Electric Bikes: How Watts, Volts, and Amp Hours Impact Performance

If you want to know how fast or powerful an eBike will be, you need to understand watts (W), volts (V), and amp-hours (Ah) as these energy measurements apply to ebike motors and batteries.

Otherwise, “attempting to compare eBike power ratings is a great way to lose your sanity. That’s because ‘rated power,’ the metric some manufacturers use, doesn’t equal a motor’s actual power output or maximum potential power output,” wrote Dan Roe in Bicycling magazine.

Rather, as an electric bike owner or shopper, you need to understand how these electrical measurements impact an eBike’s performance. This is especially helpful if you are comparing electric bikes for a new purchase. Let’s look at a brief definition of each of these electrical measurements and describe how the motor or battery impact performance.

Electric Bikes: Watts

In most cases, the watt rating on an electric bike motor describes how much energy the motor can handle (or consume) continuously. This continuous watt rating is different from a peak watt rating that describes how much energy the motor can manage (or, again consume) for short periods of time.

An electric bike motor might reach its peak watt rating when under stress, climbing a steep hill or similar.

So, when you compare electric bikes first be certain whether continuous or peak watts are being reported. In some cases, you will see both, as an example, the motor on a bike is rated for 750 watts of continuous power but has a peak rating of 1,000 watts.

CONFIGURATION IMPACTS POWER

While there can be a relationship between the wattage rating for the electric bike motor and how “powerful” an electric bike may feel, the amount of power at the wheel can differ greatly for motors with identical watt ratings depending on the eBike’s configuration.

In fact, a wattage rating may be the least indicative measurement of the those we are considering in this article since an eBike’s controller and battery can have a lot more to do with how an electric bike feels when ridden.

One of the best examples of the difference between a motor’s wattage rating and how an electric bike will perform can be found when you compare mid-drive and hub-drive electric bikes.

“A hub motor is situated in the hub of one of the bike’s wheels, providing propulsion by spinning whichever wheel to which it’s attached. As electric bikes first began gaining popularity, these were the most frequently used type of motor,” explains chapter five of “The Complete Electric Bike Buyer’s Guide.”

In contrast, “mid-drive motors drive power to the bike’s drivetrain, typically at the crankset. By directly powering the bike’s cranks, mid-drive motors work in tight coordination with the bike’s already existing gears, amplifying the mechanical advantage they provide. This becomes particularly helpful when it comes to climbing steep hills or navigating extended inclines,” according to the buyer’s guide.

Thus, an electric bike with a mid-drive motor rated for, perhaps, 350W of continuous energy may have more “power” than a 500W or even 750W hub-drive electric bike in some cases.

Electric Bikes: Volts

“Volts are a measurement of tension. How much pressure can be contained in the battery,” explained Michael Geurts, a partner at Blue Monkey Bikes, in an excellent video about electric bike power.

The electric tension Geurts is describing is really the potential power (electromotive force) in an electric bike system. Often, this electrical tension or pressure is described with a “water-flow analogy.” Basically, if you can imagine how water pressure builds up in a pipe, you can understand the voltage.

YouTube publisher, Gary Chang, has a good video describing all the parts of an electric circuit with a water-flow analogy. In the video, you will see how volts represent the pressure in the circuit.

In terms of electric bikes, “batteries usually have volts in sequences of 12 such as 12, 24, 36, 48,” said Geurts, adding, “volts pretty much mean power — how powerful a battery can be, but it also lends to top speed.”

“A 48V battery will not likely propel an electric bike 50 miles per hour because it simply doesn’t have enough pressure to rotate a wheel that many revolutions. If you climb up steep hills, a 48V battery will outperform a 36V because the 36 [volt] system will be working harder to produce the same results,” Geurts said.

It is possible to have too many volts. “If you have a 72V battery that is only propelling a system at 20 miles per hour then that means that you have unused voltage or really underutilized voltage in that system,” Geurts said.

Given what you now know about volts, you should understand why it can become very important to look at the number of volts in an electric bike battery relative to the bicycle’s entire configuration. You want the battery and motor to work together to get the desired performance.

Electric Bikes: Amp Hours

Amp-hours are an indication of the capacity of the electric bike’s battery. You might even think of amp-hours as the fuel tank or the range for the bike.

First, an ampere or amp is the base unit for measuring electrical current or load.

“A battery with a capacity of 1 amp-hour should be able to continuously supply a current of 1 amp to a load for exactly 1 hour, or 2 amps for 1/2 hour, or 1/3 amp for 3 hours, etc., before becoming completely discharged,” wrote Tony R. Kuphaldt in the “Lessons in Electric Circuits” textbook available on All About Circuits.

“In an ideal battery, this relationship between continuous current and discharge time is stable and absolute, but real batteries don’t behave exactly as this simple linear formula would indicate. Therefore, when amp-hour capacity is given for a battery, it is specified at either a given current, given time, or assumed to be rated for a time period of 8 hours (if no limiting factor is given),” Kuphaldt wrote.

For an electric bike “higher amp hours generally means higher range,” said Geurts. But it “is not an exact mathematical formula for volts and speed and amp-hours and distance…bikes and especially riders are not that precise.”

Here is an example.

“They do a Georgia Century Ride here where they actually close Georgia 400 down, which is a major expressway. …You get to choose from a nine-mile ride, a 22-mile ride, a 45-mile ride, a 62-mile ride, or a hundred-mile ride, and I did the 62-mile ride on the [EVELO] Delta and still had battery juice leftover because I did a lot of the work myself because I wanted a really good workout,” said Steve Brown in an EVELO testimonial.

Brown effectively got more range out of his Delta electric bike in the Georgia Century Ride by pedaling more. Range and even speed then is not completely based on the electric bike alone but includes the pedal power (many adults produce between 150W and 200W with their legs) and the electric bike’s drive system.

When you compare amp hours for one electric bike against another you need to understand, as Kuphaldt described, how amp-hours are expressed, and you need to consider range as a function of the entire electric bike system.

Electric Bike: Watt, Volt, and Amp Hour Resources

Hopefully, this article has helped you to better understand watts, volts, and amp hours as they impact the speed, power, and range of an electric bike. But if you want to learn more, here are several, hand-picked watt, volt, and amp-hour resources.

• “The Complete Electric Bike Buyer’s Guide,” is a comprehensive guide for understanding how to compare electric bikes.
• “E-Bike Motors, Explained,” is Dan Roe’s Bicycling magazine article. Well worth a read.
• “Ebike Volts VS Amp Hours,” is the Blue Monkey Bikes video featuring Michael Geurts which was quoted extensively in this article.
• “What Are The Differences Between Motor Types on Electric Bikes” explains the difference between the types of motors that may be used on your electric bike.
• “Which is better: 48V or 52V battery?” in this video electric bike expert, Micah Toll, explains the difference between lead-acid and lithium battery cells and how that difference impacts the batteries used for bikes.
• “How Much Power Does An Electric Bicycle Need?” also by Micah Toll.
• “Batteries And Power Systems” is chapter 11 of Tony R. Kuphaldt’s book, “Lessons in Electric Circuits.”
• “Electric Circuits, Hydraulic Analogy: Charge, Voltage, and Current (introduction)” is the Gary Chang video referenced in this article. Chang has many excellent and applicable videos on his YouTube channel.
• “How Much Battery Range Do I Need?” compares amp hours to electric bike range.
• “How do I maximize the battery life of my electric bike?” describes how to maintain and treat your eBike battery.
• “How do I increase the range of my electric bike’s battery?” explains what you can do to get more range on a single charge.
• “How Electric Bike Throttles and Pedal Assist Work” this video explains pedal assistance and throttles.