Replacing a generator is far more common than replacing engines. I've seen many boats with twice as many hours on the gen set than the engine(s). After all, once you arrive at your destination, you want all the comforts like air conditioning, the refrigerator, and the ice maker to keep humming along.

In addition, many new boats are built without a generator, but are prepped with a platform, through-hull fittings, and fuel lines for after market installations. No matter if you're replacing your generator, or installing a new one, one of the most crucial elements is installing a properly sized unit.

First of all, a generator that is undersized will not only run poorly but may damage some electrical components due to low voltage. However, a generator that is too large will have an engine with a shortened service life due to carbon buildup and inefficient fuel combustion. You need to be aware of several factors to ensure your generator is properly sized.

The philosophy for boats built during the mid 80s dictated simple power requirements that were primarily DC (direct current). At that point we were satisfied to live without microwaves and big home theater systems in our saloons. The theory then was to draw as much DC power as possible because generators in those days were heavy, bulky, and took up valuable engineroom space. Many boaters preferred (and some still do) having large alternators and battery banks rather than sacrificing the space and fuel consumption required for a generator.

As boating habits and equipment have changed, so have on board power requirements. Some owners want computers to interface with all the navigational electronics; children want to play video games and watch a DVD movie in an air-conditioned environment. And household-sized appliances and ice makers have replaced hardy systems like holding-plate refrigerators.

To be able to run this host of modern conveniences you need a properly sized generator and need to be aware of the loads when starting the unit and systems. For example, I was approached by an owner of a boat who had a yard install a new and compact 3 kW generator. All he wanted to run was a small battery charger and a 10,000 BTU air conditioning unit. While plugged into shorepower the air conditioning worked flawlessly, cooling the cabin nicely. However, once he headed away from the dock for a little cruising, he could not get the a/c to run off the generator. While the running amperage draw of the unit was within the generator's capacity, no account was made for the start-up load from the air conditioning compressor. The problem was that the load at start-up exceeded the generator's capacity by more than 25 percent. That's a lot. And even though this load was momentary it was enough to cause the generator to trip off its power supply. In the end, the 3 kW generator was replaced by an adequately sized 5 kW unit.

So whether it's a new build or an upgrade, AC electrical sizing of the generator should be the first step in the installation process. First and foremost, a generator should never run with less than a 25-percent load. So if you're cruising along with the unit running just to keep a few brews cold, you may need to turn on the air conditioning or another high load system to put the necessary load onto the unit. And for optimum performance, a load between 35 and 70 percent is desired. For larger boats with greater and varying power requirements, two generators may be the answer. A larger unit may be needed to meet higher power demands during the day such as a washer and dryer (watermakers for those that venture out of the Great Lakes), appliances, and a/c.The smaller unit can handle night loads such as air conditioning. And by using the smaller gen set for this purpose, there is not the concern of being under powered. The other option is to have two identical units that can be paralleled via a switchboard for heavy usage. A primary benefit of two identical units is redundancy for spare parts. This should not be a concern for the Great Lakes boater, unless you're planning on some extended cruising away from your home waters.

A professional will help you along in the process, but it never hurts to know how the sizing process works. It only makes you more aware of what is going on in your boat. If you're not planning to install a new generator, I recommend you still review this chart to make sure you have the right system, or if you plan on adding another AC component, you are not overloading the generator like the aforementioned example.

In order to calculate load requirements, we've included this guideline from the American Boat and Yacht Council. Please note that the ABYC assumes that not all appliances will be running simultaneously.

See Part One