Measuring Energy

This post is a part of the series An Acre of Sunshine.

It is very easy to get lost in the morass of terminology surrounding the measurement of energy. There are some good reasons for why there is so much terminology, but much of it results from the history of what was discovered when, by whom, and used for what purpose. To try to keep things as simple as possible, we will stick to only 2 units of measurement, those that I find to be most familiar to people and that are easy to work with at the human scale. The first of these is the Calorie¹, which is how we generally speak of the energy that we get from food. The average person consumes about 2000 to 2500 Calories per day. The meals, snacks, and beverages that a person consumes each day provide all of this energy. The second unit that we will use is the kilowatt hour (kWh). This is the standard unit of measurement for electricity, so all of you readers who pay an electricity bill should be used to seeing this measurement. Depending on where you live, the cost of residential electricity can be anywhere from a few cents to a dollar or more per kilowatt hour, with current 2015 prices in North America mostly being between $.10 and $.30 per kWh.

A couple of examples can explain how the kilowatt hour is measured, and put it into perspective. Say that you have a 100 watt incandescent light bulb that you want to use to light up your kitchen (a laptop computer working hard uses about the same amount of power). That rating, in watts, is a measure of how much energy is required to get the bulb to light up for any given second. With anything electrical, it is the motion of electrons  through the parts of the device, be it a lighting filament in a light bulb or a transistor in a computer, that allows them to function. It is beyond the scope of this piece to go into a deep look at the physics of electricity, but if you are so curious, here is a place to start. So our light bulb needs a continuous supply of 100 watts to stay lit. What if we wanted to talk about the amount of energy needed to keep the bulb lit constantly for a full 24 hour day? To measure it, we can just say how much time that energy was being used, and this is commonly done in hours. So to run a 100 watt light bulb for 24 hours, one multiplies watts by hours and gets 2400 watt-hours. To make the numbers more manageable, this can then be switched to be 1000 times smaller by adding the suffix kilo-, making it 2.4 kilowatt hours (kWh). 2.4 kWh is the amount of energy it takes to keep that light bulb lit for a full day.

A second very different example can be from biology. How much energy, in kWh, does it take to run a person for a day? As mentioned above, a person needs roughly 2500 calories each day. Since they are both units of energy, it turns out that one can directly convert Calories into kilowatt hours; 1 kWh is equal to 860 Calories. Applying this to our daily intake of food, a person needs 2.9 kWh of energy to do our usual goings on. So it takes just a bit more energy to run you, the reader, for one day as it does for a 100 watt incandescent lightbulb. Put another way, a person runs on about 120 watts. The kilowatt hour then acts as a very human scale measurement, in that a person typically uses some relatively manageable number of kilowatt hours of energy for a day's activities.

All of the different units for measuring energy are potentially interchangeable, and I've provided a table below that shows a few of the common units that you may be familiar with and their conversion with each other.




Now that we have established a common language and an idea of scale for discussing energy, another table can show the amount of energy found in many of energy sources we encounter day to day. Many of these particular ways of storing energy will come up again the following sections. One caveat to make about the whole fresh foods here, like chicken or potatoes, is that much of the weight of these is actually water, which doesn't contain any useful energy. So while the energy in chicken comes mostly from protein and fat, much of a whole chicken is actually water.





Previous page: Energy capture, conversion, and storage
Next page: So how much energy does a person really use?

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1 This blog will always be speaking of the dietary calorie, or kilocalorie, see here for a more in-depth explanation.

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