22 BULLETIN 102, UNITED STATES NATIONAL MUSEUM. 
requirements, and yet use the capacity of the plant but a small part 
of the total time, as developed in the next two sections. 
WHAT PEAK LOAD MEANS. 
Plotting an atmospheric temperature curve, based on Weather 
Bureau records, upside down, so that a decline in temperature will 
go upwards, as shown in Figure 10, a graphical index of the con- 
sumer’s capricious demand for heating service is obtained. The 
maximum demands are at the peaks of the curve and are, therefore, 
called peak loads. 
Referring to Figure 10, note there are only nine peak demands 
where temperatures were 15° and only one where temperature was un- 
der 9°. The peak demand requires about one-third of the heating 
equipment and is used only about one and one-half per cent of the 
total 8,760 hours of the year. 
ANALYSIS OF HOUSE-HEATING PROBLEM. 
For a number of years an eight-room brick house, 29 feet by 34 
feet — which would be an average size— -has been heated exclusively 
with gas. The room-heating equipment is a hot-air furnace with 80 
per cent efficiency ; the water heater will run about 80 per cent effi- 
ciency. For room heating, water heating, cooking, and burning the 
garbage, this house requires : 
Equivalent 
1,000 cubic 
Heat units feet manu- 
used. factured gas. 
Peak-load demand an hour during extremely cold 
weather to keep the entire house warm : 500, 000 1 
Annual consumption 280, 000, 000 560 
The annual plant capacity for the 8,760 hours in the year just to 
meet the maximum hourly demand equals 8,760 by “ 1,000 cubic feet ” 
or 8,760 “ 1,000 cubic feet.” The 560 “ 1,000 cubic feet ” actually used, 
therefore, represents 6J per cent of the total 8,760 u 1,000 cubic 
feet ” capacity needed to meet the peak demand. Therefore, the 
annual sales represent 6J per cent of the total plant capacity that 
must be held in readiness to serve and meet the peak-load conditions. 
The maximum hour peak of 1,000 cubic feet an hour does not 
occur every winter ; in fact, has not occurred for two years. About 
600 cubic feet an hour represents the maximum hourly demand 
of the usual winter, but when the extreme demand does come, if the 
gas service is unable to meet it, there will be an immediate complaint 
of the gas shortage and poor service. 
