CFM per watt (CFM/watt) is the basic unit for measuring fan airflow efficiency. It is defined as the amount of airflow (measuring in CFM or Cubic Feet per Minute) a fan can generate per 1 watt of electric power input.
Here is the most basic sense of how to think about fan airflow energy efficiency:
Fan output: Airflow or CFMs.
Fan input: Electric power input or wattage.
The general formula for calculating the airflow efficiency of a fan is very simple:
Fan Airflow Efficiency = Fan Output / Fan Input
Example: Let’s say that we have a 100 CFM bathroom exhaust fan that is powered by 20 watts. What is the airflow efficiency of such a bathroom fan?
Bathroom Fan Efficiency = 100 CFM / 20W = 5 CFM/W
That means that a bathroom fan will generate 5 CFM airflow output for every watt of electric power input.
Now, it makes sense to invest with high-energy efficiency fans. The key here is to understand how energy efficient a certain fan is.
There are many types of fans, ranging from ceiling fans, bathroom fans, table fans, and so on.
In order to get a better understanding of what is a good efficiency (good CFM per watt ratio) for a certain kind of fan, we will start by listing the average fan airflow efficiency for a particular type of fan, and note what is a good CFM/W ratio for energy-efficient fans.
Note: On average, energy-efficient Energy Star certified fans use about 40% to 60% less electricity for the same airflow output.
CFM/W Ratios For Fans (Specific Airflow Efficiency)
Here is a list of average and above-average or Energy Star certified airflow efficiencies:
| Type Of Fan: | Average Airflow Efficiency: | Good Airflow Efficiency: |
| Ceiling Fans (52-Inch) | 70 CFM/Watt | 110 CFM/Watt |
| Bathroom Exhaust Fans | 3-5 CFM/Watt | 10+ CFM/Watt |
| Hugger Ceiling Fan (52-Inch) | 50 CFM/Watt | 76 CFM/Watt |
| Window Fan | 10 CFM/Watt | 18 CFM/Watt |
| Table Fan | 25 CFM/Watt | 50 CFM/Watt |
Data Source: Energy Star Fan Efficiencies
As you can see, the airflow efficiency varies quite a bit between different types of fans.
We primarily use the CFM per watt metric to determine the ceiling fan airflow efficiency. Bigger ceiling fans can use more than 100W of electricity (spending 0.1 kWh per hour or more). You can read more about how much electricity different fans use here.
The savings energy-efficient ceiling fans offer can be substantial.
Example: An average 52-inch ceiling fan will run on about 50W and generate 3,500 CFM airflow at full speed (that’s 70 CFM/W). If you run it for a whole day (24h), it will use about 1 kWh of electricity (with US national average electricity cost of $0.1319/kWh). That means you’re paying about 13 cents/day to run a ceiling fan.
An energy-efficient ceiling fan (110 CFM/Watt) will not run on 50W. For the same airflow output, an Energy Star certified ceiling fan will run on only about 32W. Running it for a day will reduce the costs to about 7 cents/day. You are effectively saving 6 cents per day on electricity.
During the lifetime of a fan, these electricity costs do add up. That’s why it is important to check how energy efficient a ceiling fan is before you buy one, and you can do that just by calculating the CFM/W ratio.
Hope this helps illustrate what fan airflow efficiency is and how it can impact your electricity costs.