The vast majority of our customers who are purchasing an electric outboard for the first time ask us questions about range and run time. If you’re going to shell out a few grand for an electric outboard that costs ~3x more than a gas equivalent, you want to make sure it has enough range to get you where you need to go!
In this article, we’ll share some basic information about power and energy that will help you choose the right electric outboard for your boat, plus go over a few common mistakes first-time buyers make—and how to avoid them.
Understanding Power and Battery Capacity
If you’re considering purchasing an electric outboard, it’s important to have a basic understanding of how we measure power output and stored energy.
Power is measured in watts (W) or kilowatts (kW), which is the electric equivalent of horsepower. One kilowatt equals 1000 watts.
Battery capacity, or how much energy we can store, is measured in watt-hours (Wh) or kilowatt-hours (kWh). This is similar to how many gallons your gas tank can hold. One kilowatt-hour equals 1000 watt-hours.
How to Calculate Run Time
Once we know these numbers for a motor and battery, it’s relatively straightforward to calculate the theoretical run time of an electric outboard:
Run Time (hours) = Battery Capacity (Wh) ÷ Power Output (W)
For example, the ePropulsion Spirit 2 with the Spirit Battery Ultra has a capacity of 1539Wh and a maximum continuous power output of 2000W.
If you start at 100% state of charge (SOC) and run at full power:
1539Wh ÷ 2000W = 0.77 hours ≈ 46 minutes
Why Full Power Numbers Can Be Misleading
One of the most common mistakes we see first-time buyers make is only looking at range and run time at maximum power output.
Unless you undersize your electric outboard (which, given that you’re reading this, you probably won’t), you won’t be running at full power all the time.
Let’s do the same calculation at 50% power (1000W):
1539Wh ÷ 1000W = 1.539 hours ≈ 92 minutes
By reducing power output by half, we doubled our run time from 46 minutes to 92 minutes. That makes a big difference in real-world applications.
How Speed, Power, and Range Relate
Now let’s add in speed and range. While changes in power output result in proportional changes in run time, speed does not change proportionally—so range doesn’t either.
Here’s real-world testing data using the ePropulsion Spirit 2 on an 11’ Further Customs Laguna 330 inflatable dinghy with one adult onboard:
- 2000W: 7.4 mph | 46 minutes | 5.6 miles
- 1000W: 5.1 mph | 92 minutes | 7.8 miles
At first glance, that drop in speed might feel significant. Going from 7.4 mph to 5.1 mph is about a 31% decrease in speed. But range increases from 5.6 miles to 7.8 miles—a 39% increase.
That’s already a meaningful improvement, but it becomes even more clear when we reduce power further.
Finding the Efficiency “Sweet Spot”
At 750W, speed drops slightly again to 4.9 mph. That’s not a huge difference compared to 5.1 mph. However, run time increases to 2 hours and 3 minutes, and total range increases to 10.1 miles.
Now compare that to full power:
- Speed: 7.4 mph → 4.9 mph (34% decrease)
- Run time: 46 min → 123 min (167% increase)
- Range: 5.6 mi → 10.1 mi (80% increase)
The important point here is that speed, power, and efficiency are not linear. As speed increases, the power required to maintain that speed increases disproportionately.
Because of that, small reductions in speed can result in large gains in run time and overall range.
In practical terms, you’re giving up about 2.5 mph in speed, but gaining roughly 4.5 miles of additional range. For most real-world use cases—getting to and from a mooring, moving around a harbor, or general use on a dinghy—that tradeoff is usually worth it.
This is why looking only at top speed or full-throttle range can be misleading. In most cases, you’ll spend the majority of your time operating somewhere in the middle of the power curve, where the motor is significantly more efficient.
What This Means for You
It’s relatively easy to calculate run time at different power outputs using simple math. What’s much more difficult is estimating speed at those power levels, since that depends on your boat, weight, and conditions.
ePropulsion has created a boat database that’s well worth checking out, and we’re working on building our own as well.
If you need help selecting the right motor, feel free to give us a call. But first, we’d recommend checking out the next article in our first-time buyer series: battery types.