By Kevin Jeffrey

If the AC electrical load on your boat is large enough, you’ll probably need to look  beyond the capabilities of DC-to-AC inverters.  Even if you have a large inverter that can handle a large AC load at any given moment, in most cases the charging system needed to replace battery drain becomes impractical unless your large AC loads are used primarily when plugged into shorepower.

A good example of an AC load too large for most inverter installations is air conditioning.  The instantaneous AC current draw from air conditioning ranges from fairly modest to high, depending on BTU rating of the unit.  But it’s not the instantaneous draw that creates the problem for inverters, which can comfortably handle the large draw of microwave ovens, toasters, and coffee makers.  The problem lies in the length of time and the time of day air conditioning is normally operating.  Air conditioning runs for long periods of time and it operates primarily at night, when battery charging sources have a tough time replenishing the battery drain.  Generally speaking, large AC loads that run for extended periods of time require an engine-driven AC power source, typically some type of gen-set. 

Equipment weight and size are worrisome considerations for almost any multihull sailor, and cost is a universal cruising concern.  Gen-sets are quite a bit more costly than inverters, both to purchase and to install, and they need to be serviced periodically, but there are very good compact, relatively lightweight gen-sets on the market that are ideal for multihulls.  Once you’ve accepted that a gen-set is unavoidable for your situation, take some time to research the best unit for your needs. 

Power ratings begin in the 3 to 4 kw range.  Small gen-sets can operate at either 3,600 or 1,800 rpm, while larger models are almost exclusively 1,800 rpm.  This is continuous operating rpm, which is necessary to produce the required AC frequency, regardless of electrical load.  That’s why running a gen-set to satisfy a small AC load is so inefficient.  Notable exceptions to this rule are the gen-sets with VST (variable speed technology).  They adjust their speed according to electrical demand, while maintaining the correct AC frequency and waveform.  VST gen-sets are also quite pricey.

You’ll be able to choose between diesel or gasoline-fueled models—it probably makes the most sense to choose a gen-set using the same fuel as your main engine(s)—and between air or water-cooled models.  Whichever type you choose, your gen-set should be installed by a qualified mechanic, with secure mounting and proper venting and exhaust.  If noise is a big concern, choose a model with a good sound shield, or have one made, to help insulate gen-set noise from the rest of the boat.  If your budget allows, check out the Panda generators from Europe.  They are incredibly compact and come complete with suitcase-style sound shield.      

Even if you have a gen-set on board, you can still put an inverter to good use.  Use the gen-set for the larger, long-running AC loads, and bring the inverter into service for the smaller intermittent loads when you’d prefer some peace and quiet.  And don’t forget to charge your batteries anytime the gen-set is operating.  An inverter-charger is ideal for this application, since the charging current is high and it automatically switches to the charging mode when AC power is available on board.