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AskPablo: Home Energy Saving Tips

| Monday February 12th, 2007 | 2 Comments

Marketwatch.jpgLast week I was invited to the home of Stacey Delo of DowJones Online to film a segment on home energy saving tips to be aired this week on MarketWatch. We looked at a whole bunch of energy saving measures, and this week I will show you how I got some of the numbers behind them. None of these particular facts made the final cut but they are worth reading.

Screenshot Small.jpg I once read that the typical microwave oven uses more energy to power its clock than it does heating meals each day. Can this really be true? Well, I got out my Kill-A-Watt meter and tested it out. The two microwave ovens that I looked at consume 5 watts while just displaying the time (who really needs a clock on every appliance?). These 5 watts are mostly wasted in the transformer that converts the 120VAC coming out of the wall to 12VDC that the clock needs. While running, my microwave oven uses 990 watts (+/- 10). At my house we probably use the microwave about 5 minutes per day. I realize that some households cook solely with their microwave ovens and others don’t even own one. So for now an average of 5 minutes per day seems like a good assumption.
So, 990W x 0.0833 hours (5 minutes) = 82.5 Wh/day, or 0.0825 kWh/day
vs, 5W x 23.9166 hours (24 hours – 5 minutes) = 119.6 Wh/day, or 0.1196 kWh/day
So it’s true, a microwave uses more energy to power its clock than it does to cook your food! Of course you may dispute my 5 minute assumption, so let’s see where the break even point is. Adding a few minutes of cooking time isn’t going to significantly affect the standby time of 23.9166 hours, so we will keep the energy used while on standby constant at 119.6 Wh/day. To find the point at which the cooking energy equals this amount we simply work backwards. 119.6 Wh/day divided by 990W is equal to 0.1208 hours, or 7.25 minutes. This means that, if you use your microwave oven less than 7 minutes and 15 seconds, it uses more energy on that useless clock!
Another one of my facts from the segment comes straight from the Department of Energy: One CFL (Compact fluorescent) bulb will prevent 450 pounds of CO2 emissions over its lifetime.

If every household in the US replaced one incandescent bulb with a CFL we could turn off 1 coal-fired power plant
, assuming 3 hours per day of use. How did I get this? Well, if you replace one 60W incandescent bulb with a 23W CFL you are saving 37W. 37W x 3 hours/day x 365 days/year x 110,000,000 households in the US = 4,456,650,000 kWh/year, or $668,497,500! A typical coal-fired power plant generates about 500MWe. There is typically some downtime for maintenance but let’s just assume 24/365 operation. So one typical coal fired power plant generates 4,380,000MWh/year (500MWe x 24h/d x 365d/y). To find out how many power plants we could eliminate we simply divide the energy savings by the energy generated by a power plant (4,456,650MWh / 4,380,000MWh). This shows that, if every household in the US replaced one incandescent bulb with a CFL, we could turn off 1 coal-fired power plant.
And finally, I plugged the camera and lighting equipment into the Kill-A-Watt meter to find that it used 537W. We spent about two hours so we used 1.074 kWh (537W x 2h / 1000W/kW). In California the grid electricity emissions factor is 0.51 mT/MWh (or 0.51kg/kWh). So the electricity used during the filming resulted in 550g of CO2 emissions. My driving there resulted in 13.75kg of CO2 emissions (30mpg = 275g CO2/mile, 275g/mi x 50mi = 13.75kg). The total is 14.30kg and it was offset with www.DriveNeutral.org for $3.75 (1/2 mT minimum), probably making this MarketWatch’s first carbon neutral shoot!
Pablo Päster, MBA
Sustainability Engineer
View Pablo Paster's profile on LinkedIn


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  • sam

    We’ve been using CF bulbs almost exclusively in our home for about five years.
    Unfortunately, one chink in the armor of the math for CF bulbs is that they don’t always last that long.
    there seem to be some assumptions about longevity that don’t measure up in our usage.
    they seem to do better in places that are left on for long periods of time, rather than the closet with the quick on/off.
    or perhaps I just got a bad batch at some point along the way.
    Anyway, out of about twenty five bulbs in our home, I’d guess I’ve replaced eight to ten in five years.

  • http://www.AskPablo.org Pablo

    Thanks for the comments Sam!
    The life expectancy rating on light bulbs is about as misleading as the mpg rating on cars. If a light bulb is expected to last 10 years that actually means that, in lab tests, 50% failed by that point. So half the time you can expect your bulb to die before 10 years, and half the time you can expect it to die after.
    The number of on/off cycles does affect CFLs because they have small electrical components that wear out with use.
    Thanks for reading AskPablo!