues with modified square-wave inverters becomes more real.

On the other hand, devices such as hair dryers, toaster ovens and most coffee makers are not as finicky about their power source, as long as voltage and frequency requirements are met. In today’s global marketplace, many appliances are rated for 120 and 240 volts, and dual frequency at 60 Hertz or 50 Hertz.

If in doubt about a particular appliance, check the label on the device to provide you with vital information on wattage, amps, frequency and volts needed. All you need to do to figure out how many amp-hours of capacity you are going to need is consider how long you intend to use the equipment. Multiply that value by 1.1 to account for the efficiency loss in the inverter, and you should be close to your inverter-rating needs in watts.

The newest inverters have incorporated some impressive features that you may use, depending on how far you want to go with integrating the inverter into your onboard electrical system. For example, inverter/chargers are now commonplace, and I would certainly recommend one, especially if you are upgrading to modern batteries and your existing battery charger is more than five or six years old. Of the popular brands in our roundup, we focused on the ProMariner inverter-only devices, but when you look at our comparison chart, “Inverters by the Numbers,” page 84 of our December 2018 issue, you can see that the price differential between its inverter and all the other brands with integrated chargers is such that purchasing a ProMariner charger that will match nicely with these latest-version inverters is a very doable option. ProMariner does offer two slightly older-technology combination units at its online outlet store; these offer both inverter and battery charger in one case. The pure sine wave ProMariner inverter has a 2,000-watt continuous and 5,600-watt peak output rating. The modified sine wave model has a 2,500-watt continuous rating and a 7,000-watt peak rating. They are available with retail prices of $1,495 and $1,149, respectively. With those two units, the charger technology integrated into the units is quite state-of-the-art. The inverter side of the device does not offer the same capability as the inverter-only units in our comparison. So, it really depends upon your specific needs and product availability.

The newest chargers from the manufactures listed in “Inverters by the Numbers” come with three- or four-phase charge capability — step one in ensuring long battery life — and are programmable to more effectively match a specific battery technology. If you are going to invest in the latest batteries, you will want to maximize your battery cycle life, and these newest inverters/chargers have that capability. In my view, this is the compelling reason to pay a bit more and get this more sophisticated charger capability. It’ll maximize your return on investment when it comes to the battery bank.

Another relatively new inverter feature is the ability to automatically switch on and supplement generator or shore-power output when the loads on the boat reach their peak. This “cogeneration” capability, popular in Europe for some time, has only been allowed on U.S. boats under American Boat and Yacht Council Standard A-32 since 2012. Among the inverters mentioned in this story, the Magnum units, both Mastervolt devices and the larger Xantrex inverter offer this capability.

Some inverters will automatically switch to shore power when it is sensed, preventing unnecessary battery drainage. In some cases, this will also work in reverse: When shore power is lost for whatever reason, the inverter will sense this and turn on automatically. All the inverters and inverter/chargers we’ll mention here have programmable low-voltage shutdown when battery state of charge reaches critical levels. This will also help prevent damage to your expensive batteries.

To sum up, today’s inverter/chargers offer a list of features that can make them and the batteries that fuel them work more efficiently than ever. There is considerable initial cost, however, involved in designing a system that can conceivably power up the average 35- to 45-foot cruising sailboat. This is especially true if you want to have silent AC power available away from the dock and more of the conveniences from home available to you while on the hook.

That said, I am more and more convinced that it is possible, with ample storage space for batteries, to design a system using alternative ­battery-charging means, such as solar, wind and hydro power. This means you can have the luxury of home without the need to run AC generators or main-­propulsion engines just to replenish your batteries — and I love the concept of minimizing the need for fossil fuels to every extent possible. Long term, I believe that capability far ­outweighs the initial ­investment required.

Ed Sherman is a frequent CW contributor, Boat of the Year judge and is vice president of education for the American Boat and Yacht Council.