How Long Will Your Battery Last? A Simple Guide to Estimating Runtime

The Question That Ruins Trips

You are three miles from the launch. The motor starts to slow. You push the throttle higher, but the speed keeps dropping. The battery is dying—and you are not sure if you will make it back before it quits entirely.

This scenario plays out more often than it should. And the root cause is almost always the same: the angler had no idea how long the battery would actually last.

Battery runtime is not a mystery. It can be estimated with a simple formula. But that formula has variables—and understanding them is the difference between a confident trip and a nervous one.

The Basic Formula

Runtime is calculated like this:

Runtime (hours) = Battery Capacity (Ah) ÷ Average Current Draw (A)

For example:

• A 20Ah battery powering a motor that draws an average of 10A gives you 2 hours of runtime

• A 50Ah battery powering the same motor gives you 5 hours

• A 50Ah battery powering a motor that draws 25A gives you 2 hours

The math is simple. The challenge is that current draw is not constant.

Why Average Current Draw Is Hard to Pin Down

Current draw depends on several factors:

Throttle setting. A motor running at 50% power draws less current than one at 100%—but not necessarily half. Motor efficiency curves vary.

Load. A heavier kayak, more gear, or a passenger increases current draw. The motor works harder to maintain speed.

Water conditions. Wind, current, and chop all increase resistance. The motor compensates by drawing more current.

Propeller selection. A larger or higher-pitch propeller draws more current than a smaller one.

Because of these variables, the “average current draw” number is rarely a single figure. It is a range—and understanding that range helps you plan.

Real-World Estimates

Here is a practical starting point for kayak applications:

Important: These are estimates. Actual performance depends on your specific motor, battery, and conditions.

The Efficiency Factor

Not all motors are equally efficient. A well-designed brushless motor produces more thrust per amp than a lower-quality alternative.

This matters for runtime. If Motor A draws 10A to produce 8kg of thrust and Motor B draws 10A to produce 10kg, Motor B gives you more usable range from the same battery.

When comparing motors, look beyond peak thrust. Look for efficiency—how much thrust per amp—because that determines how long you can stay on the water.

What Shortens Runtime

Several factors can cut your expected runtime short:

Cold water. Lithium batteries lose capacity in cold conditions. A battery that gives 5 hours in summer may give 3-4 hours in winter.

Aging batteries. All batteries lose capacity over time. A 50Ah battery after two years may only hold 40Ah.

Voltage sag. As batteries discharge, voltage drops. The motor draws more current to compensate, accelerating the discharge.

Debris on the propeller. A fouled prop increases drag and current draw. A quick check before launching can save runtime.

What Extends Runtime

Throttle management. Running at 50-70% power instead of 100% can dramatically extend runtime. The efficiency curve usually peaks in the mid-range.

Coasting. When conditions allow, let the kayak coast instead of running the motor continuously. Small breaks add up.

Weight reduction. Every kilogram you remove reduces the load on the motor. Leave unnecessary gear ashore.

Matched battery. A battery selected specifically for your motor’s requirements avoids the inefficiencies of mismatched systems.

The Bottom Line

Estimating battery runtime is not complicated—but it does require understanding the variables. Use the formula as a starting point, then adjust for your conditions, your motor, and your battery.

And always carry a backup plan. A paddle works even when the battery does not.

Need a motor and battery matched for efficient runtime? Explore our lineup at hobbywater.com. 🔋