The red outside unit (sometimes called the birdhouse) is constructed just like a typical space heated building.  Inside, there are two small electric heating elements and a thermostat.  The unit is insulated.  The thermostat is pre-set at whatever base temperature you have requested, generally 65 degree's fahrenheit.
    The inside unit controls and measures what is happening in the outside unit.  When the temperature in the outside unit drops below 65 degree's fahrenheit, the thermostat makes contact and turns on the heating elements just like the thermostat turns on the heating plant in your own home, and the homes of you customers.  When the temperature gets back up to 65 degree's fahrenheit,  the thermostat breaks contact and turns off the heating plant; again, exactly like what happens in our home and your customer's homes.  While this is happening, the inside unit controls exactly the amount of electricity used in the outside unit to heat it, and measures the length of time heat is required.  After heat is required for a specific length of time one degree day is recorded.
    Again, exactly the same thing happens in your customer's home.  After a specific length of time that the burner is on, one gallon of fuel would have been used.
    Naturally, the colder it is outside the longer the heating plant will run.  This will mean more degree days and more fuel consumption by your customers.
    If it was very windy, heat would be lost faster than normal from the outside unit.  The heating plant would have to run longer to keep the temperature inside the box at 65 fahrenheit degrees, and we would get more degree days.  We all know that when it is windier than normal our customers will use more fuel than when it is not windy.
    Sun, or solar radiation affects the outside unit exactly the opposite as does the wind.  When it is sunny, the solar radiation helps to heat the outside unit.  The heating plant does not have to run as much, and fewer degree days would be recorded than on a day where the temperature was the same, but there was no sun.
    If you stand in front of a window when it is sunny, you can feel the heat coming into your home.  This helps heat your home, and your furnace does not have to run as much to keep your home warm.
    You can see that the Johnson Fuel Demand Meter gains and loses heat just like your home.  The unit has its own heat plant as does your home so that both our home and the meter can gain heat.  Both your home and the meter lose heat because of the weather in exactly the same manner.
    The mean temperature method of recording degree days, not taking into account the effects of wind and sun, can be most inaccurate in several ways.  When the temperature rises above 65 fahrenheit degrees and drops below 65 fahrenheit degrees, mean temperature degree days are most inaccurate.  For example:  The high for the day was 80 fahrenheit degrees and the low 50 fahrenheit degrees, no degree days would be recorded, i.e. 80 plus 50 equals 130 divided by 2, to get the mean, equals 65.  Subtracting this from 65 gives us zero, or no degree days.  However, we all know that when it was 50 fahrenheit degrees outside, all thermostatically controlled buildings would have used fuel, yet the mean temperature degree days, indicated no fuel used.  The Johnson Fuel Demand Meter would have recorded degree days under this situation.
    Another inaccuracy in the Weather Bureau high low occurs when the temperature drops form the high of the day to the low of the day, and stays near the low most of the time.
    Because the meter works as it does, it is the most accurate method of recording degree days.  "K" factors will remain more constant, larger fills may be scheduled, fewer run outs will occur, and a more profitable operation will result.