How to Choose the Right Generator for Your Boat

Got the Juice?

Alternating current puts the “pleasure” in pleasure boating. Here are some tips on how to get enough of it.

Unless you’re truly hardcore—you eat gruel boiled on an alcohol stove, read Treasure Island under the dim amber glow of an oil lamp, perform your nightly ablutions with a bucket of cold sea water (and that’s when you’re at home!)—you enjoy the benefits of alternating current. I know I do. But without a really, really long yellow cord to keep the juice flowing when away from your slip, you need a genset. Sure, inverters are handy at times, but they just don’t make it as primary sources of 120-volt juice. The good news is, there’s a genset to fit almost any boat. The tricky bit is picking the right one, and a big part of that is deciding how many kilowatts you really need.

Most people invest in a genset to run the air conditioning while underway, but there’s always a use for 120 volts even in cool weather: An electric stove is more convenient than propane or alcohol, it’s nice keeping food cold in a fridge that doesn’t need ice, and hot water 24/7 makes everyone happier. Finally, if you want to spend a couple of days swinging on the anchor, a genset lets you worry less about running your batteries down while communing with nature. Just do your anchorage neighbors a favor and don’t run the generator all night.

How Many Kilowatts Do You Need?

Some experts say the way to determine how big a genset you need is to add up the total wattage of the AC appliances you figure you’ll use simultaneously, then divide by 1,000 to get the required kilowatts to run them. Sounds good, but it’s wrong: You’ll want twice that figure, or close to it, so your genset isn’t running fully loaded all the time. Overloading the genset not only stresses out its engine, but also puts heavy demand on the alternator side, which will cause it to overheat, reduce its output and shorten its life. A genset should run at between 35 and 80 percent of its max load. Shooting for 50 percent load gives some leeway if your crew uses more AC than you planned for, or for the added demand when electric motors start; on start-up, a motor can draw two or three times as much current, momentarily, as when it’s running. Sometimes it’s more: A typical 12,000-Btu air conditioner uses about 9 amps of AC for maximum cooling, but its start-up current draw can be over 50 amps. A/Cs stop and start all the time, so you want some extra juice on hand. Ditto refrigerators, although they don’t draw as much as the mains or air conditioning.

Rather than figuring potential AC loads, here’s the lazy man’s way. Multiply your shore power inlet amp rating by 120 volts; a 30-amp service equals 3600 watts, or 3.6 kW. That’s the max you get when you’re plugged in, and if you’re not blowing the main breaker when you’re living aboard at the marina, it’s enough. Now adjust to find the rating of the genset you need: If you want to run the genset at no more than 75 percent, divide 3.6 by .75 to get 4.8—so install a 5-kW genset. (Note: This is a rule of thumb, devised by yours truly. Before laying out long green to buy a genset, consult with your electronics tech or yard manager.)

There are lots of good gensets around 5 kW, but I’d choose the Fischer Panda Marine AC 5000, a compact set that comes packed inside a sound shield the company says keeps noise at conversation levels. It can run continuously at 4.5 kW and pump out peak amps for a couple of hours without a problem, is about the size of a medium Igloo cooler (it’ll fit in a 2-foot cube) and weighs only 268 pounds. Its freshwater-cooled asynchronous alternator, also called an induction alternator, has no brushes, windings or diodes to overheat and fail, and doesn’t produce the heat of a synchronous rotor. Most generators cool their rotors with air, but water is more efficient, according to Fischer Panda, keeps dust and contamination out of the rotor, and the sound shield can be totally sealed, except for combustion air, keeping the noise inside.

The Fischer Panda is driven by a Kubota diesel. But there are plenty of gasoline-fueled gensets available: Westerbeke and Kohler both build models with low carbon monoxide emissions, ideal for boats with gas inboards. But fuel notwithstanding, many builders of higher-end outboards install diesel gensets with a dedicated tank for the fuel. One advantage of outboards is most of the possibly explosive stuff is outside the boat, bolted onto a well-ventilated transom. Why change that? Install a genset running on less-volatile diesel.

What About an Inverter?

Installing a genset is expensive and complex, a project that will almost certainly run into five figures. But an inverter costs much less; a pure sine-wave 3-kW inverter costs around $3,000 and is fairly easy for an electronics tech to install. You might have one already, incorporated into your shore-power converter. Whether a boat has a genset or not, an inverter can come in handy. It can run small-wattage appliances without starting the generator—a plus if you’re just watching TV, charging the laptop or microwaving popcorn to go with drinks and a movie.

Inverters work best with house batteries with plenty of amps. Stepping up battery voltage to AC requires lots of current to maintain wattage; for example, a 1200-watt microwave uses about 10 amps at 120 volts, but the inverter draws 100 amps from the 12-volt battery (a little less if the battery’s fully charged to 12.6 volts), plus another 10 amps or so to make up for inverter inefficiency. Generally, microwave ovens run for only a few minutes at a time, but if everyone on board wants to heat up a frozen dinner, that adds up to a lot of amps pulled out of the battery. If the engines are running and the alternators are putting out enough amps, that’s not a big deal, but if not, watch the battery voltage.

Or do yourself a favor and install a genset. A 5-kW model will charge the batteries and power a 12-volt Seakeeper, even when the engines are shut down and you’re drift-fishing. And there’s the upside of powering the A/C, or firing up the electric grill to sacrifice some burgers for lunch. (Boats from about 40 feet up use 120-volt Seakeepers. The Seakeeper 6, the smallest AC model, draws up to 2.3 kW if the seas are rough.)

I’m not a big fan of extra gear that needs maintenance and might break down, unless those disadvantages are outweighed by utility. A genset adds another engine that needs oil changes, takes up space and adds weight. But a properly cared for genset is pretty reliable, and its healthy supply of 120-volt juice certainly makes boating more fun. I’d say, if you can, go for it. If you feel nostalgic, you can always shut the genset down, light the oil lamps and go back to Treasure Island. 

What About the Alternator?

Alternator output is listed for most engines as “XX amps at WOT.” That’s a maximum output, assuming the electrical system demands all those amps. If demand is less, the alternator generates less. Most alternators are rated according to their output at 6,000 alternator rpm, with the alternator cold, around 80 degrees. (Alternator output drops at all rpms as the alternator gets hotter.) This isn’t the same as engine rpm, since there’s a ratio between engine and alternator rpm based on the relative diameters of the drive and alternator pulleys. The alternator spins faster than the engine.

A 12-volt alternator starts generating an appreciable percentage of its max rated amps somewhere around 2000 to 2500 alternator rpm, according to balmar.net. That can be as low as 1000 engine rpm, or even idle, depending on drive ratio, but as a very rough rule of thumb, figure on running the engine at least 1200 rpm before seeing much in the way of amps. The good news is, most alternators reach their maximum output potential long before the engine is running at WOT.

What size alternator should you have? Calculate your boat’s total DC loads, remembering to include recirculating baitwells, the dual-voltage fridge, electronics, radar, occasional use of the Vacu-Flush toilet, running lights, bilge, washdown and freshwater pumps—all the stuff controlled by the 12-volt circuit breakers. Make sure you have total alternator capacity at least 20 percent greater; 25 percent is better. You don’t want to run an alternator at full capacity: It’ll live a short, unhappy life.

Some long-range cruising folks, and lots of sailboaters, upgrade to a high-output alternator—one that produces 150, maybe 200 amps. But that can be tricky: It imposes big loads on the pulleys and belts, so the shafts and keys that drive those components have to be up to the job. This isn’t a job for the do-it-yourself mechanic.

This article originally appeared in the July 2019 issue of Power & Motoryacht magazine.