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  T&D > T&D Pubs > Fluorescent Lamp Retrofits: Savings or Fantasy? > Summary > Heating and Cooling Load Analysis T&D Publications Header

Heating and Cooling Load Analysis

Heating and Cooling Load Analysis calculates the annual changes in heating and cooling costs due to the retrofit of an existing fluorescent light system (Lighting Energy Savings).

In terms of heating and cooling load, all lights can be treated as electric resistance heaters. All of the energy consumed is either directly or indirectly converted to heat. Depending on the fixture, some of this heat may be radiated into the attic.

Summer

Lighting works against you when you are trying to cool your facility. More efficient lighting will reduce the cooling load on your air-conditioning (AC) system. The AC system will not be on as long. Demand charges will not be affected because the power required to operate the system will not change unless the AC compressor is downsized. Using average cost per kilowatt-hour will overestimate the AC energy savings because this figure includes demand charges. The most accurate method is to calculate the energy-related savings using the energy charges from the electric company's bill. To simplify the following calculations, we assumed all of the heat from the fixture must be removed by the AC system. The savings due to the reduced AC load are:

$ AC energy saved = (dollars per kWh)(kWh savings per year)(number of months AC used per year) (1 yr/12 mo) coefficient of performance of the AC system

= ($.032852/kWh)(21,840 kWh)(6 mo/yr)(1 yr/12 mo)/(3) = $120 per year energy–related savings

Winter

The most energy–efficient lighting system possible is desirable even when heating is required throughout the year. Lights can be considered an electric resistance heater. Electric resistance heaters are one of the most expensive ways to heat. You will save more in electricity by installing energy–efficient lights and using another form of heating. The increase in heating cost is:

Increased Heating cost = (kWh savings per year)(number of months heated per year)(1 yr/12 mo)(3.415 Btu/kWh) (1 thm/100,000 Btu)($/thm)(100%/percent heater efficiency)

= (21,840 kWh/yr)(1yr/12 mo)(6 mo/yr)(3,415 Btu/kWh)(thm/100,000 Btu) ($.5618/thm)(100%/80%) = $262 per year increased heating cost

Conclusions

The overall annual change in heating and cooling cost is greatly affected by utility rates and number of months that heating and cooling are required. In this example the annual change, an increase of $142 ($262 – $120), is small compared to the lighting energy savings of $1,459. The savings due to reduced cooling load are usually more than the increased cost of heating, especially if electrical energy costs are high and the AC system is replaced with a smaller system suitable for the reduced load.