My goal here is to explain what power means in the context of a blender and then to describe my independent measurements of Vitamix horsepower (including both input and output).
I am mainly motivated by general curiosity, but there is a practical application that relates to how fast a Vitamix blender can heat soup. Beyond that, blending is more complicated than just power, so I will not be trying to draw conclusions about performance.
This post is a bit technical, but I will try to explain it as straightforwardly as possible. If anything is unclear or confusing, please don’t hesitate to ask questions, either by commenting below or contacting me directly.
The concept of horsepower has been used for marketing since the 18th century. It invokes an intuitive picture of harnessing the work of powerful horses.
Vitamix advertises their machines as having a ~2 peak output horsepower motor, which brings to mind the power of two horses tromping around, maybe something like this: The definition of power is the rate of doing work, and work is defined as exerting a force over a distance. 1 horsepower is defined as the power that would lift 550 lbs 1 foot in 1 second. Indeed, in the early days, horsepower was used to describe how many draft horses a steam engine could replace.
However, note the “peak” in the Vitamix spec—that means that 2 horsepower is the maximum power the motor can put out. As I will show later, under typical use, the motor does not put out a full 2 horsepower. Also, the horsepower unit is derived from a horse being worked all day at a sustainable rate (i.e. healthy for the horse). For brief tasks, a horse can exert up to ~15 horsepower. Typical adult humans can exert a bit over 1 hp for brief periods and 0.1 hp for extended periods, while trained athletes can exert a few times that much (reference). So, a Vitamix blender’s power is closer to that of a competitive sprinter cyclist than a pair of horses—still not bad for the flip of a switch on your countertop.1
There are many different kinds of electric motors, with different designs and properties. Blenders typically use what is often called a universal motor, and I want to highlight some of its pertinent properties. (The following discussion is all about the case where the blender is set to maximum speed.)
Both the horsepower and the rotational speed depend on the load on the motor. In a blender, the load depends on the quantity and consistency of ingredients being blended. (Larger volumes and/or thicker consistency increases load.) So, unlike something like a light bulb that always draws the same amount of power, blender power is a bit more complicated.
The speed and output power of universal motors are approximated by this plot: (Generated with XKCD plot to indicate that that this plot is a rough estimate.)
The lowest load occurs when the container is empty, and the highest load would occur if you jammed the motor with something like a crowbar. The important things to notice about the plot are that the speed decreases with increasing load, and horsepower peaks somewhere in the middle of the load range. (Horsepower is the product of torque—equal to load in this case—and speed; if either torque or speed are near zero, their product will also be near zero.)
Input vs. Output Power
Note that there are different ways that power can be used to characterize a motor. First, there is a difference between the input electrical power, which comes from the wall socket, and the motor’s mechanical power output. The difference between input and output power is determined by the motor’s efficiency, which can range from 30–75% for universal motors.
Some manufacturers state their blender’s horsepower using the input number, whereas Vitamix states horsepower as the output.
Input power is usually measured in watts. (1 horsepower equals ~746 watts.) In electrical units watts are the product of voltage (Volts) and current (Amps):
W = V∗ A
(There is a complication with AC power, called power factor that I will ignore for now). Vitamix’s specifications for input power list 11.5 or 12 amps,2 operated at a line voltage of 120 volts, so that yields 1380–1440 watts, or 1.85–1.93 horsepower. As you can see in the horsepower vs. load plot above, the output power depends on load. (Input power also depends on load, but, unlike the output, input is approximately proportional to load.)
Measuring Input Power
What load is Vitamix using for their power input rating? Vitamix machines are UL listed (you can find the little UL symbol on the back or bottom of Vitamix machines), which means that they have been certified by UL LLC (a certification company that maintains safety standards for all sorts of products). UL 982 is a published standard that includes details about the requirements of an input power rating. It specifies that the benchmark load is “a mixture of diced carrots and water in the ratio of 2 to 3 by weight . . . to the marked capacity of the container . . . recorded 30 seconds after the first load cycle begins.” However, the UL standard also says that for appliances over 2.6 amps, the rated input power can have a ±15% deviation from the measured load test. The standard gives a benchmark load, but it also shows that manufacturers have quite a bit of leeway in their input power specification!
I used a Kill-A-Watt electricity meter to measure the input power when blending the UL-982-prescribed carrot-water mixture in the Vitamix 7500 with the wide 64-oz container. I measured 11.35 amps at 118 volts, and 1240 watts (note that the watts measurement is slightly less than V∗A because here the power factor is 0.93; VA is 1340). Those readings are well within the ±15% of Vitamix’s power rating of 12 amps. So, that’s the input power, what about the power output?
How to Measure Power Output
I was excited to realize that blenders provide a unique opportunity to measure power output without requiring any fancy equipment. When you blend liquid, the majority of the power is converted into heat via internal friction in the liquid, which raises the temperature of the liquid. (Some energy gets dissipated by sound, but that is tiny compared to the internal heating.)
This is a classic principle of thermodynamics, and it is actually almost the same setup that James Joule used to demonstrate the equivalence of work and heat in the 19th century. Here is an engraving showing his apparatus: As the weight on the right falls, it pulls a string, which turns paddles in a tank of water, and a thermometer measures the change in temperature of the water.
It turns out that a similar setup is called a water brake, and it is used for measuring things like race car engines. A blender has a built-in water brake that we can use to measure power!
Horsepower (Output Power) Results
I used a Vitamix 7500 for these tests, but I’ve previously found that all of the full-size Vitamix motors (C and G-Series) perform identically at maximum speed once they are loaded, so these results should apply to any full-size Vitamix.
I started by blending water and measuring the rate of temperature change. With the mass of water, water’s heat capacity, and the temperature change rate, I calculated heat per time, which is approximately equal to the motor’s power output. The power increased with increasing volumes of water, with the maximum at the full 64-oz capacity (in the wide container with 4″ blade). The temperature changed at 0.17°F/sec which comes to 1.1 horsepower.3 At that load, the motor was drawing 1220 watts, yielding an efficiency of ~68%. Interestingly, this means that a Vitamix is more efficient at heating water than microwave ovens, which are ~45% efficient.4
I was curious if I could measure the peak output horsepower, and to do that I needed a higher load. I decided to try a thick sugar syrup made from granulated sugar (sucrose). I found a handy formula for calculating the specific heat of sugar solutions. First I tried 67%, but that wasn’t thick enough, so I increased to 73% (73 g sugar per 100 g solution). The viscosity is highly temperature dependent (it gets thicker the colder it is), which turned out to be useful for these measurements. I cooled the syrup in the fridge, and then made a series of 10-second heating measurements, allowing the motor to cool in between. The high load means that the current draw is extremely high. I managed to both trip a circuit breaker in my power strip and set off the thermal override in the motor (on separate measurements).
I was satisfied to find a peak in the output power. That is, as the syrup warmed and its viscosity decreased, the output power first increased, and then decreased. The greatest output I measured occurred at 64°F and was 1.9 horsepower, with an input of 2050 watts (~69% efficient). The motor was able to keep this up without overheating for the 10 seconds that it took to make the measurement. I did not make too many measurements because it was too easy to overheat the motor with extended periods of these high loads. I likely missed the true peak by a little, but this was close enough to the claimed horsepower to satisfy me. Note that these measurements also disregard the power used by the cooling fan.
In order to make these measurements, I used a situation with a nearly constant load (and known heat capacity). Of course, in many practical blending applications the load is highly variable, and I believe there are situations where the motor outputs peak power that do not involve blending 64 oz of thick syrup.
Since power is the product of torque and rotation speed, we can also calculate the output torque. (I measured the speed using sound.) The 64 oz of water was spinning at 15,800 RPM yielding 0.4 ft⋅lb, and the 64 oz of syrup at peak power was spinning at 7,600 RPM, yielding 1.3 ft⋅lb.
To make it easier to look at, here are the results in table format:
|input (W)||output (HP)||RPM||Torque (ft⋅lb)|
|64 oz water||1,220||1.1||15,800||0.4|
|64 oz syrup||2,050||1.9||7,600||1.3|
Can a Vitamix blender boil water?
If you keep blending, will water eventually boil? Yes! I used a temperature probe to record the temperature as I blended 2 cups of water in the tall/narrow container, and it boiled in ~11.5 minutes:
Larger amounts of water take longer to boil, although if you double the water, the time is less than double because more water increases the load, which means the motor puts out more power.
Soup heats up faster than water in the blender because it is thicker, which increases the blender power, and because it has a lower heat capacity. (Almost everything has a lower heat capacity than pure water.) I decided to measure heating rates of a vegetable soup. I measured different volumes, as well as the difference between the 4″ blade and the 3″ blade. (The larger blade does more work per revolution, effectively increasing power.)
Note that thicker soups heat faster because they increase the load. I’ve seen some sources say that a Vitamix will heat soup to a certain temperature, but actually the final temperature depends on all of these variables: blending time, volume, thickness/viscosity, and starting temperature.
Both the input and output power of a blender depend on what is being blended. Within the range of normal operating conditions, increasing the load, by increasing the volume or thickness of the ingredients, increases the power input and output.
The power output of the motor gets converted into heating the ingredients, which can be used to heat soup or, as we have seen, to measure the power output.
I measured ~1.9 peak output horsepower—well within the ballpark of Vitamix’s stated horsepower, given that this was not a precise measurement.
2. Vitamix lists C-Series models at 11.5 amps, and G-Series models at 12 amps, but I have compared the C-Series vs. G-Series models, and there is no measurable difference in input and output power when blending the same load. On the other hand, the longer 4″ blades in the G-Series models mean that for the same ingredients the load is higher, which makes them draw more power than the C-Series 3″ blades. ↑
3. In addition to using the computed heat capacity of water, I also made a rough estimate of the heat capacity of the container/blade. I estimated their heat capacity by adding water of a different temperature, and then looking at the immediate change in temperature of the water. This is admittedly a rough estimate, but I found that it is about 5% of the heat capacity of the full pitcher of water. The reported horsepower measurements include this container/blade heat capacity. ↑
4. I measured a microwave at 44% efficient heating water. Searching around the web I found others who measured 43% and 47%. Stove top burners aren’t more efficient either. The only thing I’ve found that is more efficient than the Vitamix is a submerged-heating-element electric kettle—mine was 90% efficient. Note that this comparison of efficiency is largely academic, since cooking uses a tiny fraction of the energy consumed in modern homes. ↑
Here are a few questions about the power measurement that might come up:
Q: What about losing heat to the environment?
A: I made these measurements within 10°F of ambient temperature, where the rate of heat loss to the environment is negligible compared to the heating rates. The rate of heat flow is proportional to the temperature difference; you can see it start to be significant at the elevated temperatures of the boiling water plot (although the heating rate also decreases because the viscosity decreases with increasing temperature).
Q: What about heat from heating of the motor itself?
A: The motor does heat up quite a bit, and it is cooled by air drawn through the housing by a fan below the motor. The drive shaft does not conduct any appreciable amount heat. A section of the drive shaft is plastic (a poor thermal conductor), which is designed to break if the blades are overloaded (it is replaceable).
Q: What about heating in the bearings?
A: Yes, some of the motor’s power gets taken up by friction in the bearings, which heats them up. This is why you shouldn’t grind flour for longer than a minute: flour is not very thermally conductive, so heat from the bearings does not dissipate, and they can overheat. To test the amount of heat I ran the motor at maximum speed with an empty container for 30 seconds, then poured enough water in to cover the blades, and measured the change in temperature of the water. It was measurable (250 g water increased 2.3°C), but insignificant compared to the amount of heating that I measured in the power tests (less than 1%).