How long is the session? Electric outboard battery life explained

The question everyone asks at the boat show: how long does the battery last? And the answer every salesperson gives: it depends.

They are not dodging. An electric outboard's battery can last half an hour or a whole day, and both answers can be true for the same motor. It comes down to how hard you twist the throttle.

We pulled the runtime and range figures makers publish, so you can see the pattern and do the sums for your own boat.

The formula: read the wave before you paddle

Runtime is simple arithmetic:

Hours = usable battery capacity (Wh) ÷ power drawn (W)

Distance = hours × speed (knots)

A 1,000 Wh battery feeding a motor at 1,000 W lasts about an hour. At 250 W, about four hours. Most outboards and boats show live power draw on the display, so you can watch this in real time.

The catch is that power does not fall in a straight line with speed. Slowing down a little cuts the power a lot, so the battery lasts far more than proportionally longer.

Real figures: the small outboard line-up

Here is what makers rate their small outboards for. Note the different conditions behind each figure.

Source: MBY.

The spread is huge for similar batteries. Some of that is efficiency, but much of it is the speed each maker assumes. Mercury is open that its figure is at 25 percent throttle. At full throttle, every one of these numbers drops sharply.

The Bixpy K-1 shows the same effect in hours. On a 768 Wh battery it is rated for over 2 hours at top speed and up to 24 hours at slower speeds (Kayaks & Paddles).

Big outboards: the same curve, bigger numbers

The pattern holds at the top end. Evoy's Storm outboard, rated at 222 kW and fed by two 63 kWh batteries, is quoted at 51 nautical miles at 1,000 rpm, 27 at 2,000 rpm and 25 at 3,000 to 3,500 rpm (MBY).

Same lesson: half the revs, roughly double the range.

What drains it faster: onshore days

Your real runtime will be shorter than the brochure when:

• You carry more weight. Passengers, gear and a heavy boat all add drag.
• The wind and chop pick up. Plan the return leg as the hard one.
• It is cold. Batteries give less in cold weather. ePropulsion says its Spirit battery should not power the motor below minus 10 °C (Oceans EV).
• The hull is foul. Weed and growth add drag.
• The propeller is wrong. A propeller matched to the boat makes a real difference.

Making it last longer: ride the glide

Easy wins:

• Cruise at a steady, moderate speed. Small throttle cuts give big gains.
• Keep a second battery, if your motor allows a swap.
• Keep a reserve. We plan with around a fifth of the battery left.
• Look after the battery. ePropulsion recommends charging its Spirit battery when it reaches about 20 percent and avoiding full discharge, which it says cannot be recovered (Oceans EV).

Charging time: back in the water

Charging time depends on battery size and charger power. RAD Propulsion says its RAD40 system charges in about two hours on a fast charger, or eight hours from a standard socket (MBY). Small portable batteries usually charge overnight on a household socket.

Read how to charge an electric boat for plugs and power.

Our verdict: the throttle is your fuel gauge

So, how long does an electric outboard battery last? As long as your throttle lets it. At a gentle pace, a small outboard can run for hours. Flat out, it is a short session.

Read every range claim with its speed or throttle setting, do the simple sum with your own numbers, and keep a reserve. For more on range and boats, see how far can an electric boat go? and what size electric outboard do I need?