How much is a kWh?

Hello Readers,

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

Massachusetts Utility Net Metering Cap

Hello Readers,

Earlier in this series, I described how utility net metering caps conflict with renewable energy installation goals. In this post, I will build upon the context established in the previous post about the Massachusetts Renewable Portfolio Standard (RPS), and I will explore the purpose of and issues with the net metering cap in Massachusetts.

To begin, let us consider the question: What is the purpose of capping net metering rules in the first place? There are two possible reasons why a net metering cap would have been introduced into the regulations: (1) there are physical safety concerns related to allowing significant percentages of net metered systems to interconnect or (2) utility companies lobbied for its introduction.

Safety Concerns with Net Metering

As I described in my graduate dissertation, the addition of a large percentage of intermittent renewable generation, such as solar and wind DG systems, has the potential to cause power quality problems on a non-smart utility grid. In other words, without sufficient demand response and/or energy storage capacity incorporated into grid, the modern utility grid infrastructure may not be able to handle large swings in power output (such as those that may be associated with changing wind speeds or clouds passing over a photovoltaic system) especially if intermittent renewable systems account for roughly 20% or more of a utility company's annual peak demand.

Even so, it is important to note that both DG-scale and large-scale (non-DG) intermittent renewable systems have the potential to cause physical stress on the utility grid infrastructure. In fact, the spikes and falls in production that can be problematic for the grid as a whole have a tendency to average-out across an aggregate of many DG systems, and therefore provide more stable grid voltages compared to a single large-scale system. Again, renewable DG systems should theoretically be favorable for the grid and provide a reason to encourage as much net metering as possible.

So, again, why put a cap on net metering? Understandably, no one wants huge swings in grid voltages to cause rolling brown-outs and damage to appliances, so perhaps a cap at 15-20% of the annual peak demand may be warranted if the grid is too 'dumb' to handle the input of renewable DG systems. However, in Massachusetts the cap has been established at 1% of annual peak demand for homeowners and businesses and at 2% of annual peak demand for municipalities and other government entities (for at total cap of 3% of annual peak demand).

National Grid's Net Metering Cap

National Grid’s historical peak load of 5,131 MWs occurred on August 2, 2006 in Massachusetts Electric territory; making the 1% limit 51.31 MWs and the 2% limit 102.62 MWs. As of July 5, 2012 in Massachusetts, there are 45,244 KWs with net-metering service under the 1% limit and 11,291 KWs with net-metering service under the 2% limit. As of July 5, 2012, there are 443,588 KWs with applications in the process of being interconnected under the 1% limit and of those 64,211 KWs have returned the Schedule Z.

Excerpt from National Grid Net Metering Website

Keeping in mind that 1,000 kW equals 1 MW, it is clear that, utility companies such as National Grid, already have more applications to interconnect net metered DG systems than they have room under their 1% private cap. This begs two questions:

• Why is there a larger set aside for public entities (i.e. a 2% cap for municipalities and other government entities) in Massachusetts?
• What is going to happen to all of the private entities that are applying to interconnect DG systems in Massachusetts?

As it turns out, the Massachusetts DPU is already arranging for the creation of a waiting list for entities attempting to interconnect DG systems after the cap limit is reached (see pdf). However, what good will this do if the net metering cap does not increase? The answer is appears to be little-to-no good at all.

Lobbying for a Net Metering Cap

Ostensibly, it is plausible that utility companies would have lobbied the state legislature on the issue in order to protect their profit margins. That is, logically, offering more net metering credits to DG system owners results in a proportionate decrease in the revenue that a utility company is able to collect, and therefore a proportionate decrease in profit margins. Thus, it would be equally logical in most cases for utility companies to lobby the legislature in support of a cap on net metering credits, thereby protecting utility company profit margins.

As I said, in most cases this would be logical, and in many states this may very well be the case. However, in Massachusetts in particular this makes very little sense due to a provision known as decoupling, which connects utility company profits with the number of customers served rather than the amount of electricity consumed.

Decoupling

The Department of Public Utilities (DPU) [...] issued an Order that will begin the process of "decoupling" rates from sales volume for all of the state's electric [...] distribution utilities, in order to encourage utilities to help their customers reduce their energy consumption and take advantage of on-site renewable energy, as required by the Green Communities Act, the comprehensive energy reform law [...] signed by Governor Deval Patrick. [E]lectric utilities will file rate plans that separate, or decouple, their sales of electricity [...] from the revenues they need to collect in order to maintain the electricity [...] distribution system they are responsible for. [...] Utilities are expected to file decoupled rate plans with the [DPU] as existing rate plans expire - for most companies, by 2012 - though companies can file sooner on a voluntary basis.

Excerpt from Mass DPU Press Release

Thanks to Massachusetts' implementation of decoupling, there is little-to-no direct financial incentive for utility companies to obstruct customers looking to take advantage of renewable DG systems. Under current regulations, utility company profits will remain the same regardless of how much electricity is consumed.

Unfortunately, I do not have the personal resources to thoroughly investigate the lobbying practices of the utility companies on this matter. However, given this understanding of decoupling, it is tough to imagine that utility companies would have invested much effort or money into lobbying in favor of a net metering cap.

Although, on a related note: in a personal, off-the-record discussion with an employee of a utility company, it was suggested that part of the reasoning behind the net metering cap may simply be a lack of qualified personnel. That is, in Massachusetts, which has only recently seen a significant private sector interest in renewable DG systems with the introduction of the Solar Carve-Out of the RPS, utility companies in Massachusetts do not have enough qualified staff to safely and responsibly handle the incoming interconnection requests in a timely manner.

A Possible Solution

To summarize, capping net metering may be providing utility companies some breathing room needed to retrain current staff and/or hire qualified staff. Likewise, it may also be providing the utility companies with an opportunity to figure out how to make the smart-grid smarter (i.e. better able to handle high percentages of intermittent renewable systems).

If either of the above justifications for a net metering cap are the case in Massachusetts, that ought to be made clear to the public and the policymakers. Also, instead of setting a single net metering cap, which provides homeowners and businesses no opportunity to make plans beyond the limits of the current cap, legislators should layout a growth schedule for the net metering cap.

This growth schedule should be clearly defined and - importantly - must lead the growth of the RPS goals by at least a year. If the net metering cap remains stagnant or does not out-pace the RPS in Massachusetts: DG installers, financiers, and potential DG system hosts will continue to be frustrated and delayed by an inability to plan ahead, which will be detrimental to everyone interested in meeting the RPS goals or holding a job in the green energy industry in Massachusetts.

Requiring the utility companies to increase the net metering cap at a pre-defined rate will offer everyone involved some project planning certainty. Also - if implemented correctly - a scheduled net metering cap increase will provide utility companies the incentive they need to safely and responsibly implement beneficial smart-grid technologies, which will improve grid efficiency and reliability regardless of the success of the RPS in Massachusetts.

Sincerely,

Sean Diamond

Click Here to go back to the First Post in the Net Metering series

What is a MW or a kWh? Click Here to find out!

ColorPower - A Decentralized Smart Grid Technology

Hello Readers,

I hope you have been enjoying the on-going Global Sustainability series. This post is not in that series but instead is a weekend treat for the avid followers of this blog. Have no fear, the rest of the series is scheduled to post next week.



I was sitting in the lobby of an environmental non-profit the other week, and I happened to pick up a copy of the Spring 2012 issue of the MIT Energy Initiative's Energy Futures magazine in which I found an article about a new smart grid technology called ColorPower, which may be of particular interest for proponents of renewable energy, energy storage, and electric vehicles. The article, Tomorrow's power grid: Adjusting demand to meet supply (starting on page 11 of the magazine, page 13 of the pdf version), describes a 'smart grid' demand-side management technology that decentralizes the decision making process, giving the appliance users greater control over their electricity use and more privacy with regard to their habits.

I encourage you to read the article for yourself before proceeding any further. Here is the link again, in case you missed it: http://web.mit.edu/mitei/news/energy-futures/Energy_Futures_Spring_2012.pdf

I am particularly excited about this ColorPower technology, which is now being developed by Zome Energy Networks. As the article mentions, it could revolutionize the ability of homeowners to participate in their home energy use - especially with larger appliances. However, I think that it could also be (1) a key mechanism for electric vehicle charging and (2) a supplement and/or tie-in to energy storage devices.

In my dissertation (see the Background section), I specifically sought to avoid discussing energy storage devices that only worked in one direction (e.g. could only use electricity but not send electricity back to the grid), because on the electricity grid demand can be both too high and too low compared to supply. This is especially the case on a utility grid that includes a high percentage of renewable energy sources (such as solar photovoltaic and wind turbine generators). Thus, I was concerned that if I based the premise of my dissertation only on a technology that only had the ability to absorb extra energy when demand was lower than supply, I would be missing half of the equation.

During instances in which the supply is unable to keep up with the demand, the ColorPower systems could be used as envisioned by the system designer and described in the article - i.e. temporarily turning off or cycling down appliances based on the user preferences. In a mathematical sense, reducing demand is similar to sending more electricity onto the grid (the first half of the equation). However, I now believe that with the use of ColorPower or a similar technology much (but not all) of the role of bi-directional energy storage devices could be met with demand response, especially if it is used in combination with electric vehicles and thermal power systems.

If the ColorPower systems included a bi-directional switch, owners could not only elect to use less energy when demand is too high but could also request to use more energy when demand is too low. This may not seem practical when only considering traditional appliances. Instead let's consider non-traditional appliances such as electric vehicles and thermal mass systems (e.g. SmartBricks from Vcharge). In this case, appliance owners could prioritize their energy use so that it jumps (e.g. electric vehicles charge quicker than normal) whenever overall demand is too low and supply is too high .

As a result, if each building had some appliances set up to shutdown when demand is too high and other appliances set up to ramp-up use when demand is too low, each building could function in essentially the same role that energy storage devices filled in my dissertation. This sort of bi-directional demand response could also avoid some of the trouble with V2G systems that require EV batteries to be less than fully charged in order to function.



Well, that is all for now, I hope that I have provide you with enough links to keep you busy for the weekend.

Cheers,

Sean