Today, most powerboats in the 40+ foot size have 110 volt AC generators installed to provide power for various accessories and appliances such as microwave ovens, televisions, coffee makers, refrigerators, freezers, hair dryers and all the charging devices for lap tops, cell phones, PDAs and more.

But keeping a generator running while your boat is running can be a waste of money and add greatly to your mechanical maintenance costs. If your cruising lifestyle requires that these appliances be operable while you're anchored out, you'l be running that generator most of the time. Even with modern, underwater exhaust systems, you'll be the bane of the anchorage.

When I had my 43-foot downeast power cruiser built, I had an expert in marine electrical systems design a system that features a large inverter, a big house battery bank and a high capacity alternator. What's uncommon in a boat of this size is that it does not have a generator. Except for air conditioning, which I use only while plugged into shore power, I rely entirely on my inverter system for my AC power while running or anchored out.

Bob Campbell, the owner of Marine Electric Systems located in Severna Park, Maryland not only designed my boat's electrical system, but he supervised its installation. It has worked flawlessly for 5-years, and served us well on our 8,000 mile "Great Loop" cruise. Bob conducts seminars on this subject, and he was very helpful in providing up-to-date technical information for this article.

By designing the right inverter system for your boat, which essentially creates 110 volts of AC power from either a 12 volt or 24 volt DC battery bank, you'll save fuel and maintenance costs, and you'll enjoy peace and quiet when you shut down your main engine(s). Many new boats are now including an inverter system along with a generator as standard equipment.

It's important to understand the basics of how these systems work so that you can determine the kind of equipment you'l need as well as how to best "manage" your electrical usage. The word "system" is the key here, because an inverter alone can't do the job. If you do your homework, which requires some basic math, you'll end up with a powerful, reliable, safe system.

Determining Your Load Requirements

Designing the right system starts by understanding how you use your boat and knowing what equipment you want it to power. Just having the ability to run a microwave or hairdryer while your boat is under power is a lot less complex than having to run all your appliances while anchored out for days at a time. While there are inverter products specially designed to run a simple AC powered accessory without the need for hard wiring into your boat's electrical system, if you want a permanent, more robust system, you need to do it right.

Make a list of every piece of electrical gear you want to use. Start with all AC appliances except for your air conditioning system, which, because of its high power requirements and start-up surge, should be run off your generator or shore power only. Find the power requirements of each item in its instruction manual or on its back panel. This is usually given in amps at 110-120 volts. Now convert that number into a 12-volt power rating by multiplying it by 10. For example, a basic household refrigerator usually requires 3 amps when running on 110- 120 volts AC. The 12 VDC equivalent is 30 amps. Now estimate how long that unit will be running during a 24-hour period. If well insulated, this household refrigerator may run 40 percent of the time, which equates to 288 amp-hours of use over a 24-hour period. (30 amps X 24 hrs. X .40 = 288 ah).

Do this for every AC appliance you���d like to power, and then do it for all your 12 volt DC gear, such as cabin lights, navigation equipment, etc. (Don't multiply the 12 VDC amperage ratings by 10.) Estimate on the high side to be safe, and then add everything up. This will give you your estimated daily power requirements.

Choosing the Right Inverter

List which items you may want to run at the same time, and this will give you your peak power requirements, which determines how big of an inverter you���ll need. Inverters are usually rated in "watts," so multiply your peak DC amperage usage by 12 to convert to watts. (Watts = Amps X Volts) For example, if everything you want run by your inverter draws a total of 100 amps at 12 volts, you will need at least a 1200-watt inverter. But it's wise to specify a unit that has at least 20 percent more capacity, so in this case get yourself a 1500- watt inverter.

Inverter technology has come a long way during the past ten-years, and the cost of high performance units has actually decreased, much like computers. In fact, the cost of a full sine-wave inverter today is very close to that of a modified sine wave model. While most appliances will work okay on the latter, sensitive electronics and an odd assortment of appliances such as bread makers and laser printers will only work with full sine wave inverters. Many inverter models also include a high-end battery charger, something you'll want as part of your overall electrical system.

Batteries Are Not Included

You can't just hook up an inverter to your existing house battery and use it to power all these AC appliances with your main engine turned off. The inverter must get its electrical energy from a battery supply that is matched to your overall load requirements. Think of your house battery bank as a fuel tank. The bigger the tank, the longer you can run your engine. The bigger the bank, the longer you can run your electrical accessories. Instead of gallons, you'll want to know how many amphours your battery bank has.

The rule of thumb in determining the optimum size of your house bank is that it should be 3 times your average daily load requirement. Going back to our household refrigerator, which requires 288 amp-hours, you'll need a battery bank of about 865- amp-hours, a relatively large, heavy system. As a comparison, a top-of-the-line 12- volt marine refrigeration system that is properly insulated may require only 50 amp-hours to run all day. This rather startling comparison illustrates the benefit of carefully choosing all of your boat's electrical equipment and appliances. Take a hint from those miserly folks who own sailboats or trawlers, and outfit your power cruiser with high performance, highly efficient appliances made specifically for marine use.

Why do you need a battery bank that's three times larger than your daily load requirements? First of all, you should allow your batteries to discharge no more than 50 percent of their total amp-hour capacity. If you start with a hefty 1000 amp-hour bank, you can only use 500 amp-hours of it. Secondly, alternators will recharge the bank to 85 percent of its total capacity in the shortest amount of run time and then take 3 times longer to charge the remaining 15 percent. For practical purposes, this leaves you with a 35 percent window of power. In the case of a 1000 amp-hour battery bank, you really only have about 350 amp-hours of usable energy unless you return to shore and charge up with your battery charger or you run your engine for an extremely long period of time.

Because of today's accurate amp-hour meters, you can forget the old tried and true system of splitting your house battery into two circuits, switching from one to the other on odd/even days. Instead it���s better to create one large battery bank, which maintains its proper voltage over a longer period than one half its size. This will allow your gear to run more efficiently at a higher voltage. For example, many items will run, but not efficiently, with a voltage as low as 10.5 volts. A large battery bank will maintain its 12+ volt output over a longer period of time.

To make sure you don't run out of electrical gas,you'll want an accurate amp-hour meter that monitors your battery bank. Don���t rely on old fashioned, analog voltage meters, as they don't tell you what's really going on with your battery. Sophisticated units such as those marketed under the LINK name, will monitor your battery, telling you how many amphours have been used since the last charge. These devices are among the best investments you can make in your boat���s electrical system.

There are many types of heavy-duty, 12-volt batteries. Don't make the mistake of using an engine starting battery for your house battery. These are designed to deliver one thing: cold cranking hours. They are not efficient, deep cycle batteries, regardless of how big and heavy they are.

An economical type of house battery is the ever-popular 6- volt "golf cart" cell that can be wired in a series-parallel arrangement to create a large, powerful bank. These will provide about two to four years of heavy use. And, they have a small footprint, making their installation easier. The next step up are the more expensive, but still reasonable, ���scrubber-sweeper��� cells that will last three to five years with heavy use. Trojan is a brand that makes both types and has proven to be very reliable.

On the high-end side for those who will be anchoring out often in very remote areas, there is the 2-volt traction battery designed to operate large forklifts and tractors. The Rolls- Surrette brand is one of the top manufacturers in this premium category. But most power boaters will find the "golf-cart" or "scrubber-sweeper" varieties a good all-around choice.

If at all possible, your house battery bank should be installed near the inverter to minimize cable length and out of the engine room to reduce operating temperature. Nothing kills a battery faster than high heat (except a dead short, of course), and its key performance data is adversely affected by high temperature. Assuming proper maintenance, you can double the life of your house battery system by getting it out of your engine room.

Recharging Your Battery Bank

Most factory-installed alternators are designed to keep your engine starting battery charged. If you add a large house system, you'll want to add a high capacity alternator with a "smart" regulator that will recharge your house system while underway.

But not all alternators are created equal, so you'll want to specify one that can be properly installed on your engine and that will provide you with the right power curve for your particular requirements. For example, if you do a lot of idle speed trolling, you���ll want the alternator to deliver a good amount of its power at low rpms. Pulley ratios are important here, as is the overall power curve of the alternator itself.

Big alternators, ones that can deliver 220, 270 or even 300 amps, require as much as 10 to 13 hp from your engine, so you'll want to install it with a double-belt system harnessed to your engine's crankshaft. You'll also want to check the belt tension on a regular basis, because a slipping belt causes friction, which causes heat - a sure way to shorten the life of your expensive alternator. Leave the factory installed alternator in place to charge your starting battery and to serve as a back-up charging system.

To get the most out of this powerful recharging device, a "smart" regulator system will properly manage the rate of charge that it delivers to your battery bank. Units such as the LINK 2000R provide accurate monitoring of two separate banks and can be programmed to properly regulate your high capacity alternator, matching its output to the battery's characteristics and current condition.

While plugged into shore power, you���ll want a high performance, smart battery charger. Some inverters have built in chargers. Others are stand-alone products. Its interesting to note that the biggest chargers are rated at about 120 to 140 amps, making the alternator the more powerful charging device.

Of course, if you have a generator, it can power the battery charger, giving you an alternative way of recharging while anchored out.

Overall Results

If nothing else, a properly designed inverter system will significantly reduce the hours you need to run your generator. In some cases, like on my own boat, you may be able to do without a generator. With a 2500-watt inverter, a 915 amp-hour house battery bank and a 220-amp alternator, we can anchor out for up to 4- days and nights without having to recharge. On a 4-to-5-hour run to our next anchorage, we���ll charge up to 85 percent of our capacity and be ready for another 3 days or so. The only appliance we can���t run on the inverter is our air conditioning system, which we use only while at a marina.

There are many new, high performance inverters and smart chargers on the market. Some of the top brands to consider include Xantrex (it now owns Heart, Pro-Sine, Trace and Link) and Charles Industries.

But a good, reliable, safe system is really beyond most do-it-yourselfers. While it's hard to get electrocuted with 12-volts, using the wrong sized wire or making a mistake can cause a serious fire or damage to equipment. And since an inverter is virtually silent, it's easy to forget that 110 volts are being supplied to your AC outlets.
For more information Charles Industries,(847) 806-6300;www.charlesindustries.com; Xantrex Technology, (800) 670-0707;www.xantrex.com