The NEMA Formula
The NEMA formula was created by the National Electric Manufacturers Association. A constant of 18.5 is commonly used in the formula and reflects the percentage of heat loss that must be replaced by the heating system during a 24-hour period. In other words, the heating system will provide heat 77 percent of the time during a 24-hour period (24 X 18.5), while other heat sources will supply heat during the remaining 23 percent.
The NEMA formula consists of dividing the heat loss by 3413 (the number of Btu per kilowatt-hour) and multiplying this figure by the total annual number of degree days, which figure is then multiplied by the constant (usually 18.5). The resulting product is then divided by the difference between the indoor and outdoor temperatures. The application of the NEMA formula is illustrated in Table 4-13.
A major criticism of this formula by distributors of oil and gas heating fuels is that use of the heat loss formula gives a much higher kilowatt-hour consumption rate than use of the NEMA formula.