Hold Your Horses!

What's All This About Watts?

Firstly, a very quick recap of some high school physics…

Watts are a measure of power (named after the 18^th century Scottish inventor James Watt), which is the rate at which energy is transferred. In the context of e-bikes, watts refer to the amount of power that the motor can deliver to assist the rider. A higher wattage motor generally means more power, which translates to faster acceleration and better climbing ability.

250-watt motors are the most common wattage for e-bikes. They are relatively low-powered, which makes them efficient in terms of battery consumption, but they offer less assistance on steeper hills or with heavier loads. However, 250W motors are ideal for most of Otago’s Great Rides which are predominantly easy-intermediate graded trails with only moderate climbs.

In terms of horses, a 250-watt e-bike has 0.33 horsepower.

To provide a better understanding of the concept of power in human terms (rather than horses), let’s consider the wattage output of an average cyclist on a standard bicycle.

When pedalling at an ordinary cruising speed - typically around 15 kilometres per hour for the Otago trails - a rider generates approximately 30-watts of power which is roughly equivalent to the energy expended during a brisk walk. Increasing the speed to 30 kilometres per hour introduces more aerodynamic resistance and requires a substantial 220-watts of power, a level that most non-athletes would struggle to maintain. Climbing up a 10 percent grade slightly faster than a walking pace demands 150-watts of effort, enough to leave most riders breathing heavily and working up a good sweat!

In the world of elite cycling, professional Tour de France riders can produce an astonishing 400+ watts of power for an hour, and during the intense final sprint of a race, they can unleash over 1000-watts.

Thankfully, most visitors to the Otago trails are not riding at Grand Tour level and enjoy riding at a more sedate holiday pace compared to the Chris Froome or Greg Lemond’s of this world. So, whilst New Zealand's regulations limit e-bikes to 300-watts on cycle paths, this is more than sufficient for comfortable riding on the scenic Otago trails…. even if it doesn't quite transform us into professional cyclists with uber leg muscles and huge lung capacity.

Maximum Power Output vs Continuous Power Output

Keeping up so far?
Sounds simple, doesn’t it? But guess watt? It’s not…

The term "maximum power output" that’s described in NZTA’s regulations can cause confusion as some e-bike motor manufacturers refer to their "maximum input power", which is different i.e. input vs. output. Perhaps this is a cunning marketing ploy as the input power number is always larger than output, creating the impression that the customer is getting a more powerful motor. Obviously, this can cause misunderstandings when determining if the e-bike is permitted on the trails or not.

To further complicate matters, some manufacturers rate their motors based on "continuous power" rather than "maximum power."

It’s well worth doing some research to discover how specific manufacturers have calculated their motor output as not everyone uses the same method to calculate their bike’s rating. Many use the maximum continuous output for the motor – this is the output that the motor can consistently deliver without overheating or breaking.

To help explain this, imagine that you buy an e-bike that’s officially rated at 250-watts under the continuous power rating, but it can reach maximum power outputs of over 500-watts. Would this bike be permitted on the cycle trails? Well, in NZ, no as the maximum output is greater than 300W.

Although in this example the e-bike can peak at higher wattage (500W+), sustaining that level for more than a few seconds is unlikely. Nonetheless, this 250-watt rated bike would not be permitted on NZ cycle trails.