Horses vs watts: what size electric outboard do I need?

Walk into any boat shop and ask for a 5 horsepower outboard. Easy. Ask for a 5 horsepower electric outboard and you get a puzzled look, a spec sheet in kilowatts and a footnote saying "equivalent to".

Electric motors are rated in kW. Petrol outboards are rated in hp. And the bridge between the two is wobblier than a beginner on a longboard.

We went through the makers' own figures so you can compare like with like and pick the right size.

The maths: the clean line

The conversion itself is simple. One imperial horsepower is 745.7 W, and one metric horsepower is 735.5 W (Wikipedia).

So:

• 1 kW is about 1.34 hp.
• 1 hp is about 0.75 kW.

If an electric outboard has a 4 kW motor, the straight conversion is about 5.4 hp. That is where the clean line ends.

The makers' equivalents: a different set

Electric outboard makers rarely quote the straight conversion. They quote a "petrol equivalent", based on how the boat actually performs. And those numbers are higher.

Sources: Panbo, Barche a Motore, MBY, PDB Magazine, MBY. Straight conversion calculated by us at 745.7 W per hp.

The gap is biggest on small motors. A 1 kW electric outboard is sold as a 3 hp replacement, more than double the straight conversion. On bigger motors the two numbers move closer together. Mercury's figures sit almost exactly on the conversion.

Why the numbers differ: reading the wave

Several things explain the gap.

• Torque. Electric motors deliver full torque from standstill, so the boat gets going faster.
• Propellers. Many electric outboards turn larger, slower, more efficient propellers.
• Output vs input. Some makers quote the power drawn from the battery, others the power at the propeller. They are not the same.
• Different reference points. Petrol and electric ratings are not always measured at the same point in the drive, so the two numbers rarely line up exactly.

The practical lesson: compare kW with kW where you can, and treat any "equivalent to" figure as a maker's claim.

What size for which boat: the line-up

A rough guide by job, matching the power classes in our outboards section:

• Under 1 kW: kayaks, SUPs, canoes and small dinghies. See electric motors for kayaks and SUPs.
• 1 to 6 kW: tenders, dinghies and small boats. Torqeedo's Travel range, rated equivalent to 2 to 5 hp, is designed for dinghies and small boats up to 2 tonnes (Torqeedo).
• 6 to 30 kW: day boats, sailboats and small RIBs. Torqeedo's Cruise range, rated equivalent to 6 to 25 hp, is designed for motorboats and sailboats up to 12 tonnes (Torqeedo).
• 30 kW and up: planing boats and fast RIBs. The RAD40 and ePropulsion X40 sit here.

If you are replacing a petrol outboard, start from the maker's equivalent for your current hp, then check the battery size. A motor that matches your old petrol speed will drain a battery fast at that speed. Our guide to electric outboard battery life shows how fast.

Before you buy: check the limits

Two numbers can decide the size for you before performance does.

• Lake power caps. Some lakes cap electric motors at 500 W. See electric outboards for lake power limits.
• Licence thresholds. Many countries require a licence above a certain kW. See do you need a licence for an electric outboard?.

Our verdict: compare kW, sanity-check the hp

So, what size electric outboard do you need? Start with the job and the water, not the horsepower. Compare motors in kW, use the makers' equivalents as a rough guide, and remember that the real limit on an electric boat is usually the battery, not the motor.

For the full picture, read the electric outboards buyer's guide.