As I was writing my series on Net Metering, it was pointed out to me that most people are not especially familiar with some of the units that I used in the article. Thus, in this post, I would like to offer a primer on units that are commonly used when talking about electricity (and how they relate to renewable energy and energy use). At the bottom of this post, I have also included a glossary of Energy Industry Terminology that you may encounter while reading about renewable energy and energy use.
What's a Watt?
A Watt (W) is a basic unit of power measurement. Watts are used to measure how quickly energy is consumed by or generated by a system.
But what about all of the prefixes?
| Unit Name | Abbr. | Meaning | At This Scale |
| Microwatts | μW | 1/1,000,000th Watts | Wrist Watches |
| Milliwatts | mW | 1/1,000th Watts | Laser Pointers |
| Watts | W | 1 Watt | LEDs |
| Kilowatts | kW | 1,000 Watts | Average US Households |
| Megawatts | MW | 1,000,000 Watts | Aircraft Carriers |
| Gigawatts | GW | 1 Billion Watts | Medium Size Cities |
| Terawatts | TW | 1 Trillion Watts | Large Countries |
| Note: whether a prefix is capitalized or not can alter its meaning in some cases. |
How about a Watt-hour?
A Watt-hour (Wh) is a basic unit of energy measurement. Watt-hours are used to measure how much energy is consumed by or generated by a system.
1 Watt-hour is the amount of energy that would be consumed by or generated by a system if the system operated at power level of 1 Watt for a time period of 1 hour. Note that, all of the prefixes listed above apply to Watt-hours the same way that they apply to Watts (e.g. 1 kWh = 1,000 Wh).
| Energy Used | At This Scale |
| 15 Wh | Compact Fluorescent Bulb for 1 hour |
| 250 Wh | Xbox 360 and LCD Television for 1 hour |
| 1 kWh | Average US Household for 1 hour |
| 23 kWh | Nominal Rating of Electric Vehicle Battery |
| 12 MWh | Average US Household for 1 year |
| 20 GWh | Small US Town for 1 year |
| 57 TWh | Total Massachusetts Electricity Consumption (2010) |
| 3,750 TWh | Total US Electricity Consumption (2010) |
Watts or Watt-hours?
Despite the similarity in name a Watt-hour is very different from a Watt!
For example, If I asked you, "How much power are the appliances in your house using right now?" Your response should be something like, "They are consuming 800 W."
Likewise, If I asked you, "How much energy do the appliances in your house use in a year?" Your response should be something like, "They normally consume about 9,000 kWh."
If you used the two units interchangeably (such as answering the first question as "800 Wh" instead of "800 W), it would be the same as confusing any other two units.
For example, if I asked you, "How fast were you driving when the cop pulled you over?" and you answered, "About 30 miles." In this case, your answer simply does not make sense!
Appliance Power
The more Watts an appliance is rated at, the more energy it consumes. You can think of this as describing how 'hungry' an appliance is. That is, how much energy does the appliance need to consume in order to do its job? In many cases, a higher power rating means an appliance can work more effectively. Of course, a higher rating does not necessarily mean that an appliance is doing a better job, sometimes it can mean a lack of efficiency. To put this in perspective, let me offer two familiar examples.
| Example 1: Microwaves A 600 W microwave will typically take longer to cook your TV dinner than a 900 W microwave. In this case, the 900 W microwave is 50% more powerful than the 600 W microwave, which allows 50% more energy to be applied toward cooking your TV dinner each second that the microwave is running. In this example, the added power means the job is done more effectively. Example 2: Light bulbs A 100 W incandescent light bulb can often be replaced with a 25 W compact fluorescent light bulb and essentially the same amount of light will be provided. With each light bulb, enough light is provided to read a book each second that the light is turned on. In this example, the incandescent bulb is more powerful but less efficient, so a lot of energy is wasted by heating up the coils in the bulb. |
Reading an appliance's power rating can also, sometimes, be a bit misleading. Some appliances - such as microwaves - have straight forward power ratings. (e.g. A 900 W microwave should be using 900 Watts of power when it is in use.) However, other appliances have power ratings listed that represent their maximum energy use.
Refrigerators, for example, are typically plugged in and 'turned on' all of the time, but a 720 W refrigerator is not constantly using 720 Watts of power. Instead, refrigerators may use their rated power for 5 minutes and then use no power for 25 minutes before switching back on again. In this case, a 720 W refrigerator would have a peak power consumption of 720 W and an average power consumption of 120 W (or 720 W x 5 minutes/30 minutes).
Generator Power
The more Watts a generation system is rated at, the more powerful it is. You can think of this as describing how 'strong' the generator is. That is, would energy being generated feel like a blast from a fire hose? Or, would it feel more like a squirt from a small water pistol?
Electric generator power ratings can also be a little tricky. For most types of generators (such as coal power plants, nuclear power plants, wind turbines, etc.), the so-called nameplate capacity indicates the maximum possible power generation under optimal conditions. However, solar photovoltaic (PV) generation systems can be especially confusing, because they have two nameplate capacities: an AC nameplate capacity and a DC nameplate capacity.
The AC nameplate capacity of a PV generator is relatively straightforward; it indicates the maximum AC power output that the system's inverter(s) can produce. On the other hand, the DC nameplate capacity of a PV generator indicates the expected cumulative DC power output for all of the solar PV modules under Standard Test Conditions (STC) without applying any derates, which in some cases compound to result in an actual output that is much higher than the DC nameplate capacity (see description below).
| Solar PV Industry: Standard Test Conditions (STC) |
| In the PV industry Standard Test Conditions (STC) describe the production of a solar PV module
at 25°C (77°F) at sea level with 1000 W/m² of incoming solar radiation
(irradiance). While changes in atmospheric pressure can contribute minor changes in the actual output, changes in temperature and irradiance significantly alter the actual output of a solar PV module at different locations and throughout the year (and even throughout a single day). Depending on the location and time (and the amount of moisture and other particulates in the atmosphere) an array of solar PV modules may receive 100-1400 W/m² of solar irradiance during the daytime. This range includes irradiance both less than and greater than the STC irradiance. Greater irradiance will allows for a derate ratio that is greater than 1. Also, the cells in solar PV modules are better able to transfer electricity in cooler temperatures, which also allows for a derate ratio that is greater than 1. As a result, on a hot (cloudy) summer day a solar PV module will probably have much lower output than the DC nameplate capacity. Conversely, on a cold (sunny) winter day a solar PV module is likely to have an actual output that is much higher than the DC nameplate capacity. |
In any case, it is important to remember that the nameplate capacity of a generator is a nominal power rating, so it alone will not tell you how much energy a generator produces in a year. To determine a generator's annual energy production, you will need to multiply the nameplate capacity by the capacity factor and the amount of time in a year (i.e. Nameplate Capacity [W]x Capacity Factor x 8,766 hours = Energy Production [Wh]).
Other Terminology
Well, unfortunately, it would take an entire book or a semester course to explain all of the nuances of power, energy, voltage, and current related to energy generation and consumption. However, to help you along, I have put together a brief glossary of terms that are commonly used in the energy industry. I decided to put them in a logical (rather than alphabetical) order, so that you can read the glossary a bit like a book.
| Glossary of Energy Industry Terminology |
| Nameplate Capacity | The maximum* potential power output or consumption for which a generator or appliance is rated to operate under optimal conditions (so called because this is often the nominal value listed on the name plate of the system). |
| Derate | A multiplier that describes what portion of a nominal system output remains intact after the effect of a non-optimal or non-standard condition is considered (often used to calculate the impact of changes in operating temperature or other environmental variables). |
| Capacity Factor | The Actual Power Output compared to the Nameplate Capacity over a period of time (typically as a %). |
| Availability Factor | The portion of time that a generator is physically able to operate (as opposed to being offline for maintenance; typically as a %). |
| Peak Power | The maximum amount of power that is actually generated or consumed during a defined period of time. |
| Average Power | The total amount of energy generated or consumed during a defined period of time divided by that amount of time. |
| Load Factor | The Average Power divided by the Peak Power for a defined amount of time (normally used to describe how well a utility grid is utilized; typically as a %). |
| Direct Current (DC) | Refers to power that is generated, consumed, or transferred at a steady voltage during normal operation (commonly used in batteries and solar photovoltaic modules). |
| Alternating Current (AC) | Refers to power that is generated, consumed, or transferred using a fluctuating voltage, which causes the current to alternate directions (commonly used in household appliances and long distance utility lines). |
| AC Voltage (VAC) | The voltage range over which AC power fluctuates (in the US, household electrical outlets nominally operate at ~120VAC, which means the actual voltage fluctuates between +120V to -120V many times per second). |
| Grid Voltage | Refers to the AC Voltage that must be maintained on a utility grid in order to ensure safe and reliable operation (neighborhood utility lines typically operate between a few hundred volts and several thousand volts). |
| Grid Frequency | Refers to frequency with which Grid Voltage fluctuates each second (in the US, household electrical outlets nominally operate at ~60Hz, which means that each second the Grid Voltage fluctuates through 60 cycles from +120V down to -120V and back up to +120V). |
| Inverter | A device that turns Direct Current (DC) power into Alternating Current (AC) power (a common component of solar photovoltaic generation systems). |
| Converter | A device that turns Alternating Current (AC) power into Direct Current (DC) power (often referred to as "AC Adapters" and are used in appliances and electronic devices that run DC power or rechargeable batteries). |
| Transformer | A device used to increase or decrease voltage (higher voltages are used to transport power over long distances with minimal losses; lower voltages are needed to operate common appliances). |
| *For solar photovoltaic modules, the Nameplate Capacity typically indicates the DC power output during standard test conditions of the module during its first year of operation. |
I hope that you found this post useful. In the future, I will try to link to this post whenever I have included one of the terms listed above.
Cheers,
Sean Diamond
