At home, there are few limitations on your power usage. If you overload one circuit with appliances you can blow a breaker, and if you habitually leave everything turned on you might not like your power bill -- but most of the time you don't have to think much about it. That's not the case on a boat, where your power is sharply limited by the capacity of your alternator or your solar/wind charger. Before you add a fish finder or other device to the mix, it's prudent to know how many amps it draws.
Amp/hour calculations quickly become second nature to boaters, because that's how you measure your system's electrical capacity. An item drawing one amp, running for one hour, results in a total drain of one amp/hour. If you have a total of 6 amp/hours to spare, you could run your 1-amp device for six hours or a 500-milliamp device -- in other words, one-half amp -- for 12. This calculation is especially important with items such as fish finders, which will be used for extended periods as you cruise.
The simplest method of comparison for fish finders, like other electronic devices, is to compare the manufacturers' stated power draw. This information can usually be found in the product's manual, or on the packaging if you're in a retail store. If you can't physically look at the product, try the manufacturer's website. Most companies make their user manuals available online in PDF format, for easy reference. Although these numbers are reasonably accurate, you can verify them by visiting hobbyist forums online and comparing other users' experience with the brands and models you're considering. Two models with comparable power ratings might perform differently in real usage.
A second, more accurate way to gauge a fish finder's power usage is to connect it to a power-usage monitor from manufacturers such as Kill a Watt. Leave the fish finder set up and turned on for several hours, as the monitor records its power usage. Each model reports its power usage data differently, but most can report your fish finder's amperage. Some fish finders include an AC adapter, while others run only from 12 volts. You can use a 12-volt power supply for testing purposes, but remember to test the power supply separately and subtract its usage from the fish finder's consumption.
Features and Power Consumption
Try to compare fish finder models of similar features. A model with an extra-large, backlit, high-resolution screen will have higher power consumption than a rival unit with a smaller screen and lower resolution. You'll need to balance each model's power requirements against its features, and decide how much emphasis to place on each. The variance between models can be substantial. For example, Garmin's high-end Echo 550C has a 5-inch color screen and draws 1 amp. Lowrance's 4X model, with a smaller screen, draws only 200 ma or one-fifth that current.
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