Enter the current draw of each piece of electronics, the size and type of your battery, and how long you usually fish. The calculator shows an estimated runtime and the battery capacity you should look for.
Where to Find the Current Draw
Your fish finder and sonar manuals list current draw in amps (A), usually in the specifications section. Some manuals list power in watts instead. To convert watts to amps on a 12V system, divide the watts by 12. For example, 24 watts divided by 12 is 2 amps.
Why Battery Type Matters
You should not drain a lead acid or AGM battery much past half its rated capacity if you want it to last, so the calculator only counts 50% of its capacity as usable. Lithium iron phosphate (LiFePO4) batteries can use most of their rated capacity, so the calculator counts 90%.
Real-World Factors
Runtime in real life is often shorter than the estimate. Screen brightness, cold weather, extra sonar channels and an ageing battery all increase draw or reduce capacity. The size suggestion already adds a 20% margin, but if you fish in cold weather, size up again.