Inverters 101

By Carolyn Shearlock, copyright 2012 . All rights reserved.

The basics of inverters

Two points I want to make right at the beginning of this article:  I am NOT a marine electrician and this article is NOT intended to provide all the answers you may need on the topic of inverters.  It’s designed to be an introductory article, and yes, in places I have deliberately kept the explanation simple and not discussed all the possible variations.  The goal is to keep it non-techie!

Note that some of the most important things to know are at the bottom of this article, so if you’re considering purchasing an inverter, be sure to read the final few sections.  I don’t recommend any specific models, for two reasons:  everyone’s needs are different, and models are constantly changing.

Additionally, I’m discussing US/Canadian/Mexican appliances and power needs (110/120 volt) — I have very little experience with European 220/240 volt systems!

What’s an Inverter?

Most boats have 12-volt DC power on board, from batteries.  Things that you plug into a cigarette lighter run on 12 volts.  Things that have a “normal” household plug run on 110/120 volts AC.

An inverter takes the 12-volt DC power coming from the batteries and converts it into 110/120 volt AC power that you can plug “regular” plugs into.

Portable vs. Whole Boat Inverters

Portable inverters are very simple.  They plug into a cigarette lighter and have one or more “regular” outlets, such as that shown at right.  They come in several different styles — some have cords that go to the cigarette lighter, some have more than one outlet, some (such as this one) also have a USB outlet for charging electronics.

Portable inverters are convenient as there’s basically no installation, they’re easy to put wherever you need them and they’re relatively cheap.

The down side is that you can’t use them for anything that needs a lot of power (more on this below).  Microwaves, blenders, and vacuum cleaners are just some of the things that usually won’t work on a portable inverter.

For larger electrical loads, you need a permanently-installed inverter.  They are usually installed by a marine electrician — because of the electrical currents involved with both fire and electrocution a possibility if not done right, it’s not something to do yourself if you’re not trained.  Every installation is different, it seems, but basically this type of inverter is larger and installed in a locker in an out-of-the-way location.  It’s permanently wired to the boat’s batteries (often, it’s a combination inverter/charger that also handles charging the boat’s batteries from shore power).  Most times, there is a switch to turn the inverter on when needed.

Depending on how your boat is wired, you may have to plug things into the inverter (as in the case on our new boat, Barefoot Gal). Other boats (such as our previous boat, Que Tal) have the inverter wired to  regular household-type outlets in convenient locations in the boat that you can plug things into.  If so, you’ll usually have to both turn the inverter on and flip a switch on the “AC” power panel to energize those outlets.

Inverter Size

Inverters are sized in watts.  Luckily, most appliances are also labeled as to how much power they draw in watts.  For example, my microwave is 1200 watts. The inverter must have a larger watt rating than the item you’re using, including the “start-up load” (discussed below).

Some appliances, however, only state their electrical “draw” in amps.  But if you know the amps, you can calculate the watts.  Use the equation:

amps x 120 = watts

So, a regular household appliance — say a mini-blender — that draws 5 amps is the same as 600 watts.

In general, portable inverters range in size from 75 watts to 400 watts.  Most cigarette lighters can handle the 75, 150 and 200 watt inverters and associated loads.

However, a 400-watt load can be too much for many, and will blow the internal fuse or trip the breaker.  If you are thinking of buying a 400-watt portable inverter, make sure that your cigarette outlet(s) can handle it.  The receptacle itself, the fuse, the wiring and the breaker on your electrical panel all have to be capable of handling the load — and many aren’t.  (A 400-watt load is going to pull ± 35 amps DC.)

Because of this, once you get over 400 watts, almost all inverters are designed to be hard-wired into the boat’s electrical system.

“Start-Up Load”

Okay, now I’m going to confuse the issue a little more.  In the last step, I talked about determining the load of the appliance you intend to use with the inverter.  But its regular “running” draw isn’t the only thing you need to know.

Most electrical motors use more power for just a few seconds when they are first turned on.  It can also be a bit of an issue for some electronics, although it’s not usually near as large of a problem.

Figuring out the initial surge draw can be tough.  Manufacturers almost never state this and I’ve never seen a “rule of thumb.”

There are two ways to deal with this:

  1. Don’t use an inverter that’s just barely adequate for the running load — use at least the next size larger.
  2. Better inverters usually can handle a brief surge — look for this is the specs.  For example, a 400-watt inverter may say “peak surge: 1000 watts.”

If the Inverter Doesn’t Work . . .

Okay, there are LOTS of possible reasons.  But here’s a quick list of things to check.

Portable inverter:

  • Breaker is ON for the cigarette lighter.
  • Switch on the inverter is ON.
  • Fuse in the inverter isn’t blown (there is usually a fuse in the tip of the cigarette lighter plug) — if it’s blown, you had too large a load for the inverter and/or the socket.  Sometimes there’s a reset button on the inverter, too.
  • Did it overheat?  If it feels hot to the touch, you may just need to let it cool down.  Try to find a cooler place to use it — or at least one with better air flow.
  • There may be an inline fuse between the socket and the breaker — if so, it is blown?
  • Just too small an inverter to power the item.
  • You did turn on the appliance, didn’t you? (Yeah, I’ve done this . . .)

Hard-wired inverter:

  • Inverter is ON.
  • Outlet breaker ON at the AC panel.  AC master switch ON.  If the outlet breaker trips, the load was too large for the circuit.
  • Ground fault not tripped . . . check all outlets on the circuits, not just the outlet you’re using.
  • Just too small an inverter to power the item.
  • You did turn on the appliance, didn’t you? (Yeah, I’ve done this . . .)

Pure Sine Wave vs. Modified Sine Wave

Oh, geez . . . more tech-y stuff.  But it’s important.  Make the wrong choice and you can fry expensive electronics.

AC power that you get ashore from the power company has a true sine wave form — the red line in the drawing at right.  But it’s complicated to change power from DC current to that smooth wave form.  So for years, inverters used what is called a “modified sine wave,” or step function, such as the blue line.

The problem is that certain electrical devices don’t like a modified sine wave.  Electronics and battery chargers (such as for cameras, cell phones and cordless drills) are notorious for not running properly with modified sine wave inverters.  Some displays (such as TVs and monitors) will have funny lines on them, audio equipment may have a buzz, and we totally fried the charger and battery for a cordless circular saw.

For galley purposes, variable speed motors — mixers and blenders come immediately to mind — generally perform much better with pure sine wave power.

The good news is that over the last 8 years or so, “pure sine wave” inverters have come on the market.  They tend to be more expensive, however.  Most will state somewhere in the technical specs that they are “pure sine wave” although sometimes you really have to dig for the information.  Generally, if an inverter is marketed for charging electronic devices, they are pure sine wave . . . but it’s not a guarantee.

My recommendation is that if you’re getting an inverter, get a pure sine wave one.  Consider the cost of what you’ll be plugging into it.  The equipment that we ruined by opting for a “cheap” inverter (it was a portable) cost far more than what the better inverter would have cost.  And if you’re going for a hard-wired inverter, I definitely would opt for the pure sine wave.  Chances are very good that somewhere on the boat, you’ll plug in something that is sensitive to the wave form.

Power Output

Due to less than perfect efficiency in changing power from one form to another, it’s highly likely that items will be slightly less powerful than when plugged into shore power.  For us, anything we put into the microwave took about 25% longer to cook when running on the inverter.  And after a summer at anchor, Dave would inevitably forget to set the timer for a minute less when microwaving popcorn on shore power and the first batch back at the marina would be burnt!

Two More Things to Think About

Two more things that are really beyond the scope of this article, but that I want to point out:

  • Before getting a large inverter and planning to run a bunch of appliances from it, make sure that you’ve got the battery bank and charging ability.  Just because your inverter can handle an air conditioner doesn’t mean that you have enough power for it.
  • Inverters generate heat.  Most have internal fans, which can get loud.  But you don’t want one without a fan, as it will shut down due to overheating.

Once again, I’m not an electrician.  Prior to our first trip on a friend’s boat, I had no clue what an inverter was, or why I needed to switch it on before using the microwave.  I learned a lot as we bought our boat and cruised.  I’ve only scratched the surface here, but hopefully this information will give you a basic understanding of inverters and the questions to ask before buying, installing, or using one.

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Comments

  1. Carol Watson says:

    Thanks Carolyn, I’ve had all this in my head but not so organized. I do deal with European 220/240, but the principles are the same and this makes it less head-spinning. Appreciate the effort.

  2. Thanks for the explanation, we are in Australia so have 220/240v but I now understand the principle of how it works a lot better. We have a hard wired inverter so I may have to dig around in our pile of manuals and do a bit of reading about it.

  3. Jeff Janacek says:

    I recommend folks steer away from the Xantrex brand.

  4. Once again, many thanks! There are so many details to wrap my head around when it comes to cruising! It’s not like life will suddenly be all relaxing and painting toenails! Just trying to learn Spanish has been a challenge. One more detail is a bit clearer. Compared to trying to learn the extremely painful, in depth electronics at our local Ham radio club, this was a pleasure!

  5. I have a hardwired inverter on my houseboat with a remote. While I’m away should the inverter be left on ? I’m hooked up to shore power but want the inverter to maintain my refrigerator in case of a power outage. Thanks for your advice.

    • In theory, there’s a slightly higher risk of fire with every item you leave on. In reality, if the inverter is in good working order, it’s not a LOT bigger risk since you’re already leaving the shore power on.

    • Ken Arnold says:

      You would need a “Generator Switching Relay” (pricey) to run Shore and Battery/Inverter to the Fridge (at the same time) and have it switch on for Shore power outages. You can’t run it on both at the same time.

      You’re inviting a Fire.

  6. Thanks for the Inverter Basics course!!

  7. Very well written and thoughtful article. You tackled a big issue. A lot of boaters just say “No” to inverters, but that is their “loss”. Using an inverter complicates the battery usage for sure. We use an energy monitor to keep track of the “Amp Hours” used. But that is for another article.

    Love that coffee in the morning without the genset running or waiting for hot water on a propane stove.

  8. You state that: “. . . a 400-watt load can be too much for many, and will blow the internal fuse or trip the breaker. If you are thinking of buying a 400-watt portable inverter, make sure . . . . (A 400-watt load is going to pull ± 35 amps DC.)”
    It’s the parenthetical expression from the partially quoted paragraph above that I am having trouble understanding. Will you please explain it to me?
    Thank you for your considered response,
    Steve

    • 400 Watts / 12 Volts = 33.3 Amps .: 400 Watts / 24 Volts = 16.6 Amps : 400 Watts / 120 Volts = 3.3 Amps ; 400 Watts / 220 Volts = 1.8 Amps

      As you can see above, different voltages will “draw” different Amps.

      The internal fuse of these 12 Volt portable inverters is rated for 35 amps so the fuse will open (power disconnected). Remember, the portable is plugged into a cigar plug, which is always 12 Volts and probably only has a 20 Amp fuse. If the same 400 Watt appliance is plugged into a 120 AC receptacle, this same appliance will only draw 3.3 Amps.
      Hope that makes sense.

    • Steve —

      Watts = volts x amps

      By doing a bit of math, that means Amps = watts / volts

      So with a 12-volt DC system (some boats use 24 volts or 32, but 12 is far more common), 400 watts is going to be just under 35 amps . . . 400/12 = 33.3 amps — I say +/- 35 amps because of variations in the voltage and also how much voltage is lost due to inefficiencies.

      Hope that helps!

  9. Most certainly. I substituted 120V for the required 12volts. I saw that I was off by a factor of 10 but even that did not turn on the light bulb in my head. I seem to be having more of those events these days.

    Thanks for the kind patients you have all shown in your response.

    Fair Winds and Following Seas To Us All – in the days to come

  10. Thanks for the great info. This may explain my camera issues.

  11. Great article Carolyn. Thank you!

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