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Calculating Your Electrical Load

Your patterns of electricity usage vary based on how and where you sail. Obviously, liveaboards will need different charging sources and battery capacity than the occasional day sailor.
Estimating how much electricity you use helps determine what charging sources are needed to supply enough electrical power on a routine basis and approximately how much total battery capacity is required. Try to be as accurate as you can with your estimate, but keep in mind that all you are really trying to do is determine what category of electrical use you fall under. Your actual patterns of electricity use will undoubtedly vary with how and where you use your boat, so you want to have a power system that can handle times of heavy use yet not be oversized for normal conditions.  

Step 1  In order to visualize your electrical needs, you must set up an appliance load chart similar to the one shown in the sidebar. List all the electrical appliances you would like to have on board. If you plan on upgrading your boat soon, this is a good time to think about future electrical demand. List your DC loads first, then your AC loads that will be run from an inverter. This allows you to assess potential DC charging sources as well as various methods of providing AC power. If you plan on making blue-water passages, it's a good idea to create a chart for time in port and a separate one for passage-making.

For autopilots, and other onboard appliances, the current draw depends on how hard the autopilot is working when operating.
Step 2  Next to each appliance note the average current draw. I use current draw (in amps) here instead of power draw (in watts), since battery capacity is rated in amp-hours instead of watt-hours of energy.Average current draw takes into account the fact that for some appliances, such as autopilots, the current draw varies with how hard the appliance is working when operating. Average draw also accounts for appliances with multiple speed or other operation settings. Keep in mind that current draw varies widely between various makes or models of the same appliance. Try to find the actual current rating for your appliances under normal operating conditions from the owner's manuals. Your appliance supplier should be able to provide this information. 

A digital system monitor is a very handy way to determine the exact current draw of all onboard appliances.
Another convenient method for finding the exact current draw of all appliances already on board is with a digital system monitor. Begin by ensuring all loads and charging sources are turned off, then turn on individual appliances one at a time. You may be surprised at what you find. I was once going through this procedure with the skipper of a cruising yacht, and he was happily noting the modest amp draw of each appliance when the first mate, unbeknownst to us, turned on her 1500-watt hair dryer which was plugged into the inverter. The monitor reading jumped to over 100 amps and the skipper nearly fell overboard from shock. 

A digital monitor can also alert you to potential problems. On another boat I found that marine growth on a seawater intake caused a water pump to draw twice the current it normally would with a clear intake. 

Step 3  In the next column, bring time into the equation by noting how many hours each appliance is used on an average basis. Try to take into accountthat some appliances are used every day while others are only used occasionally, and that some appliances are used mostly in port and some are used exclusively at sea. To calculate average daily hours of use for occasional loads, calculate average hours of use per week and divide by seven days. 

Don't be intimated by electrical matters on board. By determining your electrical load you've won more than half the battle of surveying your power needs.
Step 4  Now multiply the average power draw in column 2 by the average daily hours of use in column 3 to get your average daily electrical energy consumption in amp-hours. The relationship between electrical current,time, and electrical energy is: A x t = E where A is current in amps, t is time in hours, and E is Electrical Energy in amp-hours. It's interesting to see how some appliances with high current draws, such as hair dryers and microwave ovens, consume less electrical energy than you'd think, because they are on for only short periods of time, whereas a low-draw appliance such as a standard anchor light can consume a surprising amount of electricity when operated nightly for 10 or more hours. 

Determining your electrical load is a crucial part of accurately surveying your power needs. Later on in your survey you'll find out what you need to do to comfortably and consistently to satisfy your electrical demand. If it turns out that the power sources or battery capacity you require are too costly or not able to be accommodated on your boat, you'll need to reduce your electrical load accordingly. Switching to more efficient appliances, such as a low-drain anchor light that uses a fractio nof the energy of a standard model, might be the answer. If, at the end ofthe survey, you find that your system gives some power to spare, you may be tempted to add appliances or use existing appliances more often.  


Sample Appliance Load Chart


12-Volt DC Loads
ApplianceAvg. current draw (amps) at 12-VoltsAvg. use
Avg. 12-Volt consumption
Laptop computer3.0 2.0 6.0
Anchor light (standard)
Stereo/tape deck2.02.04.0
Bilge pump4.00.10.4
Modest-size refrigeration5.012.060.0

115 Volt AC Loads Run on a 12-Volt Inverter
ApplianceAvg. current draw (amps) at 12-VoltsAvg. use
Avg. 12-Volt consumption
Laptop computer3.0 2.0 6.0
Microwave oven800.2520.0
Coffee mill100.010.1
Coffee maker
(drip only)
Hair dryer1200.056.0


Total DC Load
Total average daily load 63.0 amp-hours per day at 12 Volts DC

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