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 For autopilots, and other onboard appliances, the current draw depends on how hard the autopilot is working when operating. | |
 A digital system monitor is a very handy way to determine the exact current draw of all onboard appliances. | |
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.
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
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