Methods
The following methods were used to reach the conclusions promoted by this web site. All of the data about AM broadcast stations in the US was down loaded from the FCC database on April 23, 2009. The stations under consideration are only those authorized for night time operations. Applications, Construction permits, Critical Hours, Plans, Pre-sun rise or Post sunset stations were not included for consideration. (source: http://www.fcc.gov/mb/audio/amq.html)
FCC rules used to establish transmitter power
In accordance with FCC rules (CFR 73.1560(a)), all stations must operate with not less than 90% nor more than 105% of there authorized power. Therefore, the licensed power levels are a good indication of what transmitter power is being used by any given station. (source: http://edocket.access.gpo.gov/cfr_2008/octqtr/47cfr73.1560.htm)
Converting transmitter power to AC mains power
All transmitters suffer losses when converting AC mains voltage to RF current feed to a transmitting antenna. This is known as "AC to RF efficiency." The average AM solid state transmitter is 53.6% efficient based on manufacture's data. The efficiencies are slightly higher for higher powered transmitters and slightly lower for lower powered transmitters. The manufacture's considered were Harris Corporation, Broadcast Electronics, Inc, and Nautel.
The authorized station power is divided by the average transmitter efficiency to give the approximate power consumption from the power grid
Determining the stations operating at night
All of the stations authorized to operate at night were compiled (see attached AM data .pdf).
Adding all of those stations transmitter powers together
The sum of the all of the authorized station powers (in kilowatts) was then divided by the average transmitter efficiency.
Converting AC mains power to Kilowatt hours (kWh)
That product was then multiplied by 6 to derive the kWh for the six hour operating period from midnight to 6 am.
Since this project is only considering a radio station's transmitter, it is safe to say that the estimates of power consumption are on the low side because other loads such as air conditioning are almost always present but are not considered here.
The aggregate power for each class was then divided by the number of stations in that class to give the "average" station for that class. For class A stations, the average station is 42.3 KW, for class B 3.72 KW for Class C 0.96 KW and for class D 0.069 KW carrier power.
Data used to make power savings claims
The US DOE estimates the average cost of electricity in the us is $0.11 per kWh (soure: http://www.eia.doe.gov/cneaf/electricity/epm/table5_3.html)
Data used to make environmental claims
The US EPA estimates that for every 1 kWh of electricity produced in the US, 1.7 pounds of CO2 is released into the atmosphere. This figure was used to derive the statements regarding CO2, miles traveled and equivalent gallons of gasoline.
- Gasoline equivalent use calculated by using the amount of CO2 produced from burning 1 gallon of gas. That is figured at 19.6 pounds of CO2 per gallon. (source: http://www.epa.gov/OMS/climate/420f05001.htm)
- The average car in the US gets 21.3 MPG, therefore the result of the above multiplied by 21.3 MPG nets the equivalent road miles
The US EPA estimates that for every 1 kWh of electricity produced in the US, 0.00412 pounds of SO2 and 0.00348 pounds of NOx is released into the atmosphere. (source: http://www.eia.doe.gov/pub/oiaf/1605/cdrom/pdf/e-supdoc.pdf