One solution to the problem of having AC electrical power on board is a dedicated genset. An alternative, however, is the use of an inverter for changing DC power to AC. The compact, quiet inverter allows many voyagers access to AC, even while on the hook or making passages. The down side of marine inverters is the relatively low amount of current they can provide efficiently.

Ten years ago, we had an inverter on board essentially to run a small TV and a computer. And on more than one occasion the availability of AC power came in handy. For example, once we were surrounded by a pod of humpback whales, and the batteries on the camcorder went dead. I plugged in the AC adapter and continued to tape using a long extension cord. Another time in mid-ocean the mainsheet bail on the boom broke, and my hand drill was in a drawer jammed shut by shifting equipment. Frustration mounted until I remembered my 3/8-inch AC drill was available. I fired up the inverter and used that drill to make the repairs.

If you want to take advantage of the cost savings, convenience, and wider selection of AC equipment, maybe an inverter is in your future. To select the right inverter you need to answer at least three questions: what size and type do you need, and what impact will the inverter have on your battery requirements? I wish I could tell you that the answer is “six” or some such number, but, as with most things in life, if we want convenience we have to do some soul-searching. Inverter size

To size the inverter, make a list of the AC-powered gear you might want on board. Then list the power requirements for each piece of gear. These requirements are often listed on the box or on the UL label attached to the equipment. In some cases the power requirements are given in watts and other times in amps and voltage. Both are useful, and they are related by the following equation, in which power (P) is equal to the amperage (I) multiplied by the volts (E) or:Equation 1: P = IE

Electrical engineers supposedly have good reasons for designating current and voltage with the letters I and E, but then what can one expect from a field where they measure things in Gauss and Henries? Table 1 lists some representative values for various pieces of equipment often found on boats. Right now you can ignore the last two columns in this table. We will fill in these columns later when we discuss the impact on your battery bank.Once you have identified the equipment you want on board, then you need to determine which equipment is likely to be used at the same time. Try various combinations and sum the total power requirements for each combination. Select the largest total power requirement to size your unit. Keep in mind that marine inverters come in sizes ranging from 50 to 2,500 watts. Because of this size limitation, you may want to adjust your combinations and see that some equipment never operates on the inverter at allfor instance, the water heater. This can be done by splitting your AC panel and only wiring selected loads to the inverter portion of the panel.

Now, as you know, inverters take power in and send power out. The numbers in table 1 are power out, and since there are some losses through the inverter there is a difference between power in and out. That difference is called efficiency, and the relationship between power and efficiency is given in equation 2. To select the correct-size unit, you need to convert power out from table 1 to power in by dividing by the efficiency of the unit. These units usually operate between 80% to 96% efficiency, depending on the load, so 90% is a good average to use.

Equation 2: power in = power out/efficiency

This equation will increase your power requirements somewhat and will give you the size of your inverter. The next step is to decide on what type inverter is best for your application.Inverter type

Inverters come in three basic types: a true sine wave, a modified sine wave, and a square wave. The theory of inverter operation is to break the DC voltage into pulses. These pulses are then passed to a power transformer that increases the voltage. The simplest inverter merely turns the DC power on and off and a square wave is generated (seefigure 1). The output voltage, or height of the wave, is dependent on the input voltage. Since the voltage in a battery bank can vary from 10 to 14 volts, the peak output voltage varies significantly.

This variance increases the area under the voltage curve and makes some AC equipment act oddly or not at all. Generally speaking, square-wave inverters have difficulty with equipment that produces loads other than pure resistance. One way to correct the problem is to equalize the area under the curve by altering the pulse length. This produces the modified sine wave. As the peak load increases, the pulse width is shortened to produce the correct area under the curve. Conversely, as the voltage drops the pulse width widens until there is no zero voltage time left and a square wave results.

The modified wave form produces satisfactory results for most AC equipment, although microwave ovens behave unpredictably, they do at least function. Fluorescent lights and dimmer switches may not operate properly. Dimmer switches generally lose their ability to dim, operating only in full off or full on. If you need this gear to operate correctly, or in the case of sensitive equipment, like the equipment shown below the bar in table 1, a true sine-wave inverter is necessary.

The true sine wave generated by more expensive inverters produces power that is cleaner and steadier than the normal power supplied by many electric utilities. Further note that all inverters may still have problems with some power supplies and rechargeable devices that require power be available before they start to operate. Since the inverter is looking for demand before it operates, the two pieces of gear do nothing but argue over who goes first.

Finally, almost all inverters generate these wave forms through the use of high-frequency oscillators. High-frequency oscillators can cause interference with some sensitive equipment like SSB or ham radio, weatherfax, and loran receivers. Recently, several manufacturers have come out with modified sine-wave equipment that uses line frequency that eliminates this problem but increases the size, weight, and cost. Another way to eliminate the interference problem is to turn off the inverter when using the sensitive equipment. Battery impact

Now that we have selected the size and type of inverter it is time to see if our battery bank is sufficient to handle the loads. The size of the bank depends on the current demand, the amount of usage and how often the bank is recharged. For this discussion we’re talking about deep-cycle, not starting, batteries.

Deep-cycle batteries are rated by reserve capacity. Reserve capacity is measured by attaching a standard 25-amp resistance load to a fully charged battery for 20 hours. The open-circuit voltage is then measured, and, using the standard voltage percent charge curve shown in figure 2, the reserve capacity at 10.5 volts is calculated.

The reason they use a standard 25-amp load is because discharge rate depends on current draw. The larger the current demand, the more rapid the rate of discharge. For example, if you have a single 100-amp-hour battery and draw a steady 25 amps, it will take four hours to discharge it to 10.5 volts. When a 50-amp load is attached to the same battery, you might expect it to discharge in two hours. Not sobecause the current draw is larger than the standard, the discharge will take place in much less than two hours. Now here comes the tricky part: if your 100-amp total is made up of two 50-amp-hour batteries, your discharge is back to two hours again, since each battery supplies only 25 amps to the total.

Current demand on a battery bank greater than the 25 amps is normally not much of a problem for most DC equipment because their current draws are quite low. However, an inverter can draw as much as 200 amps. With this kind of a load you can almost see the top of your battery suck down. Thus it is better to build your inverter battery bank with eight or even 10 smaller batteries than a couple of very-large-capacity batteries. Keep the load on a single battery below 20 amps by increasing the number of batteries above the total inverter current draw divided by 20.

Equation 3: total batteries = total inverter current draw/20

The term amp reserve capacity is a little misleading in that a 100-amp-hour battery cannot supply 100 amp hours of usable power. Remember, the test for reserve capacity draws the battery down to 10.5 volts. Unfortunately for battery users, most DC equipment is designed to operate more efficiently when the voltage is greater than 12 volts. If we look again at the typical battery voltage discharge curve, figure 2, we will see the percentage of power left when the battery voltage is 12 volts is 50%. Thus the real usable power is that power above 50% discharge, not the 98% or so discharge used for the rating test.

It can also be seen from this figure that a rapid discharge occurs above about 13 volts, or until the battery is about 95% charged. Thus, most systems are designed to operate between 50% and 95% charge, or about 45% of the battery’s rated amp-hours are really usable on a day-to-day basis. Therefore, the reserve capacity of our battery bank needs to be larger than our total amp hour draw between charges divided by 0.45.

total amp hour draw/0.45

Now all we need to do is figure out our total amp-hour draw between charges. The first step in this process is to estimate how much DC power will be used per day. Return to table 1 and estimate the hours that you intend to use the AC equipment each day. List that data in column 3 of table 1. This will allow you to calculate the values in column 4 using the following equation. Equation 5: amp-hours = (hours of use)(power)/12

The total value for column 4 will give you the total amp hours your inverter will draw from your batteries per day. You will then need to construct a table similar to table 1 but this time include all the DC equipment including the inverter draw. Notice in table 2 that power is being rated in amps.

Normally the power usage is dependent on the boat use. This table has divided the use into three columns. “Passagemaking” assumes low engine use and no shore power; “anchored” assumes no engine use and no shore power. “Dockside” is connected to shore power full time. Fill in the columns with the numbers appropriate to each use.

Once the daily total power output is calculated it is next necessary to account for daily power input. How about a third table? The difference between the total usage in table 2 and the generating capacity in table 3 is your power deficit for passagemaking and anchoring. Take the largest of these deficits as your total amp-hour draw for use in equation 4 to size your battery bank. Making decisions

At this point expect to do some juggling of these numbers. How you wish to juggle is a highly personal decision. This deficit can be made up by decreasing usage, increasing generation, making more frequent trips to a dock, or increasing battery capacity. For example, it is possible to get by on almost no battery capacity: just turn off all power equipment and leave the engine running while you swing on the hook.

With all these electrons running in and out of a battery, some people may feel better if they had a way to keep a close eye on the level of power left in their batteries. One way to do this is with an E-meter or battery-monitor system. These systems generally run from $200 to $500 and take the guesswork out of available power. However, measuring the no-load voltage with a quality digital multi-meter and using the curve in figure 2 will do the same thing for around $50.

So, granted all of this has taken a little soul searching, but once the decision is made, you will never again have to face life without your frozen daiquiri.