AskPablo: Home Energy Saving Tips

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 for $3.75 (1/2 mT minimum), probably making this MarketWatch’s first carbon neutral shoot!
Pablo Päster, MBA
Sustainability Engineer
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