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 the 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 the remaining 23 percent.

The NEMA Formula consists of dividing the heat loss by 3413 (the number of Btu’s per kWh), 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.

KWh = { ( heat loss / 3413 ) x (annual degree days) x (constant, usually 18.5) } / { difference between indoor and outdoor design temperature}

Example of using the NEMA Formula:

Calculate the necessary quantities:
heat loss = 35,000 Btuh
annual degree days = 5542
constant = 18.5
inside design temperature = 70
outside design temperature = 0

and substitute in the values:

35,0000 Btuh
------------- = 1030

1030 x 5542 x 18.5
------------------ = 15,086 kWh
    70 – 0

A major criticism of this formula by distributors of oil and gas heating fuels is that the use of the Heat Loss Formula gives a much higher kWh consumption rate than using the NEMA Formula.