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I have three energy reduction measures.

L3JckwGJx8 | Posted in Energy Efficiency and Durability on

Electricity is the only energy power.
One measure saves 25% . The second saves 7.5% and the last one saves 10%. What is the total saving?

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  1. jklingel | | #1

    First off, are you sure these savings are cumulative, and therefore independent, and even real? If so, then take off the 25%, then take off 7.5% of the 75% balance, and finally 10% of that balance. However, I suspect that these savings, if even real, are not independent and cumulative. After all, if you installed enough of these types of things, pretty soon you'd have a (nearly) 100% savings.

  2. GBA Editor
    Martin Holladay | | #2

    John gave you the right answer.

    If a house has only a TV (33% of electrical usage), lights (33% of electrical usage), and a refrigerator (33% of electrical usage), what happens when you install a TV that saves 25%, lights that save 7.5%, and a refrigerator that saves 10%?

    Well, if you used to use 1,000 kWh a month (333 kWh for the TV, 333 kWh for the lights, and 333 kWh for the refrigerator), you will now use 250 kWh + 308 kWh + 300 kWh = 858 kWh a month.

    That is a savings of 14.2%.

    But this is just one example. You should think in kWh, not in percentages.

  3. user-943732 | | #3

    Actually, there isn't any right answer here except -- it depends.

    Are the percent savings the percent of specific end uses that add to the total, like Martin assumed.? Or are they each % savings calculated relative to the entire energy use? If it's the latter, then the question becomes whether they interact with each other -- which typically happens if there are HVAC measures but not so much for envelope measures.

    Example, you calculate savings from insulating your walls at 20% of your usage and insulating your attic at 10% of your usage and replacing your windows at 8% of your usage. If you do all 3, then you should save 20% + 10% + 8% = 38% -- the savings from each measure are independent. But if you were savings the 20% from heating system replacement instead, then you would save 20% + .8 * 10% + .8 * 8% = 34.4% because you need to adjust the savings from the shell measures for the change in heating system efficiency.

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