Boat Inverters and Inverter/Chargers: Sizing, Safe Use and What a Pro Must Install

Write down each AC device and its watts (on its data tag). Add up only what will run at the same time. BoatUS gives the example of a 200-watt TV plus an 800-watt microwave: you need at least 1,000 watts continuous. Motors and some electronics draw a short surge at startup (that TV might briefly want 500 watts), so include the surge. Running things one at a time lets a smaller inverter do the job.

An inverter is limited by the batteries behind it. BoatUS suggests a battery bank of at least 20% as many amp-hours as the inverter's watt rating as an absolute minimum: a 2,000-watt inverter needs at least 400 amp-hours. Running it near full output for an hour uses roughly 200 amp-hours from a 12-volt bank. Air conditioning or a water heater on an inverter usually needs far more battery and charging than most boats have; plan those for shore power or the generator.

Modified sine wave units cost less, but BoatUS notes they can make radios and audio buzz, upset some TVs and computers, and suit motors and pumps poorly. Pure sine wave inverters cost more and are the better all-round choice, especially for anything with a motor or sensitive electronics.

When you hire the installer, these are the basics BoatUS (a marine surveyor writing to ABYC practice) lists, so you can ask about each one:

• Location: dry, ventilated, easy to reach, away from heat such as exhaust, and within the maker's limits for temperature and humidity.
• Near the batteries but not above them: short cables are safer and cheaper, but charging lead-acid batteries give off corrosive gas.
• Never in a gasoline engine compartment: inverters are not ignition-protected and can ignite fuel vapor.
• A fuse on the positive cable close to the battery, of the type and rating the inverter maker specifies. BoatUS says a typical installation puts it within 7 inches of the battery and uses a Class T fuse rather than a cheaper ANL type; follow the maker and ABYC.
• A battery disconnect switch in the positive cable, so the inverter can be shut off for service or an emergency.
• Cable size per the maker's chart for the inverter's current and the cable length.
• A chassis ground wire as large as the DC positive cable. BoatUS says a missing or undersized one is among the most common problems surveyors find.
• Chafe protection and support about every 18 inches along the cable run.

If the inverter feeds the boat's outlets, BoatUS says there must be a visible way to tell that the inverter is on (an indicator light or remote panel) and a warning placard at the main AC panel stating that the boat has an inverter. That warning exists for a reason: Practical Sailor reported boatyard technicians getting shocked after unplugging only the dock cord, because an inverter was still feeding the panel.

Backfeeding means AC flowing the wrong way into wiring that is supposed to be dead. Never:

• Make or use a cord with a plug on both ends to push inverter power into a boat outlet or the shore power inlet.
• Connect an inverter's output and shore power or the generator to the same circuit without a proper transfer switch.
• Plug a portable inverter's output into the boat's wiring at all; use it only for appliances plugged directly into it.

A backfed shore inlet can put 120 volts on exposed pins and on the dock, and can energize wiring a technician thinks is off.

On US boats, an inverter/charger must bond the AC neutral to the green ground wire when it is inverting, and lift that bond when shore or generator power is supplying the boat, so there is only one bond point at a time. Blue Sea Systems explains this prevents voltage differences that can cause shocks or nuisance GFCI trips. Most marine inverter/chargers do it internally; ask your installer to confirm yours is set up correctly. This is not a DIY change. Small stand-alone inverters are the ones to watch: depending on the model and voltage version, the output neutral may be left "floating" from the factory, and a GFCI outlet wired after it may then give no shock protection until the installer makes the bond the manual describes. That is exactly what the test in the next step catches.

Outlets in wet areas (galley, head, engine room, deck) are normally protected by GFCI outlets or breakers on a properly wired boat. With the boat on inverter power (shore power off), press the TEST button on each GFCI outlet: power should cut off, and RESET should restore it. A plug-in tester helps. If a GFCI does not trip on inverter power but does on shore power, stop using those outlets and have the neutral-ground setup checked. Some inverters list GFCI types that work best with them in the manual.

With the inverter off:

• Check that the vents and fan openings are clear of dust, lint and stored gear; inverters make heat.
• Look at the battery cables and fuse holder for discoloration, melted insulation, green corrosion or loose-looking lugs. Don't tighten heavy terminals yourself on a live bank; note it and call the installer.
• Feel the cables after a heavy load (a microwave run). Warm is OK; hot is not.
• Read any fault codes on the remote panel and look them up in the manual.

• Switch the inverter off when you don't need it; many draw power just sitting on.
• Watch battery voltage on big loads, and let the low-battery shutdown work instead of overriding it.
• Don't use it to power heaters or other big loads unattended.
• Keep the area around it free of rags, fuel cans and cleaning chemicals.

Before anyone works on the AC panel or outlets: unplug shore power, switch off the generator, switch the inverter off at its control, and open its DC battery switch. Then confirm with a tester that the circuit is dead. BoatUS says to always disconnect the inverter's DC power and its AC connection before working on the AC system.

Switch the inverter off and open its DC disconnect so it does not drain the batteries. If you use the inverter/charger to maintain the batteries on shore power over winter, confirm the charger settings match your battery type and check it periodically. Keep the space dry and ventilated.