Nearly every retail electric bicycle and e Bike conversion kit is listed at a specific power level, such as a “500 watt electric mountain bike” or a “250 watt ebike conversion kit”, yet often this power rating is misleading or just plain wrong. The problem is that manufacturers don’t use the same standards to name their motors, and consumers often don’t understand the differences.
What’s a Watt?Let’s start with some definitions and a bit of a physics lesson. A “watt” is a unit of power, named for Scottish Engineer James Watt. Watts can be used to measure the instantaneous power output (or input) of a machine, such as the electric motor on your ebike. The number of watts used by an electric motor at any moment equal the voltage supplied by a battery multiplied by the current flowing from the battery to the motor. So an e motor connected to a 24V battery being supplied with 10 amps of current would be powered at 24*10=240 watts.
As you can see, calculating the peak power of an ebike is simple. You just multiply the voltage of the battery by the maximum current the e bike can handle. The maximum current is determined by the ebike controller, and is usually somewhere between 15-30 amps. An e bike with a 48V battery and a 20 amp peak controller would theoretically be capable of a nominal 960 watts of instantaneous power.This is where things get complicated though, because ebike manufacturers don’t always rate their parts this way.
Lies! Deception! Blasphemy!
This happens for a number of reasons. The most common cause is to skirt importation laws. Many European countries limit imports to electric bicycles with a motor rated at 250 watts or less. 250 watts is not very much power by ebike standards. Professional cyclists can put out more than 400 watts on leg power alone. So in order to clear their electric bicycles for import to as many countries as possible, many ebike manufacturers rate the components on their e bikes much lower than what they are in reality.
Meet “250 watt” motors.
Here is a great example of a 250 watt electric bicycle conversion kit. It comes with all the parts except the battery, a pretty standard motor rated by the vendor as “250” watts, and a pretty decent price of about $250 including shipping. But when we look at the specifications, we see the 36V controller has a peak current limit of 15A. Doing the math shows us that 36V * 15A = 540 peak watts. This is very common in the industry. E Bikes sold with “250 watt” motors often come standard with 36V batteries and 15 or 20 amp controllers. As we saw, a 15 amp controller would mean the actual peak power supplied to the motor is closer to 540 watts and a 20 amp controller would be over 700 watts.
Yea, “250 watts” my tuchus!
How do e bike manufacturers get away with this? One way is to rate the motor for “continuous power” instead of “peak power”. The difference between continuous power and peak power is that continuous power essentially means power a motor can safely handle for an indefinite amount of time without damage or overheating the motor. A “250 watt continuous” motor, theoretically, could run forever at 250 watts without overheating, but any more power would cause it to eventually overheat. If the motor is truly a 250 watt motor by definition, then running this motor at 251 watts would eventually cause it overheat.
Is it ok for e bike companies to rate their motors this way? Technically yes, if the numbers are accurate. But most of the time a “250 watt continuous” motor can handle more than 250 watts continuously, meaning the numerical naming convention is inaccurate and misleading.
The problem here isn’t the morality of underrating e bike specifications (this is one of the few times you usually get more than you pay for), it’s that this often confuses customers and makes comparing different motors much more difficult.
How can you best use power ratings?When comparing e bikes or e bike kits, it is important to know first of all if you are comparing continuous or peak power. When someone advises that a 220 lb rider would likely need at least a 1,000 watt motor, he or she usually means 1,000 watts of peak power, as in the amount of power the ebike should be able to produce to drive the rider up a hill.
A 500 watt electric bicycle conversion kit may be listed as a 500 watt kit, yet a closer inspection could show that the kit comes with a 48V battery and a 20 amp peak controller. The math shows us that this kit is in fact capable of putting out 48V x 20A=960 watts, essentially a 1,000 watt kit. What might have initially appeared to be too weak (advertised as 500 watts) is actually an approximately 1,000 watt peak kit, perfect for our 220 lb rider we used in the example about above.
Lawmakers don't really know eBikes!This is also an interesting example of how nonsensical many electric bicycle laws are. Limiting the wattage of e bike motors doesn’t necessarily limit how powerful they can be. Even though a motor is marked as 250 watts (and even if it may actually be a true 250 watt motor), anyone could connect it to a 48V battery and run 20 amps through the motor to achieve 1,000 watts of power. Of course this could eventually damage or destroy the motor, but it is still demonstrates how it is entirely possible from a practical standpoint.
In fact, direct drive motors such as the Nine Continent are often listed as 500 or 1,000 watt motors, but many people have had success running them at over 3,000 watts by drilling out the cover plates to provide additional air cooling to the motor. Other modifications such as increasing the gauge of the wires carrying power to the copper windings can help maximize the useful power output of these strong, underrated motors.
Peak Watts are Watt matter!
These examples should reinforce the take-home message here: when you are looking into an electric bicycle or e bike conversion kit, always calculate peak watts in your mind -- Volts x Amps (allowed by the Controller) to do a fair comparison of the actual power you can expect out of any e bike setup. That way you’ll know what type of power level you’ll really experience when you’re ready to twist the throttle.
The Throttle and eBike Evolution
Do you need a Throttle on an electric bicycle?
For most people, the short answer is … YES! Twenty years ago, when I first became involved with electric bikes, all eBikes in America had a throttle. And people loved them as you could still pedal just like your regular bicycle, but also power them like a motorcycle -- or seamlessly apply electric power to your pedaling to help flatten the hills, cut the wind, or just cruise around. What a joy, and a revelation! For riders who were a little out of shape, had an injury, or were intimidated by hills, you could ride like a kid again!
For about ten years, every US electric bike was configured in this manner. The throttle also enabled you to feather in a bit power if you were slowly cruising by an art show or a farmer’s market, and gave you a ride home if you were too pooped to pedal. Enter PEDAL ASSIST -- the function where power would be automatically added to your pedaling effort. This was a nice additional feature, as long as you didn’t ditch your wonderful throttle. Most early pedal assist systems had just one power level, which boosted your pedaling effort about 50%, helping conserve battery life, though limiting top speed. But many manufacturers started offering bikes that were ONLY pedal assist which was a turn-off for many prospective ebike riders. Why in the world would they ditch the beloved throttle? The answer was simply that throttle bikes were illegal in Europe whose market size dwarfed the secondary US market. Europe also had a 250 Watt limitation on the size of the motor, so many bikes were pedal assist only and just 250 Watts of power. These early manufacturer’s didn’t want to design a US specific machine.
Around 2010, as the US market matured and models were designed specifically for America, most electric bikes integrated both features, and added multi-level pedal assist -- giving the rider the best of both worlds. The better Hub motors got more efficient, powerful and totally dependable. And the better brands employed higher quality Lithium Ion batteries with advanced chemistries providing longer life and lighter weight. However, many models still positioned the battery on a rear rack, causing the bike to be unbalanced toward the rear. In 2013 and 2014, new designs with more creative and better battery integration began appearing bringing back the balance of what a bicycle needs to be. In addition, the electronic parts all became modular and plug and play so they could be easily replaced if necessary. In my opinion, an electric bike with the battery on the rear rack is simply ... obsolete.
The Mid-Drive and the Hub Motor
Enter the Mid-Drive motor, another design for keeping the electronics balanced and at a low center of gravity. This design adds power to the gears of the bicycle which can yield a very high efficiency for the rider, and eliminate the weight of the motor in the wheel. This is particularly ideal for the off-road Mountain bike rider switching elevations in demanding terrain. Though the mid-drive can have many advantages, the throttle is usually eliminated, and pedaling is necessary to get the full benefit of using the gears. One key issue with mid-drive motors is that they are limited by the strength of the chain and the sprockets. To date, almost all systems use standard bike chains and sprockets, which were never designed for motorized use, so the power has to be kept quite low and inevitably, a crank drive will lead to increased maintenance on the drive chain and gears. This is a newer and emerging technology, going through many design modifications as it matures. That being said, there are some amazing off-road mountain bikes on the higher end of the price scale.
At this point, the trusty hub motor (originally invented by Nikola Tesla), with a throttle and pedal assist is a refined and mature system. A good sealed hub motor is zero maintenance, and now with ‘quick disconnect’ plugs, changing a tire is as simple as a standard bicycle. And for most riders who just want to add power to their pedaling, they are simply more fun.