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 are nerd speech for POWER
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 ebike 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 ebike can handle. The maximum current is determined by the ebike’s controller, and is usually somewhere between 15-30 amps. An ebike 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.
Maximum continuous power versus maximum power
Why do manufactures offer different "power ratings"? This happens for a number of reasons. A 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 ebikes much lower than what they are in reality.
Meet “250 watt” motors for electric bikes
A great example are so called "250 watt electric bicycle conversion kits". They often come 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. Ebikes 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 electric 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 ebike 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 ebike 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 ebikes or ebike 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 are ignorant about ebikes (among other things)
This is also an interesting example of how nonsensical many electric bicycle laws are. Limiting the wattage of ebike 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.
These examples should reinforce the take-home message here: when you are looking into an electric bicycle or ebike conversion kit, always calculate peak watts in your mind (volts x amps) to do a fair comparison of the actual power you can expect out of any ebike setup. That way you’ll know what type of power level you’ll really experience when you’re ready to twist the throttle.