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Saturday, April 5, 2008
LCD vs CRT monitors in terms of energy savings
You asked for specific information regarding the energy cost savings and pay-
off time for a flat screen vs. a CRT monitor. As has come up in several
comments, there are many specific factors that go into calculating this
answer. Among them are:
Time Usage – how often your monitor is on (and for a CRT is it asleep or
awake?), hours used per day, days used per year
Energy usage – the wattages of the specific two monitors you wish to compare
Local energy costs – the cost charged per kW by your electric provider
While I cannot answer your question exactly (without further details), I can
tell you how to make the calculation yourself.
First, you need to calculate the kW used per year for each monitor. The energy
usages can then be converted into costs by multiplying by your energy rate.
The difference between the two costs reflects the cost savings you will recoup
after using a flat screen (instead of a CRT) for one year. You can apply this
annual savings to the price difference between the two models to determine how
long it will take to pay off the more expensive flat screen.
kW saved per year
The Energy Efficiency and Renewable Energy Network (EREN) is part of the US
Dept of Energy. At the EREN website they list the wattage of many appliances.
http://www.eren.doe.gov/consumerinfo/refbriefs/ec7.html
The wattage of a CRT monitor is listed as 150 watts (awake) or 30 (asleep).
This means that in energy use, one hour of awake time = five hours of sleep
time for the EPA example monitor.
According to IBM, the T Series flat panel monitors consume 3-4 watts asleep and
30 – 65 watts awake.
http://www.pc.ibm.com/us/accessories/access_promo/flatpanel/tour/
Note that the awake energy use changes by a factor of more than 2, depending on
screen size. For a flat screen monitor which uses 3.5 watts (asleep) and 50
watts (awake) 1 hour of awake time = 14 hours of sleep time.
For an example solution (for a CRT) see below:
Time Use – hours/day: 6.5 (to account for sleep time)
Time Use – days/year: (52weeks/yr – 2weeks vacation/yr) x (5days/wk) = 250
days/year
Monitor Watts: 150
Cost per kWh: $0.07
[(hours/day) x (days/year) x (watts)] / 1000 watts/kW = annual kWh consumption
(kW) x ($/kWh) = annual energy cost
[(6.5hr/day) x (250days/yr) x (150watts)]/1000 = 243.75 kWh annual energy
consumption
(243.75 kWh) x ($0.07/kWh) = $17 per year
A CRT monitor costs $17 per year in electricity.
Using the above calculations for a flat screen monitor with the following data:
Time Use – hours/day: 6.2 (to account for sleep time)
Time Use – days/year: (52weeks/yr – 2weeks vacation/yr) x (5days/wk) = 250
days/year
Monitor Watts: 50
Cost per kWh: $0.07
A flat panel monitor costs $5.40 per year electricity.
To do this yourself
To determine the exact answer to your question, perform the above calculations
for each of the two monitors, and compare your results. To determine your
annual energy usage, use the wattage values printed on the back/bottom of the
two monitors you are comparing. To determine the Time Usage for a CRT that
goes to sleep, use an average value of time based on the % time the monitor is
awake/asleep.
Additional Information
Many resources can be found where the energy efficiency of flat screen monitors
is touted. Several of these sites are mentioned in the comments. Others are:
UC Irvine News, March 14, 2001
http://www.today.uci.edu/ucinews/0314f4.html
National Renewable Energy Laboratory
http://www.nrel.gov/sustainable_nrel/energy_saving.html
Other Notes:
Be careful when using specific energy savings figures from a Manufacturer. As
shown above, there are many factors that go into these final figures, so
anything quoted is just an estimate, and Manufacturers have an incentive to
make their product more attractive.
While some of the energy used by a CRT is generated as heat, a CRT is generally
not as efficient as a real heater. For heating purposes it will still be more
efficient to spend those kW on a heater not a CRT.
While the EPA Monitor Power Management Calculator is helpful in computing cost
savings on the basis of certain criteria, this calculation does not allow you
to calculate costs based on monitor type.
http://yosemite1.epa.gov/Estar/consumers.nsf/content/powercalculator.htm
I used the following search terms: flat, screen, panel, monitor, “energy use,”
efficiency, watts, utility, cost, electricity
Enjoy your new monitor! (whether it's a flat screen or a CRT)
While it doesn't add to the energy savings, the flat screen sure looks cooler :)
off time for a flat screen vs. a CRT monitor. As has come up in several
comments, there are many specific factors that go into calculating this
answer. Among them are:
Time Usage – how often your monitor is on (and for a CRT is it asleep or
awake?), hours used per day, days used per year
Energy usage – the wattages of the specific two monitors you wish to compare
Local energy costs – the cost charged per kW by your electric provider
While I cannot answer your question exactly (without further details), I can
tell you how to make the calculation yourself.
First, you need to calculate the kW used per year for each monitor. The energy
usages can then be converted into costs by multiplying by your energy rate.
The difference between the two costs reflects the cost savings you will recoup
after using a flat screen (instead of a CRT) for one year. You can apply this
annual savings to the price difference between the two models to determine how
long it will take to pay off the more expensive flat screen.
kW saved per year
The Energy Efficiency and Renewable Energy Network (EREN) is part of the US
Dept of Energy. At the EREN website they list the wattage of many appliances.
http://www.eren.doe.gov/consumerinfo/refbriefs/ec7.html
The wattage of a CRT monitor is listed as 150 watts (awake) or 30 (asleep).
This means that in energy use, one hour of awake time = five hours of sleep
time for the EPA example monitor.
According to IBM, the T Series flat panel monitors consume 3-4 watts asleep and
30 – 65 watts awake.
http://www.pc.ibm.com/us/accessories/access_promo/flatpanel/tour/
Note that the awake energy use changes by a factor of more than 2, depending on
screen size. For a flat screen monitor which uses 3.5 watts (asleep) and 50
watts (awake) 1 hour of awake time = 14 hours of sleep time.
For an example solution (for a CRT) see below:
Time Use – hours/day: 6.5 (to account for sleep time)
Time Use – days/year: (52weeks/yr – 2weeks vacation/yr) x (5days/wk) = 250
days/year
Monitor Watts: 150
Cost per kWh: $0.07
[(hours/day) x (days/year) x (watts)] / 1000 watts/kW = annual kWh consumption
(kW) x ($/kWh) = annual energy cost
[(6.5hr/day) x (250days/yr) x (150watts)]/1000 = 243.75 kWh annual energy
consumption
(243.75 kWh) x ($0.07/kWh) = $17 per year
A CRT monitor costs $17 per year in electricity.
Using the above calculations for a flat screen monitor with the following data:
Time Use – hours/day: 6.2 (to account for sleep time)
Time Use – days/year: (52weeks/yr – 2weeks vacation/yr) x (5days/wk) = 250
days/year
Monitor Watts: 50
Cost per kWh: $0.07
A flat panel monitor costs $5.40 per year electricity.
To do this yourself
To determine the exact answer to your question, perform the above calculations
for each of the two monitors, and compare your results. To determine your
annual energy usage, use the wattage values printed on the back/bottom of the
two monitors you are comparing. To determine the Time Usage for a CRT that
goes to sleep, use an average value of time based on the % time the monitor is
awake/asleep.
Additional Information
Many resources can be found where the energy efficiency of flat screen monitors
is touted. Several of these sites are mentioned in the comments. Others are:
UC Irvine News, March 14, 2001
http://www.today.uci.edu/ucinews/0314f4.html
National Renewable Energy Laboratory
http://www.nrel.gov/sustainable_nrel/energy_saving.html
Other Notes:
Be careful when using specific energy savings figures from a Manufacturer. As
shown above, there are many factors that go into these final figures, so
anything quoted is just an estimate, and Manufacturers have an incentive to
make their product more attractive.
While some of the energy used by a CRT is generated as heat, a CRT is generally
not as efficient as a real heater. For heating purposes it will still be more
efficient to spend those kW on a heater not a CRT.
While the EPA Monitor Power Management Calculator is helpful in computing cost
savings on the basis of certain criteria, this calculation does not allow you
to calculate costs based on monitor type.
http://yosemite1.epa.gov/Estar/consumers.nsf/content/powercalculator.htm
I used the following search terms: flat, screen, panel, monitor, “energy use,”
efficiency, watts, utility, cost, electricity
Enjoy your new monitor! (whether it's a flat screen or a CRT)
While it doesn't add to the energy savings, the flat screen sure looks cooler :)
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