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Performing an Energy Audit to Size Your Lithium Power System on a Sailboat

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Ready to build out your new electrical system but don’t know where to start? An energy audit of your sailboat will give you a clear picture of just how much energy you anticipate using. This will allow you to make an informed decision about the size of your battery bank and what charging method you plan to use.

What is an Energy Audit?

Connor Smith explaining how to do a power audit

An energy audit allows you to identify the loads of all the electronics on your boat. This can include things like a microwave, coffee maker, lights, refrigeration, navigation, autohelm, and more. If your energy audit is done properly, you’ll be able to accurately size your marine electrical system. Once you understand how much energy you’re using each day, you can easily determine how much energy you need to store and how much energy you need to generate to keep up with your usage.

How Do You Perform an Energy Audit on your Sailboat?

There are two methods you can use to perform an energy audit on your sailboat. Both methods require the use of a battery monitor so that you can track all the power that is coming into and going out of your batteries. With a BMV-712 and a touch screen, you will easily be able to see if your inverter is on or off, how many AC loads are being pulled, and the current state of charge of your batteries.

battery monitor bmv-712 on a sailboat

The first method of performing an energy audit is looking for the power usage of each of your appliances and then estimating the daily duration of use for that appliance. To begin this power audit, turn off all the electronics on your sailboat. The current power draw will be your parasitic or “vampire” loads, which will be the baseline. Then, one by one, turn on each appliance to determine how many Watts of power it is pulling during normal usage.

Although appliances such as microwaves and coffee makers will have large instantaneous power draws, they’re only used for a few minutes at a time. On the other hand, lights and outlets draw smaller amounts of power but are used for extended periods of time. By calculating the instantaneous power draw for each appliance and then estimating how much that appliance will be used in a 24-hour period, you can determine how much power you need.

coffee pot power audit diagram

The second energy audit method allows the battery monitor to do the work for you. While at anchor, turn off your battery charger and go about your day using all your appliances. At the end of the day the battery monitor will show the cumulative effect of the day’s usage of power. Be sure to separately calculate your energy usage at anchor and while sailing as instruments such as navigation and autohelm can change your power usage.

power audit comparison for a sailboat at anchor versus a sailboat undereway

Once you’ve calculated your estimated days of power usage for being at anchor and sailing underway, choose the larger of the two values and calculate the number of amp hours. This number will tell you how many batteries you need. It’s generally helpful to size your system up about 30% to leave room to grow. Additionally, the amount of solar you intend to add to your boat, as well as if you plan to utilize alternator charging, will change the number of batteries that you need. With more power going into your system, there is more flexibility in the amount of stored energy you need.

Want to Learn More?

Check out the full Marine 101 series on our YouTube channel or through the Academy page on our website. For further questions or help designing the perfect marine power system, our technical sales team would love to help! You can reach by calling (855) 292-2831 or emailing [email protected].

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