Inverters

by BoatUS

If your idea of getting away from it all is taking it all with you, onboard AC power is a must. With the use of a 110vAC power source you can operate the microwave, TV, hair dryer, or any other appliance that you'd rather not do without.

110vAC power is available three ways: as shore power, or through the use of either a generator or inverter. Your choice of which to use will depend on your power requirements. Assuming you don't want to stay tethered to the dock, let's compare generators (gensets) to inverters.

Gensets produce large quantities of continuous power, and are ideal for running air conditioners, refrigeration, and other high-load, long-duration applications. They will also charge the ship's batteries while providing all your power needs. However, gensets are mechanical devices and require engine-type maintenance. And although modern technology has rendered the generator's old smoky, noisy, monolithic stereotype obsolete, gensets do vibrate, require a fuel supply and exhaust system which consume space, and aren't all that quiet. If your needs are simpler - or at least more intermittent - an inverter might be the tool of choice. Compared to gensets, inverters provide an economical, maintenance-free, and relatively compact source of AC power. Because of their solid state circuitry, inverters no longer use mechanical vibrators and are therefore quiet and non-intrusive. Although the inverter produces a modified sine wave which is less pure than the true sine wave of a genset, the power from most of today's inverters is "clean" enough to run even very sensitive electronics, including computers.

Whether an inverter, genset, or a combination of both is the best solution for you requires an honest accounting of your power needs and an understanding of how these AC sources work.

How Inverters Work

Inverters work somewhat like battery chargers in reverse: they convert 12vDC power from a battery, through modern circuitry and a step-up transformer into 110vAC current. The ship's batteries are the inverter's fuel tank, and by nature, inverters are real gas guzzlers. You can only draw upon the juice left in the tank without recharging, which is why the inverter's optimal application is handling lighter, intermittent loads. Since the inverter places such a huge drain on the batteries, it is strongly recommended that you have a separate, dedicated engine starting battery. You wouldn't want to sacrifice your engine starting for the sake of a cold drink.

Because inverters and battery chargers can share certain electrical components, many units are available with both these capabilities. Of course, you need to be drawing power from an alternate source (either shorepower or a genset) to charge the inverter’s batteries. Most of today’s inverter/chargers are "smart;" that is, they contain an AC sensing circuit that will switch the inverter to charge mode when in the presence of an alternate power source. They can also safeguard and prolong the life of your batteries. While charging, the inverter/charger will monitor the batteries' level of charge, backing off the power when it senses the batteries are nearing full capacity. Since overcharging is the leading cause of death among batteries, this is an important feature to consider.

Choosing an Inverter To select the appropriate inverter for your needs, take a hard look at what your needs actually are. The key is to determine your maximum requirements at any given time. This refers to the wattage drawn by each appliance, the duration that it is used, and the number of appliances you want to operate simultaneously. The most accurate way to tote up the watts is to check the data plate on the appliance itself. This is especially important for items like hair dryers, which can vary in wattage from 500-1,500 watts, depending on the model. Keep in mind too, that some equipment, like blenders and power tools, will draw a power surge for a few seconds when they are first switched on. Modern inverters will generally accommodate these surge loads, as long as you don't switch everything on at the count of three.

To determine the maximum power you require, and the size inverter you need, make a list of all the equipment you will run, and the amount of time it will be in use. If the plate states the power in amps, convert to watts using this formula: Volts x Amps = Watts

Choosing Batteries

Your car won't go far without gas, and the same is true of inverters and batteries. The type and size batteries you use are critical to the proper operation of the inverter. An engine starting battery is designed with numerous thin plates, providing a high surface area needed to produce the short, powerful energy burst to start the engine. Inverters, however, are constantly discharging and recharging their batteries, demanding a different battery configuration. Deep-cycle batteries contain thick, coated plates designed specifically for this type of load.

Although golf cart batteries share the same characteristics as deep-cycle batteries, their use is discouraged because they are not intended for, nor suited to the marine environment. To determine the size and number of batteries you'll need, let's look again at the chart. This time, we'll be adding up the amps for each appliance: in our example, the sum is 408.5 amp hours. We still want that waffle iron - what does it cost in battery power? You can determine the amp hours drawn by any appliance by using this formula (always round up): AC watts ÷ 12 x 1.1 x Hours of Use Example: 13" TV: 50 watts (taken from appliance data plate) ÷ 12 x 1.1 x 2 hours= 9 amp hours

If the data plate lists AC amps rather than watts, no problem, just use a different formula: AC Amps x 10 x 1.1 x Hours of Use= Amp hours

In the earlier example, we figured that we needed 408.5 amps to meet our power requirements. So, we simply determine the type and number of batteries we need to total a capacity of 408.5 amps, right? Well, nearly. Nothing's perfect, and batteries are no exception. Batteries enjoy a subtropical 77°, and hotter or colder conditions can reduce their efficiency by 20-50%. So instead of selecting batteries having a 408.5-amp capacity, we'll play it safe and look for 50% more amps, in this case, a total of 613.

One more wrinkle: when figuring the amp hours you need, don't forget that your AC appliances aren't the only items on your batteries' dance card. The batteries still have to handle your DC appliances too - including 12vDC lights, electric head, pressure pumps, etc., so take a hard, thorough look at what you're running.

Just as you monitor your car's fuel gauge, you must be aware of the status of your batteries. Methods of checking the battery's juice level can range from using an inexpensive hydrometer, to purchasing one of the excellent monitoring systems offered by inverter manufacturers. Some of the features available include low battery and overload warnings and the number of amp hours consumed, displayed on a remote panel.

Only a complete accounting of your power needs will determine the source of AC power that's best for you. Whether the answer is a genset, an inverter, or a combination of both, the technology is available to make you feel like you never left home.