Hallway lights energy use summary
• First what assumptions did we make?
– All lights had the same wattage bulbs
– Number of hours lights were on during the day
– All bulbs were on all the time
– All fixtures had the same number of bulbs in them
– Electricity costs $0.11 per kilowatt hour
Hallway lights energy usage
• Room KW /day yearly cost
• TCCW 1 49.9
1,976
• TCCW 2 78.6
3,156
• TCCW 3 74.1
2,975
• TCCW 4 66.6
2,672
Total for TCCW
10,779
Hallway lights energy usage
• Room KW /day yearly cost
• Snell B 14.0
562
• Snell 1 64.7
2597
• Snell 2 13.6
547
• Snell 3 58.96 2367
• Snell 4 19.5
781
Total for Snell
6,854
(3686.77)
Horse power
• Arose as a result of the invention of the steam
engine. People needed a way to compare the
power of a steam engine to that of the horses
it was replacing.
• Confusing unit there are too many different
definitions!
BTU
• BTU: British Thermal Units - an energy unit
– the amount of heat required to raise the temperature
of one pound of liquid water by one degree from 60°
to 61°Fahrenheit at a constant pressure of one
atmosphere
• Used in the power, steam generation, heating and
air conditioning industries and the energy
content of fuels.
• However, BTU is often used as a unit of power,
where BTU/hour is often abbreviated BTU.
– So you need to watch the context!
Back to Watts…..
• A human climbing a flight of stairs is doing work
at a rate of about 200 watts.
• A typical household incandescent light bulb uses
electrical energy at a rate of 25 to 100 watts,
while compact fluorescent lights typically
consume 5 to 30 watts.
• A 100 Watt light bulb consumes energy at the
rate of 100 joules/second.
• After 1 hour, this light bulb uses 100 watt-hours
• 1 kilowatt (kw) is 1000 Watts
Examples
• In a certain room in your house, you use a 100 W
light bulb. This light is on for 5 hours every day.
How much energy does it use?
• 1 W = 1 J/s and there are 5hours x 60min/hour x
60 sec/min = 18,000s in 5 hours so the total
energy used is 100 j/s *18000s = 1.8 x 10 6 J.
• Lets assume the same lighting level can be
achieved using a 30 W compact florescent bulb.
How much energy is used by the compact
florescent bulb?
Examples
• Total energy = 30 j/s x 18000 s = 5.4 x 105 j.
• So how much energy is saved every day using
the compact florescent bulb? Take the
difference between the energy used by the
two different light bulbs: 1.8 x 10 6 j - 5.4 x 105
j = 1.3 x106 j.
• Lets look at this in something you might be
able to relate to better than joules---dollars!
Example continued
• After 5 hours, our 100 W light bulb uses 500
Watt-hours, or 0.5 Kwh. The 30 W bulb will
use 150 Watt hours or 0.15 Kwh.
• Assume electricity costs 11 cents/Kwh
(roughly the average cost in the US in October
2010). So it costs .5 KwH x 11 cents/Kwh = 5.5
cents every day to run the 100 W light bulb
and 0.15Kwh x 11 cents = 1.65 cents every
day to run the compact florescent.
Example continued
• So in a year, the 100 W light bulb costs you 5.5 cents/day
X 365 days/year = $20.00 and the 30 W bulb costs costs
you 1.65 cents/day x 365 days/year = $5.50.
• Savings of 14.50/year per 100 W light bulb!
• There are now 13 Watt LEDs that can replace 100 W light
bulbs.
• These would cost 2.61 per year. That’s a 17.39 per year
per bulb savings.
How do CF bulbs work?
• made of glass tubes filled with gas and a small
amount of mercury.
• produce light when the mercury molecules are
excited by electricity running between two
electrodes in the base of the bulb.
• mercury emits ultraviolet light, which in turn
excites the tube’s phosphor coating, emitting
visible light.
Limitations of CF bulbs
• Take longer to turn on
• Do not work with dimmer switches
• Contain mercury, a known toxic substance
Mercury in CFLB
• 4-5 milligrams per bulb, about the size of a
period.
• Old mercury based thermometers contain
about 500 milligrams.
• Should be disposed of carefully at recycling
centers, not in the trash.
• If broken, specific steps need to be followed to
be safe, but you do not need to call a hazmat
crew.
Is the government taking away your
incandescent light bulbs?
• Provision in the Energy Independence and Security Act of
2007 – signed in to law by president George Bush
• Set energy efficiency standards for light bulbs that many
incandescent light bulbs to not meet –it does not “ban”
current generation of light bulbs, simply sets energy
efficiency standards for light bulbs sold and used in the
US which will phase out use of the current generation of
incandescent light bulbs.
• Incandescent light bulbs currently exist that are
affordable ($2.99 for a two pack) and meet the new
energy standards-so you don’t have to buy the CFLB.