Evaluation of Electricity Consumption in the Manufacturing Division

Electricity consumption can be expressed in terms of either physical units, most commonly kilowatthours (kWh), or a common thermal unit, most commonly British thermal units (Btu).

The physical unit is meant to give a clear understanding of the amount of a particular energy source being used, while the thermal unit is a measure of convenience used to aggregate or compare various energy sources measured in different physical units. Converting kWh of electricity to Btu is not a trivial issue, because the amount of input energy needed to create a kWh of electricity is far greater than the amount of useful energy in the kWh at its point of use. Therefore, meaningful conversions of electricity use from kWh to Btu can be given in terms of:

• Site (point-of-use) electricity, at the universal value of 3,412 Btu/kWh. This value is useful to engineers, energy managers, and others trying to evaluate energy efficiency.
• Primary electricity, at a value that reflects the content of the energy inputs used to produce the electricity. This rate is most useful to policymakers and analysts who are considering global resources and environmental issues.

For convenience and consistency, a factor is traditionally used to convert point-of-use electricity use to primary electricity: 10,447 Btu/kWh for 1985, 10,324 Btu/kWh for 1988, 10,352 Btu/kWh for 1991and 10,280 Btu/kWh for 1994. These factors represent the average energy input to the generation process for fossil-fuel utility plants in the United States for their respective year, as given in EIA's Monthly Energy Review (DOE/EIA-0035(95/03)). However, the reader should understand that the true conversion values for the range of electricity estimates are unknown. Applying the single value to the range of electricity estimates in this report provides only a rough approximation of primary electricity because:

• For some type of utility energy inputs--hydroelectric, wood/waste, wind, and solar (thermal or photovoltaic), there is no generally accepted conversion rate.
• The fossil-fueled, nuclear, and geothermal generation processes have known but different conversion rates, so the overall conversion rate for these energy sources is a function of their mix.

A particular problem with manufacturing is that industries tend to be clustered much more than households and businesses in general. The electricity-intensive industries, such as aluminum, are concentrated in areas with inexpensive electricity, usually hydropower-generated. Thus, the accuracy of the overall conversion factor given above varies across industry groups. Estimates of primary electricity using this conversion factor should thus be treated with caution. They should be considered rough alternative measures to site energy as indicators of the importance of electricity as a manufacturing energy source.  Because of these intractable conversion problems, this report excludes energy intensity estimates where primary electricity is a component.

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