Tacklebox ’n Tools
Test version

Whole-Boat Power Budget Calculator

Add up your trolling motor, fish finder, livewell and lights, and see whether your batteries last the whole day on the water.

How to use it

  1. Set your day and trolling motor. Hours on the water, motor size and how you fish. Pick "No trolling motor" if you don't have one.
  2. List your electronics. An example boat is loaded. Change the amps and hours, remove what you don't have, and add the rest.
  3. Describe your batteries. Say whether the motor has its own battery, then enter each battery's type and amp-hours.
  4. Read the results. See how long each battery lasts, what uses the most power, and the battery size that covers your day.

Your day and trolling motor

hours
Speed control

Electronics and accessories

Amps are typical figures at 12 volts. The label or manual on your own device beats any estimate.

Your batteries

How is your boat wired?

How the numbers are worked out

Electronics

Each device uses amps × hours × quantity amp-hours a day. A shallow-water anchor is counted per use instead. Devices marked "all day" follow your hours on the water until you change them.

Starting figures come from manufacturer specifications where we could find them, listed under Sources. The stereo, phone charging and incandescent light figures are estimates. Swap in the number from your own device.

Trolling motor

It uses the same model as our Trolling Motor Power Calculator: the manufacturer's maximum amp draw, scaled by speed setting, running for the share of the day you choose. Variable speed uses max amps × (setting ÷ 100)^1.6. 5-speed motors use 17%, 25%, 40%, 55% and 100% of maximum, which are rough estimates.

Batteries

Lead-acid batteries deliver fewer amp-hours under heavy draw, so each battery's rating is adjusted with Peukert's law (k = 1.25 flooded, 1.15 AGM, 1.05 lithium), using the motor's running current when the motor is on that battery. When everything shares one battery, the motor and electronics current are added, which leans conservative.

"Lasts" is how long the battery keeps up with your day before reaching the share you are willing to use. "Battery size for your day" is the rated amp-hours that covers it, per battery.

What it leaves out

  • Charging from your outboard's alternator while you run. If your electronics are on the engine's starting battery, results here are the worst case. Keep enough in reserve to start the engine.
  • Cold weather and battery age, which both cut capacity
  • Wind, current and weight, which change how hard the motor works

Planning estimates. Follow your equipment manuals for wiring, fuses and breakers. Keep the engine's starting battery separate from the trolling motor bank.

Sources