Understanding E-Bike Power, Range, And Energy

What’s the difference between watts and watt-hours, and how far can I go?

Trek Super Commuter
Treck Super Commuter battery and Bosch Mid-Motor pedal assistBosch

All new fields bring with them a new vocabulary. Take cars, for example: Carburetor, throttle, quarter panel, differential, and a few thousand other terms were created or repurposed to label the new technology. Similarly, there are a few new terms and concepts that you’ll have to learn to talk knowledgeably about eBikes.

Power, of course, is a familiar automotive concept. It’s a measure of how quickly you can do work. More power generally means you can accelerate more quickly or climb a steeper hill without slowing down. With cars and motorcycles, for example, power is usually measured in horsepower, and we all know the difference between a motorbike with 5 hp and one with 200.

Another unit for power is the kilowatt, generally used with the metric system and—in the US—to measure the power of electric motors. One kilowatt is defined to be about 1.34 hp. You can just as readily say a small motorcycle with 10 hp has about 7 kilowatts.

But when it comes to eBikes, a kilowatt, a full 1,000 watts, is often just too big a unit. Instead, eBikes are generally rated in unprefixed watts, with power ranging from 250 to about 900 watts. If you want to think about that in terms of horsepower, that’s about a third of a horsepower to about 1.2 hp. Just as with cars, an eBike with more power can accelerate more quickly or carry more weight at speed up a steep hill.

What do watts on an e-bike actually mean?

To get some perspective for what power means in human terms, we can look at how many watts a typical rider uses on a standard bicycle. Just pedaling along at a typical cruising speed for a non-sporting rider (about 9 mph) takes about 30 watts, about the same energy as normal walking. Go up to 20 mph, and thus rapidly increasing aerodynamic loads, and 220 watts is needed—more than most non-athletes can sustain. Riding up a mild 10 percent grade at just faster than a walking pace takes 150 watts—enough to have most riders breathing and sweating heavily. Tour de France quality racers, though, can put out 400 watts for an hour, and somewhere over 1000 watts on the short sprint to the finish at the end of a race. The 750 watts allowed for e-bikes under US regulations means we can all have the legs and lungs of a professional bicycle racer.

The other important concept with eBikes is energy. Power is about acceleration, but energy is about range. In a conventional automobile, energy is stored in the form of gasoline and converted by the car’s engine into power. Accordingly, in a car, we don’t explicitly think about energy, only the number of gallons of gas we have and how far that’s likely to carry us.

E-bikes are much more direct. Energy is stored in the battery and fed to the bike’s electric motor to create power. The units of electric power are watt-hours, which is how many watts can be delivered for an hour by the battery. Typical eBike batteries range from about 300 watt-hours (abbreviated WH) to about 1,000 WH. The last would more often than not be expressed as 1 kilowatt-hour (kWh), kilo being the prefix indicating 1,000.

As with cars, eBike’s mileage varies. The US Environmental Protection Agency (EPA) originated a system to rate electric cars and to compare them against gasoline cars, something it called MPGe (miles-per-gallon-electric). An electric car typically gets 90 to 130 MPGe. Most eBikes get somewhere around 2,000 MPGe on this measure, roughly 20 times more efficient with electricity than a Tesla Model S.

But if you like being able to calculate your range, a more important number is the number of watt-hours used per mile of travel. Depending on the bike, your travel speed, and how high the electric assist is turned up, that number is typically somewhere between 10 and 20 WH per mile. That tells you a typically small 300-WH battery might take you somewhere between 15 and 30 miles on an eBike. If you want to do long rides on a weekend at a high-boost level, that might not be enough, and you should get a bike with a bigger battery. Or it might be perfect for your 8-mile round-trip commute.

Of course, most eBikes have a simple bar and/or percentage gauge indicating the state of their battery charge. With that, you’ll quickly get a feel for how far 10 percent or 25 percent of battery charge will carry you, and will get an intuitive sense of how far you can go with no math required. And unlike an electric car, you can always just pedal without assist if were to misjudge and run out of battery.