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Energy Equivalents

Discussion in 'Energy, Environment, and Policy' started by mknox, Jul 8, 2013.

  1. mknox

    mknox Well-Known Member

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    One of the things we do in my business (electric utility) to help customers understand units of electricity is to try and relate units back to common things that they may understand. For example, we may say that if "x" customers all change their light bulbs to CFL, it is the equivalent of taking "y" homes off the grid for a year. Or we may say running that 1,500 watt heater for an hour uses the same power as a 100 watt light bulb uses in 15 hours.

    I've had a number of people ask me how much energy my car uses, and often get blank stares when I talk of kWh's or even dollars from people not familiar with electrical concepts. They do get it when I say I used to put $75 of gas into my old car every 4 days, and now I use about $75 in electricity per month, but how does that compare with other electrical appliances?

    Just for fun, I looked at three months worth of energy use on my car and on my electric water heater. Crunching the numbers I have found that driving 1,200 miles per month (14,400 miles per year) would use exactly the same amount of power that my water heater uses for a family of three.

    This is using April, May and June data, so it may be a bit worse in the winter, but it's probably safe to say that driving a Model S 12,000 miles a year (kind of a benchmark number) would use the same energy as a residential electric water heater.

    I drive a lot more than that, so I will use more power than my water heater, but I thought this would make a good talking point when discussing EVs with the uninitiated.
     
  2. deonb

    deonb Active Member

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    Wow. Crazy. I must admit I didn't believe that for a second - thought for sure you missed a decimal in there!

    But after looking into it - you're totally right - an electric water heater uses about 5000 kWh of electricity per year. That comfortably gets a Model S over 12'000 miles.

    Electric water heaters are crazy inefficient!

    Just shows again - you want to burn something to generate heat, and use electricity to make things turn. If you do it the other way around (ala ICE vehicles), you inevitably have very large losses.
     
  3. ElSupreme

    ElSupreme Model S 03182

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    Well it really isn't that electric water heaters are inefficient. It just requires a whole LOT of energy to heat water. In fact the nature of electric heaters is that they are very nearly 100% efficient. As all the waste energy ends up being heat. The water tank will lose heat over time, but the method of heating doesn't change anything.

    And NG is cheaper per unit of energy than electricity. But much harder to produce anything but heat with.
     
  4. deonb

    deonb Active Member

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    Yes, but a heat-based electric power plant is only 41% efficient at best. If instead of generating heat at a power plant, to boil water, to spin a turbine, you cut out the turbine and just heat up the water directly, you'd always be better off.

    Well, ok... maybe you shouldn't cut out the turbine if your fuel source is Nuclear :).

    Of course if your power source starts off as something other than heat then this doesn't apply.
     
  5. Merrill

    Merrill Active Member

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    Mknox; Wonder if you can help me with a kWh conversion problem. I just got my separate meter for the Tesla and charged it last night. I started at 38 miles left and ended with 188, my meter says I used 51kWh. Yet when I use Tesla's calculator is says I would use 43kWh. I charge with the 14-50 Nema at 39amps. Is there a formula I can use to accurately calculate the kWh used. Any help would be appreciated.
     
  6. clea

    clea Member

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    There are other ways to look at it as heat is something that can be transferred as well. For instance my house is heated/cooled by using a geothermal heat pump which is based on transferring the heat between the house and the ground but another advantage of this transfer is that i can take some of the excess heat to pre-heat the water in my hot water tank.

    And to tie it to the OP, I have not tried to quantify how much it costs on average to heat the water for my household as it is not a simple equation nor is it the same when I am heating the house as opposed to cooling it in the summer. So mknox, as much as I like your original example, I will have to find another metric to describe the energy usage of the car ....
     
  7. ElSupreme

    ElSupreme Model S 03182

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    5MWh (or roughly 15,000 miles in a model S 335Wh/mile) is also equivalent to heating an Olympic sized swimming pool (50m x 25m x2m) about 3.1 degrees F (1.7 C).

    Another illustration of the amount of energy required to heat water.
     
  8. mknox

    mknox Well-Known Member

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    Sure. I was just trying to think of ways to put the car's electrical usage in terms an average consumer might understand using an analogy. Using my example, you could say that if you drive 12,000 miles (20,000 km) a year, you'll use about the same energy as an electric water heater. I'm sure there's lots of others: how many loads of washing/drying clothes, how many roasts cooked in the oven and so forth.

    - - - Updated - - -

    Sent you a PM
     
  9. VolkerP

    VolkerP EU Model S P-37

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    ... as is produced by a standard 5kWp solar photovoltaic array in my neck of woods. That's a complete southern half of a ridged roof top (we have small houses here :wink:)
    If you use this example, correct for your local insulation value. I use 1000kWh per year and square meter.
     
  10. mknox

    mknox Well-Known Member

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    Yeah, that's good too.

    Here in Ontario, we have a Feed In Tariff program that pays a significant premium over the regulated cost of power for solar and wind. If you're just worried about offsetting costs (not kWhs) you could actually go smaller.
     
  11. scaesare

    scaesare Active Member

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    Comparisons like this are indeed fun, and useful for helping to familiarize folks with what energy usage "looks like".

    I had fun relating how amazing the superchargers were to some friends/family: "The Supercharger supplied more energy to my car in under an hour, than my entire 4,100 square foot house with family of four uses an entire day, on average."

    (I average about 75KW/day home energy usage).
     

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