29 Apr

Size IS Everything!

How to select the right inverter for your boat.

By Don Wilson
TECH DOCTOR DON WILSON has worked in technical capacities in the marine, automotive and RV fields and for the military since 1989 and has extensive experience in designing and troubleshooting onboard electrical systems. A former customer service manager dealing with electronic issues, Wilson currently serves as a technical instructor for the RV industry’s RVIA Trouble Shooters Clinics and is a full-time sales application engineer for Xantrex Technology, Inc. SIZE IS EVERYTHING! is the fourth installment of TECH DOCTOR articles being presented by the Boat and Yacht Report.

In an industry where size is everything—larger engine, smaller accessories, lighter weight—it’s easy to make inaccurate assumptions and create undesirable problems for yourself. Unfortunately, in my line of work I have witnessed many instances of undersized inverters being used by boaters in an effort to save a buck. When operating below recommendation, the electrical system is far from efficient and can prove troublesome.

So, the million dollar question for this edition of TECH DOCTOR is: What size inverter SHOULD I have in my boat? And the obvious answer is…well, it’s really NOT so obvious. The truth is, this decision all depends on exactly how you want and/or intend to use your inverter. Let’s consider some questions to help you understand how to make the best choice for your unique application.

How much load (electronics/appliances) do you want to run when disconnected from shore power?

It’s important to look at the wattage draw of ALL of the appliances, plus to leave some room for future additions. If you have a 1000W A/V system, for example, and install a 1000W inverter, you’re probably asking for problems. You see, inverters are rated for their optimum performance under optimum conditions. If the inverter gets hot, or the battery voltage drops, the inverter’s ability to feed a constant 1000W may be compromised. Also, surges in power draw (such as turning on a motor) can affect the ability of the inverter to maintain proper output.

Consider this example and accompanying formula:

If you are going to plug in two devices simultaneously, add up the total wattage of both devices, and then add at least 50% more to account for peaks or spikes in the power draw.

1 monitor = 100 watts

2 portable lights = 100 watts

Recommended size of inverter: 200 watts (100 watts + 100 watts) x 0.50 = 100 watts; therefore adding the extra 100 watts results in a 300 watt inverter being necessary.

Obviously if you know for sure that two loads are never going to run at the same time, your calculation can include the higher draw load exclusively. My advice? Use common sense here, but always err on the side of the inverter. After all, nobody wants to have to remember to turn off the surround sound when using the microwave to pop the popcorn! If there is any possibility of the loads running concurrently, include both in your calculations.

 What are other important considerations?

Some loads, like motors or other inductive loads, have an extremely high demand at start-up, some as high as five times their rated power (check with your manufacturer for more information). Most inverters have a fairly high surge rating which is its ability to feed short-term high power to get these loads started. There’s no sure fire way of knowing the surge rating without testing since such data is not labeled.

While high frequency inverters are cheaper and more efficient, low frequency inverters can surge better, and for a longer period of time. However, depending on your load mix, it may be better to use a larger, high-frequency inverter than a smaller, low-frequency inverter to provide instant start-up current for some loads, plus available power to run all loads at once.

What is a sine wave inverter? Does it have anything to do with inverter sixing?

It is not directly related to inverter sizing; however you must take into account the type of electronics and appliances you want to run using the inverter. (See installment #3 of TECH DOCTOR: The Inside Story, posted April 24th, 2011.)

True Sine Wave inverters produce AC power that is similar to power available from the public utility grid system. They are expensive compared to Modified Sine Wave inverters, but they produce quality output that operates the most sensitive and sophisticated electronics. Modified Sine Wave inverters produce AC power that is sufficient to run most electronics. However, the laser printer, fax machine, plasma television set and medical equipment may not run properly with Modified Sine Wave power.


Computer system 200 Microwave oven 1200
Power drill 400 Large vacuum 1200
Coffee maker 900 Circular saw 1500
Toaster 800 Blender/juicer 1000

To size it all up? I hope these simple tips will help you select just the right inverter for your boat’s specific requirements.

NEXT UP FROM DON WILSON: Three Tips To Maximize Your Electrical System

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Posted by on April 29, 2011 in Uncategorized


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