Clarifying a Few Clean Tech Terms

With all the talk of energy systems, next-generation technologies, climate science and other fascinating topics, we’re bound from time to time to be confused by all the jargon. As esoteric as they may seem, many of these terms are critical to understanding the discussions they tend to cloud. I’ve laid out a few of the most popular and most misunderstood below.

Watt (or kilowatt, etc.) versus Watt-hour:

For starters, there are a thousand Watts in a kilowatt, a thousand kilowatts in a megawatt, a thousand megawatts in a gigawatt, and a thousand gigwatts in a terawatt (and on and on). The difference between a Watt and a Watt-hour is the difference between a rate of power and energy. The Watt, named after Scottish engineer James Watt, is a unit of power equivalent to one joule of energy per second. Thus, a Watt is most easily understood as a rate. A Watt, kilowatt or megawatt is simply the rate of output at any given point in time.

A Watt-hour, on the other hand, is the unit of energy associated with that rate over one hour. For example, if a power plant with a maximum capacity of 100 megawatts (MW) operates at full capacity for an hour, it will generate 100 megawatt hours (MWh); for two hours, 200 MWh; and for a year – 8,760 hours – 876,000 MWh.

And while we’re talking about power plants –

Capacity versus capacity factor:

The term “capacity” is most commonly used to refer to a power plant or generating facility (as well as individual solar panels and wind turbines). The capacity of a power plant, also referred to as “nameplate capacity”, is the maximum power, or output rate, at which the plant can generate electricity. So it makes sense that capacity is measured in terms of Watts, usually as megawatts for a power plant.

Capacity factor is also used to refer to power plants, solar panels and wind turbines. The capacity factor is the ratio over time of a plant’s actual output compared to its capacity. Many plants operate at maximum capacity during times of peak demand (in other words, they put out electricity at a rate that equals their capacity), meaning that our 100 MW plant does churn out 100 MW at times. Over a year, however, the capacity factor is usually substantially lower than 100%. If this plant averages 50 MW output over an entire year, for instance, it will have a capacity factor of 50%. The capacity factors of average industrial wind turbines have traditionally been between 20-30%, although they have recently begun to reach 40-50%. Capacity factor is huge when analyzing the cost effectiveness of electricity generation.

Parts per million (ppm):

In climate science, one of the main indicators that humans are impacting the composition of our atmosphere is its proportion of carbon dioxide (CO2) to other gases. Measured in parts per million (ppm), historical concentrations of CO2 have peaked between 280 and 300 ppm during warmer interglacial periods, like the one we’re in now. You may have heard of the organization – the number 350 refers to ppm atmospheric CO2, which represents a level at which many scientists believe we would be able to avoid the worst impacts of climate change. Too bad, since we’re currently averaging 394 ppm and increasing by over two ppm per year. Parts per million can of course be used as the metric for any atmospheric gas.

Biogenic carbon versus fossil carbon:

One important point that comes up about CO2 emissions, particularly with respect to biofuels (more on this in a future post), has to do with the type of carbon that’s being burned to create the CO2. Fossil carbon is the stuff most commonly talked about – this is the CO2 that’s emitted by burning fossil fuels, essentially carbon that’s been stored underground for millions of years. Without a dedicated effort on our part, almost none of it would reenter the atmosphere on a timescale that concerns us. Burning it adds directly to the amount of CO2 in our planet’s otherwise natural carbon cycle. Which brings me to biogenic carbon.

Before the carbon in fossil carbon succumbed to time and pressure, it existed in the form of living organisms. Trees, puppies and dandelions are all composed of carbon, which, in simple terms, was extracted from the environment to become part of them. This carbon is no different from the carbon in the atmosphere insofar as they are both part of the natural carbon cycle – they are biogenic. When the organisms decompose, most of the carbon is restored to the earth and the air. Biogenic carbon is one of nature’s great lessons in recycling.

The Verdict: This stuff can be confusing. Don’t mislabel as experts people who throw around big words just because they do. And don’t let the jargon mislead you – take control and learn it!

Surely there are more words and phrases that confuse. Please drop us a note and we’ll keep a running list for another post soon.



2 responses to “Clarifying a Few Clean Tech Terms

    • Good question – essentially, why bother with unit conversions?

      1 kWh actually equates to 3,600,000 joules. Remember that 1 Watt equals 1 joule per second. From this, we can determine that 1 minute of energy at a rate of 1 Watt equals 60 joules (just 1 joule per second times 60 seconds; essentially 1 Watt-minute, although no one uses this metric). Then we can multiply by 60 to go from 1 minute to 60 minutes, bringing us to 3,600 joules per Watt-hour. Finally, we must multiply by 1,000 to go from a Watt-hour to a kilowatt-hour, leaving us with 3,600,000 joules per kWh.

      So I would say there are probably two main reasons for sticking with kWh over joules. First, joules are so small that we’d be dealing with ridiculously large numbers. The watt-hour allows us to scale up to kWh, MWh, etc. without having to convert into joules first. Also, converting from the rate determined in Watts/kilowatts, etc. to Watt-hours/kilowatt-hours is more intuitive that converting back into joules since they’re just factors of 1000. I said this stuff was confusing – thanks for asking, because the more we understand, the more power we have to make the best decisions.

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