The method used to estimate savings from making lighting retrofits is very simple. Determine what the operating cost is with the existing system, estimate what the cost would be with a retrofit system, and the difference is the savings. Knowing which lighting systems should be considered for replacement by what other lighting systems, will take a lot more effort.
The common denominator of all lamps is wattage. Wattage is additive, so no matter what the lamp and fixture combination is, if you know (or can estimate) the total wattage, you can determine the operating cost. The basic formula is:
(Watts x Hours /1,000) x $/kWh = Operating Cost per hour
Be sure to include the electric Demand Charge and all other costs that make up the actual average cost per kWh.
An office has 50 – 4 lamp 4 foot fluorescent fixtures that are on 14 hours per day. What is the daily cost to light the office at $0.08/kWh?
4 lamps x 40 watts/lamp + 40 watts for ballasts = 200 watts per fixture
200 watts/fixture x 50 fixtures = (10,000 watts/hours x 14 hours) / 1,000 watts/kW = 140 kWh per day
140 kWh x $0.08/kWh = $11.20 per day to light the office
50 – 4 lamp T8 fixtures on 14 hours per day at $0.08/kWh
160 watts/fixture x 50 fixtures = (8,000 watts/hour x 14 hours) / 1,000 watts/kW = 112 kWh per day
112 kWh x $0.08/kWh = $8.96 per day to light the office with the retrofit system
savings = $11.20 – $8.96 = $2.24 per day
Often the number of fixtures can be reduced, or perhaps 2 lamp fixtures could be used instead of 4 lamp fixtures, which would substantially increase the savings potential.
A warehouse has 25 – 400 watt metal halide fixtures and 20 – 2 lamp 4 foot fluorescent fixtures that are on 24 hours per day. What is the daily cost to light the warehouse at $0.08/kWh?
Metal Halide = 25 x 450 watts/fixture (lamp + ballast) = 11,250 watts
Fluorescent = 20 x 100 watts/fixture (lamps + ballast) = 2,000 watts
11,250 + 2,000 = (13,250 watts x 24 hours) / 1,000 watts/kW = 318 kWh per day
318 kWh x $0.08/kWh = $25.44 per day to light the warehouse.
25 High Efficiency Metal Halide fixtures at 350 watts/fixture and 20 – 2 lamp T8 fixtures.
Metal Halide = 25 x 350 watts/fixture (lamp + ballast) = 8,750 watts
Fluorescent = 20 x 80 watts/fixture (lamps + ballast) = 1,600 watts
8,750 + 1,600 = (10,350 watts x 24 hours) / 1,000 watts/kW = 248 kWh per day
248 kWh x $0.08/kWh = $19.48 per day to light the warehouse for a savings of $5.96 per day.
The Compact Fluorescent (CF) lamp was developed as a direct replacement for the standard incandescent screw-in light bulb. It is a self-contained lamp/ballast with a miniature electronic ballast built into its base. The following table shows equivalent light output for a given wattage. Determine energy savings by comparing the difference between the wattage for same lumen output. The basic formula is (Watts Saved x Number of Lamps x Hours of Operation x Average Electric Costs). Be sure to convert the watts to kilowatts (kW); then multiply by hours to get kWh, and Average Electric Costs in $/kWh, to get an answer in Dollars. (Dollars per day, month, year, etc., based on the Hours of Operation period used.)
Payback is determined by the first cost of the replacement Compact Fluorescent lamp as compared to the much lower cost of the incandescent lamp. Since the CF will last 10 to 20 times longer than the incandescent lamp, the CF may actually have a lower first cost, even if there is no labor factored in for changing the lamps. Assuming the same fixture can be used (not in all cases) there is no fixture replacement cost to be considered.
The Compact Fluorescent will have a lower CRI than the incandescent, and the CF is physically larger than the incandescent, so the CF is not an option for all applications of incandescent lighting.
The standard fluorescent lamp is the T12, or ‘tube’ 12 watts per foot. A standard 4′ T12 is rated for 40 watts, with the other 8 watts used by the ballast. A 2 lamp fixture will have one ballast and a 4 lamp fixture will have 2 ballasts. It is common practice to round-off the wattage rating to 100 or 200 watts for 2 or 4 lamp fixtures.
The T8 lamp represents a 30% reduction in energy usage over the T12. It is smaller in diameter and puts out similar lumens (2,250 to 2,950 lumens per lamp) using 32 to 34 watts total instead of 48 watts. The ballast must also be retrofitted; the same fixture may be used in some cases.
T12 lamps may use magnetic ballasts or older style electronic ballasts designed to be used with T12 lamps. T8 lamps can ONLY use new electronic ballasts designed specifically for T8 lamps. For more information on ballast see Ballast in the GE Lighting Training in Energy TechPro IC.
Almost all new fluorescent lighting systems are T8. Where electric costs and lighting run hours are high, a T12 to T8 retrofit represents a substantial energy savings potential. For commercial spaces, it may be the single most significant energy savings measure that can be taken. However, due to the relative high cost of retrofits, the vast majority of existing fluorescent lighting is still T12. Paybacks of 3 to 5 years are typical.
Estimate savings by counting fixtures and calculating wattage savings. Determine first costs by consulting with a manufacturer’s rep or lighting consultant on what options are available and weather or not a fixture can be retrofitted or must be replaced.
The most common metal halide lamp is the 400 watt. Manufacturers have developed new lamps that use as little as 320 watts to produce about the same lumen output and offer the same life-time. Most of the new lamps can use the same fixtures and ballasts, and are therefore a direct replacement. Only the higher initial cost of the lamp must be considered. This would seem to be a ‘no-brainer’ for metal halide retrofits.
T5 fluorescent has been around since the 1990’s, but it has been mostly available in Europe, does not directly retro-fit with existing 4 foot fixtures, and has been very expensive first cost. However, T5 has a better CRI, better life maintenance, can be dimmed, no re-strike time, and can produce similar amounts of lumens for less wattage. And as with all lighting systems, first cost comes down with volume and market acceptance/competition.
For conversion from incandescent to compact fluorescent or LED, see Exit Signs
All interior lighting watts are eventually converted to heat. A reduction in lighting watts will result in the same reduction in heat. This is good for cooling loads, and may be bad for heating loads.
1 watt = 3.413 BTUs and 1 kWh = 3,413 BTUs/Hour
Depending on the size of the retrofit and the local climate, lighting changes may not make a significant impact on heating and cooling system operation, but a thorough lighting analysis should also include a consideration of the impact.
1 Ton of cooling = 12,000 BTUs. Therefore, for every (12,000 / 3.413) = 3500 watts reduced in lighting load, there is an equivalent savings in cooling system size and run time (run costs). Air conditioning systems use about 1/3 of a watt to move a watt of heat. That means that a lighting kWh reduced, is actually 1.3 kWh reduced when including the cooling cost.
On the heating side, those reduced BTUs now must be replaced by the heating system. Most heating systems are so over-sized, there will probably be no problem handling the load, but there will be a higher cost in heating BTUs to replace the lost lighting BTUs. In most parts of the country, heating BTUs from natural gas are cheaper than lighting BTUs from electricity so there is still a net savings. About the only places for concern may be something such as a warehouse, where the only “space heating” comes from the lighting system. Upgrading the lighting system could result in a space being subject to freezing that used to stay well above freezing.
Sources: www.energytechpro.com, Text and Example Formulas by Bob Fegan 11/2002; Incandescent to Fluorescent Table from DTE Energy Training Materials compiled by Claudia Gabay-Jones, LC, CLEP, 7/31/2002; rev Bob Fegan 10/2005;