Under Executive Order, and in keeping faith with my latest decree of being disciplined about my boat projects, I have declared March to be Electrical Systems Month.  This means I have 12 days to figure out how I’ll tackle my electrical projects throughout March, projects that need to be accomplished if I’m going to be a successful cruising photographer.  These projects include: an inverter, new electrical distribution panel, batteries, solar panels, and rewiring.  Let’s start with selecting an inverter.

Right now I have two 12 volt batteries powering my DC electrical system.  My AC system is supplied by “shore power” from the dock, which also charges my batteries.

While off the grid and sailing, my only power comes from the batteries, which can only be charged from my engine alternator.  See a problem for long-term cruising?  Normally I wouldn’t care to not have AC power, but I’ll need to charge camera batteries, charge my laptop, and power my printer to run a photography business from my vessel.

Additionally, there’s my sailmaking (not sewing) machine, drill, and portable vacuum that require AC power – things I’ll only occasionally use and could really get by without having.  These systems require an inverter to convert my batteries’ DC power to AC power, and this power will need to be replenished by a source other than my engine (for now, solar).

Determining Power Needs  

So how much AC power will I need?  Let’s make a list of all my AC systems to figure this out.

Laptop: .7A, 85W
Printer: 83mA, 10W
Sailrite: 1.5A, 180W
Corded drill: 5.2A, 624W
Dremel: 1.15A, 138W
Jigsaw: 4A, 480W
Space heater: 12.5A, 1500W
Dirt Devil: 4A, 480W
Battery charger: Negligible
2-qt slow-cooker: 1A, 120W

Adding all this up, we come up with 3,712 Watts.  This is ridiculous!

I’m not even bringing my space heater with me once I set off, since I have alternate means of heating my cabin if I need to and plenty of warm clothes.  I only have four plugs on the boat and won’t be using all of these at once.

So I really just need an inverter to power my corded drill at 624W, my highest energy consumer.  For the few minutes I’ll ever be using that, I can unplug my other AC systems.  In fact, I don’t ever anticipate having more than two AC systems plugged-in while off the grid (gotta save battery power).

So we’re looking for an inverter with two outlets built-in and a capacity of at least 624 Watts plus a safety pad (we’ll get into DC requirements once we start looking at batteries).  I won’t be running any systems with AC induction motors; those motors need inverters at least three times their rated power due to start-up requirements.

Pure Sine or Modified Sine Inverter?  

Alternating current power, as the name implies, alternates between positive and negative polarity.  Power you get from the electrical company alternates that voltage smoothly, resulting in a wave that looks like a sine (if you remember anything from trig class).  Inverters can provide the same wave, but at a higher cost due to the higher-quality components.

A cheaper inverter will produce a “modified” sine wave, which looks more like steps as the inverter cycles between positive and negative voltage.  A lot of AC-powered systems can run just fine off of this modified wave, and inverters providing that are notably cheaper.

But there are some systems – high-end electronics and motors – that are best operated with a pure sine wave.  Add that to our list of requirements (and reach deeper into the pockets).

What about AC wiring?

Serious boaters with serious yachts have inverters with two power inputs – one from the batteries and one from the shore power (and maybe even a generator).  Their inverter’s output is then wired into the AC side of their electrical distribution panel to power the numerous outlets spread throughout the boat.  When plugged into shore power, the inverter knows this and bypasses itself, allowing the shore current to go straight to the electrical bus.  When shore power is removed, the inverter senses this and immediately starts converting power from the batteries for uninterrupted AC power.

There are two electrical outlets in Saoirse, one in the galley and one next to the quarter berth.  They’re about nine feet away from each other.  And I have no AC systems that need to be continuously operated, meaning that I don’t need uninterrupted service when switching between shore power and battery power.

Now my list of inverter requirements does not include automatic power switching or dual inputs.  I just need a simple inverter with one input (from my battery bank) and two three-prong plugs for my AC systems (and combined with a heavy-duty extension cord where needed).

For technical specs about how to wire these systems, check out Nigel Calder’s Boatowner’s Mechanical and Electrical Manual.


Of course, the inverter has to be rugged and built to marine standards, with coated circuit boards, overheat/undervoltage/overvoltage/overload protection, and some method of being able to monitor volts, amps, and watts.

My reasons for not including an inverter with built-in battery charger is because if the inverter fails, it is almost certain that the battery charger has also died.  I’d rather keep the battery charger separate from the inverter, at the cost of a little more space used but with money saved.

With all of that said, I’ve been eyeing the Xantrex PROwatt 1000, which rings in at about $240 (cheapest at Amazon).  This model offers 900W of power with 89% efficiency and a surge rating of 2000W.  There are two GFCI outlets on the front and a monitoring system.

Simple, but not too simple.  Check back later for installation and integration with new batteries, distribution panel, and solar panel.

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