Your Generator Should Be the Backup — Not the Plan

Marine generators are useful. There are times when having one aboard is extremely valuable.

But for a modern cruising boat, we don't believe the generator should be the primary source of everyday electrical power.

Generators are expensive to operate, maintenance-intensive, noisy, and inefficient when they're run for hours just to support a relatively small load like overnight air conditioning.

A generator makes a great backup.

It shouldn't have to be the plan.

Running a Generator All Night Just for Air Conditioning

One of the biggest problems with traditional marine air conditioning is that it often requires the generator to run all night.

The generator itself may be capable of producing 8, 10, 12, or 15kW, while the air-conditioning system may only require a fraction of that output much of the night.

The generator doesn't care.

Whether you're pulling 2kW or 10kW, the engine is still running. You're still adding an hour to the hour meter every hour. You're still circulating cooling water, operating pumps, consuming fuel, heating the engine, and putting wear on every component in the system.

And diesel generators generally operate most efficiently when they're carrying a meaningful portion of their rated load.

Running a large generator lightly loaded for eight hours just to keep the cabin cool means you're operating a machine capable of producing far more power than you're actually using.

You're essentially wasting much of the generator's capability while still consuming its finite operating life.

The Real Cost Adds Up Quickly

Fuel is the easiest cost to see.

Let's use a simple cruising example:

• 3-month cruising season
• 90 nights
• 8 hours of generator runtime per night
• 0.5 gallons of diesel per hour
• $6 per gallon

That's:

720 generator hours per season

At 0.5 gallons per hour:

360 gallons of diesel

At $6 per gallon:

$2,160 in fuel for one cruising season

And that's just the fuel.

It doesn't include oil, filters, impellers, belts, coolant, fuel filters, repairs, service calls, or the eventual replacement of the generator itself.

If your generator burns closer to 0.75 gallons per hour under your particular operating conditions, the same season would consume 540 gallons of diesel—or $3,240 in fuel.

Generator Hours Have a Cost Too

A marine generator doesn't last forever.

If we use 10,000 operating hours as a general expected service-life benchmark, those 720 hours in a single three-month season represent about 7.2% of the generator's entire operating life.

Cruise that way for three seasons and you've accumulated:

2,160 hours

That's 21.6% of a 10,000-hour operating life spent largely keeping the air conditioning running at night.

Think about the replacement cost of the generator itself.

If replacement and installation eventually cost $15,000, using 7.2% of its expected life represents roughly $1,080 of generator life consumed in a single season on a simple straight-line basis.

At a $20,000 replacement cost, that's about $1,440 per season.

That's before maintenance and repairs.

The hour meter is effectively a meter counting down toward the next major repair, overhaul, or replacement.

Maintenance Doesn't Stop at Fuel

Marine generators live in a difficult environment.

They require routine maintenance including oil and filter changes, raw-water impellers, fuel filters, belts, coolant service, heat-exchanger cleaning, and other periodic maintenance.

Then there are the unexpected issues: starters, pumps, injectors, electrical components, exhaust systems, cooling problems, clogged strainers, and corrosion.

Every additional hour you run the generator brings all of those maintenance intervals closer.

A generator sitting ready as a backup can be extremely valuable.

A generator running eight hours every night becomes another engine you're continuously maintaining.

Then There's the Noise

Even a well-installed marine generator makes noise and vibration.

You may have excellent sound insulation, good engine mounts, and a high-quality generator, but you're still sleeping aboard a boat with an internal-combustion engine running.

At anchor, that affects more than just you.

Generator noise carries across quiet anchorages, particularly at night.

One of the best parts of battery-powered air conditioning is being able to turn the generator off, cool the boat quietly, and actually enjoy the anchorage.

Don't Just Move Generator Runtime to Daytime

There's another mistake that's easy to make.

You install a large lithium bank so you no longer need to run the generator overnight. You enjoy eight hours of silent air conditioning.

Then you wake up and run the generator for several hours to recharge the batteries.

You've improved your night's sleep, but from an energy standpoint you've mostly moved the generator runtime from nighttime to daytime.

That's better than listening to it while you sleep, but it misses much of the point.

The real goal should be to reduce total generator dependency, not simply reschedule it.

That's why battery capacity and charging sources need to be considered together.

Solar Should Do the Heavy Lifting

Whenever possible, we prefer solar to be the primary source replacing daily energy consumption.

Solar has no fuel bill, almost no noise, very little maintenance, and no hour meter.

A properly designed solar system can spend the entire day gradually putting energy back into the battery bank while you're anchored, sailing, swimming, or exploring.

You don't need to start anything.

There is no engine running.

The sun comes up and charging begins.

That's why we say solar is king for cruising boats.

High-Output Alternators Are a Great Supplement

High-output alternators are another particularly effective option.

If you're already running the propulsion engine to move to another anchorage, that's an excellent opportunity to recharge the batteries at the same time.

Many boats also use engine heat to produce domestic hot water.

Now that engine runtime is accomplishing multiple things:

You're moving the boat.

You're charging the batteries.

You're making hot water.

That's a much more productive use of engine hours than starting a dedicated generator solely to produce electricity.

Where the Generator Fits

None of this means we don't like generators.

A generator is an excellent tool when used for the right job.

Several cloudy days, unusually high electrical consumption, a problem with the solar system, extended time at anchor, or the need to run very large AC loads can all justify starting the generator.

That's exactly why it's valuable to have one.

But ideally, the generator fills in the gaps rather than carrying the entire electrical system every day.

Build enough battery capacity to comfortably get through the night. Install as much practical solar as the boat can support. Take advantage of a properly designed high-output alternator when the propulsion engine is already running. Use shore power when it's available.

Then keep the generator for the times you actually need it.

A generator is a very good backup. It just shouldn't have to be your primary source of power.