<blockquote><em>”Time is what prevents everything from happening at once.”</em> – <a href=”http://en.wikipedia.org/wiki/John_Archibald_Wheeler”>John Archibald Wheeler</a></blockquote>
<h2>Choosing Time Scales</h2>
One of the things we’re most proud of about <a href=”http://www.wattzon.com”>WattzOn</a> is that we allow our users to directly compare energy consumed on wildly different time scales. Like, flying twice a year vs. owning a television for 10 years. Simplifying the message of energy use to a single unit helps clarify the relative impact of each lifestyle choice. That’s why we measure energy impact in power (watts), which represents the rate at which you are using energy (just like speed is the rate at which you travel a distance).
To calculate watts, we are converting all profile answers into the amount of energy (in joules) you are using per time (in seconds). Choosing the time scale, however, is not necessarily clear. Are you driving 100 miles a week or 100 miles in 2 hours? In presenting your energy data to you in power, we are often creating a constant rate out of something that is not constant — and, how we choose to do so has a significant impact on the watts we present.
There is no <em>right</em> answer for most of these calculations; tracking personal energy choices in watts is our unique concept. So, we must make informed choices that seem to make the most scientific sense, while still maintaining the usability the website.
<h2>If I Could Save Time in a Paper Cup…</h2>
To demonstrate the time issue, let’s consider the energy use of a <a href=”http://www.wattzon.com/stuff/items/ke0v6ifnyujweukn61iijs86v6/kb0j33kz1b5oow8sdy70bmjv4b”>paper cup</a>. Based on estimates of the materials, manufacturing, transportation, and disposal, we have a reasonable assumption that a cup has an embodied energy of approximately 2.2 MJ (as is currently in the EED). To convert that to watts, we have to divide by time. But, what time scale best represents the duration of that cup’s “consumption”? We’ve looked at 4 different options:
<li><strong>Usage Time</strong> – The amount of time you actually spend using the cup for its intended purpose (i.e. drinking out of it). This seems to most accurately capture the idea of using watts to track energy consumption. But, under this model, the paper cup watts should only be added to your total watts for the time you are using the cup and then be deleted, making it difficult to track the impact of choices.</li>
<li><strong>Usage Rate</strong> – The rate at which you purchase a cup (in this case, once per day). The time for this option is easily determined since it’s already in the number (1 day). But, asking people to estimate their usage rate of items, especially things they may use irregularly, adds to the difficulty of using the site.</li>
<li><strong>Existence Time</strong> – This method spreads out the embodied energy of the cup over its own lifetime – the amount of time between manufacturing and disposal. However, it is difficult to know how long the cup has been around. Also, this method would give a higher wattage to the first cup out of the bag than to the last one.</li>
<li><strong>Lifetime</strong> – This way would spread the amount of energy in the cup out over <em>your</em> entire life. Perhaps the most accurate way to contemplate the energy impact our lifestyle choices, but I don’t have to tell you that it would be VERY difficult to ask people to accurately guess how many paper cups they’ve used since infancy.</li>
Though there are merits to each, the option chosen has a tremendous impact on the wattage of items, as you can see in the following table:
<tr class=”emphasis” style=”text-align: center;”>
<td><strong>Paper Cup Time Scales</strong></td>
<td>Usage Time (1 hour)</td>
<td>Usage Rate (1 day)</td>
<td>Existence Time (1 month)</td>
<td>Lifetime (70 years)</td>
Right now, we are tracking disposable goods (like our paper cup) with the “Usage Rate” model. It gives us the greatest balance of ease of information entry for users and understandable calculations. But, as noted above, this model does not work as well for items you may use irregularly (like, say, a notepad). Since nothing on WattzOn is static, we are continually exploring this concept (along with many, many others) . So, as we contemplate time, we welcome you to join us and give us your thoughts on how to improve our data!