We’ve just finished updating the inverters on Wind Horse so we thought it might be timely to discuss the logic of how to chose an inverter.

There are a number of choices available. The question is, which is the right unit with the best features for your intended application.

To begin with, most boats these days are fitted with combination inverters and battery chargers. Both functions usually are in one box. We want to see an efficient battery charger, hopefully with the ability to equalize our batteries (for more on equalization see Offshore Cruising Encyclopedia). Different styles of batteries have varying charge requirements, so you will want to make sure the charger can be adjusted to fit your needs. Also, check charging capacity. Generally speaking, a minimum charge level of at least 10% of the battery capacity is required.

This is the new pair of our Mastervolt inverter chargers just installed. Noise is often an issue with chargers. Depending on the location of the unit(s), you will want to make sure you can tolerate the noise when it is running, especially at night. We always try to mount our units away from bulkheads.

On Wind Horse the inverters are mounted with vibration/noise isolation mounts, which bolt to a free-standing aluminum frame.

Finally there is the issue of AC capacity for DC charging, either from shore power or from the generator on board. Modern inverter chargers operate somewhere in the range of 70 to 85% efficiency when in charging mode. With a big charger and modest AC power sources, it is easy to overload the circuit, especially with other AC requirements online as well.

This is where a feature called “power sharing” comes into play. Some models of inverter chargers are smart enough to sense other AC loads. They look at amperage being used, and then back off the charging demand as other AC loads come into play, so that the shore power or genset circuit is not overloaded.

There’s a wonderful corollary to this if you have a genset and not enough load. This smart charger function can be used to maintain generator loading at a higher level than might otherwise be the case. Take our set-up, for example. If we are doing a laundry cycle, the washer or dryer by itself only loads the genset to about 20 or 25% of capacity – not good for the genset. With the chargers going, they can bring the load up to 75% with some simple settings. The genset is happier under load and we get some battery charging at the same time. If we turn on the watermaker, or one of the air conditioning units, the battery charger function senses the increase amperage load and backs off, maintaining a constant overall load on the genset.

Now let’s look at the inverter side. Most modern inverters produce a modified sine wave form of AC. We’ve found this works for computers, AV gear, and other heavier loads without too much hassle. Capacity should be based on your AC requirements and your ability to sustain the load with either battery bank size or alternator(s) on the engine under way.

On most of the boats we’ve done the last 15 years we fitted 2500-watt units, as these were the biggest available. Wind Horse has a pair of 4000-watt models, so we have the same capacity as the genset.

Inverter capacity is a function of battery bank size, ambient temperature, and inverter rating. The higher the voltage the inverter operates on, the more capacity it will have. The same applies to heat. Good ventilation is critical to prolonged periods of high output. If you are powering along, with the alternator keeping the batteries topped up, the inverter will be able to output more power than if you are sitting on the hook with lower voltage from batteries only. Our 4000-watt models have a hot continuous rating of 3750 watts.

Check both the surge (or starting) capacity of the inverter as well as the long term rating at various temperatures. We always use the warmest (lowest) rating when checking on size. It also helps to verify with users that in the real world the inverter will actually do what its rating indicates. We have found that sometimes inverter manufacturers exaggerate a bit.

Some inverters can be “stacked” or ganged together. The inverter matches the wave form of stacked units so they act as one. This is ideal for higher loads, as you can turn on one two or more inverters when the loads are high, and just run a single unit when they are low.

In our case, the Mastervolt units could not be stacked, so we have to separate the output to feed two different sides of the AC panel. This meant that both inverters had to be on, even though we might just be using a small percentage of each inverter’s capacity.

This is inefficient in terms of battery drain, so Todd Rickard, at Yacht Masters on Lake Union, came up with a simple switch system.

Each inverter output goes through these selector switches. These are set up so either inverter can do both sides of the panel if loads are light, or they can do their own sides of the panel when loads are higher.

Also, check the idling current. This will vary with both inverter type and setup within individual units. If the idling current is low enough, the inverter(s) can be left on without excessive drain on the batteries.

On Wind Horse we use the inverters under way to supply all our AC needs, including running the air conditioning and watermaker. Our Electrodyne alternators supply up to 10kW of DC power, with a nice high (27V) line voltage to keep the inverters happy.

Some recent inverters are available with a new feature called “power assist”. In this case, they sense when the genset or shore power circuit is being overloaded and add in some of their own capacity, drawing from the batteries, to help out with these extra AC loads.

This is usually built in, but we had to install external switches because of the wiring setup of our units.

Al McNeir, the chief electrician at Yacht Masters, is installing these external transfer switches.

They are pretty complex looking, but so far seem to be doing the job.

In our case, if we’ve got the washer, dryer, watermaker and some air conditioning drawing power, from time to time we’ll exceed our 8kW genset capacity. This typically happens for a small percentage of the time, usually when the dryer goes into heat mode for a few minutes. With power assist, the inverter senses this genset (or shore power) overload, and kicks in with some extra amps. When the AC power need drops down, the inverter backs off. This allows us to use a smaller genset, which is more easily kept at a high load most of the time (lightly loaded gensets are not happy). It is also helpful when we are connected to less than robust shore power.

A word of caution on this subject. As we mentioned earlier, we’ve found that suppliers tend to exaggerate their capabilities. If you are only using a small portion of the rated capacity – for charging or AC power – this is not going to be a problem. But in a situation like we have with Wind Horse, where we really need both the AC and DC rated outputs, it is hard to know short of actually installing a unit if it will do the job. The reputation of the manufacturer will give you a clue. Better yet, if you can find boats actually using the gear in a similar manner to what you have in mind, you can verify in the real world what is happening. Installers who sell a variety of gear will also be able to give you a feel for what you can expect.

The key issue is to be realistic about your needs. And, if in doubt, upsize the capacity of the unit just to be sure.

Our application is not the easiest for this gear. Our large battery bank – 1500 amp hours at 24V (10 amp hour rating) demands maximum output on charge cycle. Under way in the tropics we will often run for long periods with watermaker and air conditioning plus washer or dryer – a six to eight kW load. We’re just starting to use these new inverters from Mastervolt and there are some wrinkles to iron out. We should have them going properly in a few weeks and will report back on how they are doing.