CFM per watt (cfm/watt) is the basic unit for measuring the air flow efficiency of a fan. It is defined as the amount of airflow (measured in CFM or cubic feet per minute) a fan can generate per 1 watt of electrical power input.
Here’s the most basic idea of how to think about fan air flow energy efficiency:
Fan Output: Airflow or CFM.
Fan Input: Electrical power input or wattage.
The general formula for calculating the air flow efficiency of a fan is very simple:
Fan Airflow Efficiency = Fan Output / Fan Input
Example: Let’s say we have a 100 cfm bathroom exhaust fan that is powered by 20 watts. What is the air flow efficiency of such a bathroom fan?
Bathroom fan capacity = 100 cfm / 20 W = 5 cfm/w
This means that a bathroom fan will generate 5 cfm of airflow output for every watt of 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, including ceiling fans, bathroom fans, table fans, etc.
To get a better understanding of what is a good efficiency (good CFM per watt ratio) for a certain type of fan, we will start by listing the average fan air flow efficiency for a particular type of fan, and note That’s a good CFM/W ratio for energy efficient fans.
pay attention: On average, energy-efficient Energy Star certified fans use about 40% to 60% less electricity for the same airflow output.
CFM/W Ratio for Fans (Typical Air Flow Efficiency)
Here is a list of average and above-average or Energy Star certified airflow capacity:
|Fan Type:||Average Airflow Capacity:||Good Airflow Capacity:|
|Ceiling fans (52-inch)||70 cfm/watt||110 cfm/watt|
|bathroom exhaust fan||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 Capabilities
As you can see, airflow efficiency varies greatly between different types of fans.
We primarily use the CFM per watt metric to determine ceiling fan airflow efficiency. Large fans can use more than 100W of power (expending 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: On average, a 52-inch ceiling fan will run at about 50W and generate 3,500 CFM of airflow at full speed (that’s 70 CFM/W). If you run it all day (24 hours), it will use about 1 kWh of electricity (with the US national average electricity cost of $0.1319/kWh). This means you are paying about 13 cents/day to run a ceiling fan.
An energy efficient ceiling fan (110 cfm/watt) will not run at 50W. For the same airflow output, the Energy Star certified ceiling fan will only run at about 32W. Running it a day would reduce the cost to about 7 cents/day. You are effectively saving 6 cents per day on electricity,
During the lifetime of a fan, these electricity costs add up. That’s why it’s important to check how energy efficient a ceiling fan is before buying it, and you can do this simply by calculating the CFM/W ratio.
Hope this helps clarify what a fan’s air flow efficiency is and how it can affect your electricity costs.