Electric Boat Motor Power: How Many kW Do You Really Need?
How much motor power you need depends on your boat's weight, hull and how you use it, not the horsepower of your old petrol engine. A practical sizing guide.
TL;DR
- Electric boat motor power is almost always less than the petrol engine you are replacing, because electric motors deliver full torque instantly and most boats spend most of their time well below full throttle.
- What matters is matching power to your boat’s weight, hull type and real-world use, not copying a horsepower number off the old engine.
- On my own boat I cruise at just 200–300 watts, a tiny fraction of the motor’s peak, which is exactly why a modest battery lasts me a full afternoon.
Forget the horsepower of your old engine
The most common mistake when going electric is to take the horsepower figure off the petrol outboard and look for an electric motor with the same number. That almost always leads to buying far more power than you need.
Petrol engines are sized around their peak output and burn fuel inefficiently across the rev range, so people over-spec them for the rare moments they want full throttle. An electric motor behaves differently: it delivers maximum torque from the very first turn, so a lower-rated electric motor often pushes a boat as well as a larger petrol one in everyday use. The number that matters is how much power you actually draw at the speed you normally travel. That is usually a small slice of the motor’s rating.
kW and HP: the simple conversion
Electric motors are usually rated in kilowatts (kW), petrol engines in horsepower (HP). The conversion is straightforward: 1 kW is about 1.34 HP, and 1 HP is about 0.75 kW. So a 3 kW electric motor is in the region of 4 HP.
Manufacturers also like to quote an “equivalent HP” for electric outboards, claiming a motor performs like a petrol engine of a higher horsepower. For example 1 kW is often referred to as 3 HP equivalent. There is some truth to it. Thanks to electric torque and props that are designed for torque rather than high speed, these motors produce thrust similar to higher-power petrol engines. However, treat these equivalence claims as a rough guide rather than gospel, and focus on the actual kW figure and your real power draw.
Sizing by weight and use
What size electric motor you need for your boat comes down to your boat’s loaded weight and how you want to move:
- Displacement cruising (pushing gently through the water): very little power needed. A light boat at a relaxed pace may only ask for a few hundred watts.
- Heavier displacement boats or windage in a breeze: more power for headroom against wind and chop, even if you cruise gently.
- Planing (climbing on top of the water to go fast): a large jump in power, because getting onto and holding the plane is demanding.
For most lake and coastal cruisers who are happy at displacement speed, the realistic power requirement is modest. The bigger battery, not a bigger motor, is usually what buys you a better day out.
Here is a helpful rule of thumb to determine your boat’s power needs: Weigh your boat the way you will actually use it, not as it sits empty in the brochure: motor, battery, water, gear, food, and everyone aboard. Then determine as follows:
Rule of thumb: 1–2 kW per tonne
- 2 kW per tonne for displacement hulls
- 1.5 kW per tonne for sailboats
- 1 kW per tonne as an auxiliary motor
- 1 kW per tonne extra for current and coastal conditions
These figures are not really about top speed. They are about keeping a safety reserve, enough power to hold position against a current, point into a gust, or manoeuvre confidently around a dock instead of fighting the boat. Treat them as a ceiling, not your cruising speed: in day-to-day use you will likely draw only a fraction of this maximum, which is exactly the case on my own boat.
My setup: a 6.5 m boat at 200–300 watts
My own boat is a Varianta 65: 6.5 metres long with around 5.4 metres at the waterline, weighing around 900 kg with all my gear and me aboard. I use it on lakes and almost never run the 1 kW electric outboard at full throttle. In normal use I sit at roughly 200 watts, occasionally nudging up to 300.
That tells you a lot about real-world power needs. The motor is capable of far more ( a safety reserve is nice ), but a light displacement hull moving at a calm pace simply does not ask for much. The payoff shows up in endurance: my 1.2 kWh ePropulsion battery holds 1,200 watt-hours, so at 200 watts it sustains me for about six hours (1,200 ÷ 200), and even at 300 watts it still lasts around four hours. That is comfortably a full afternoon on the water from a battery I can carry in one hand.
You may have noticed that my 1 KW motor is actually slightly underdimensioned for my boat weight. The lesson is that peak power is almost irrelevant to how I actually boat. What counts is the cruise draw, and keeping it low is what turns a small battery into a long day.
Try it yourself on the range calculator: drop a pin, configure your boat with different motor and battery combinations, and see your range drawn directly onto the map.
Power and range are two sides of the same coin
Choosing a motor is only half the picture. The power you draw + the battery capacity is exactly what sets your range. A motor sized sensibly for gentle cruising sips energy and travels far; an oversized setup run hard drains the battery quickly. If you want to see how a given power draw translates into distance, read how far an electric boat can go.
For quick answers on the questions that come up most when sizing a motor, see do electric boats have horsepower and how long an electric boat motor will last.