∫∫ The Right Way to do Measurement and Verification (M&V)

With the release of the latest McKinsey report on Energy Efficiency [1], the Federal Government, the Non-Profit world, and the Private sector have come together in promoting energy efficiency as a viable means of fighting climate change and increasing corporate (and household) profits simultaneously. But unlike solar electricity, which can be metered and hence easily measured and verified, electricity saved through energy efficiency is more difficult to track.

Before investing in any major efficiency upgrade, corporations should ask exactly how the proposed savings will be Measured and Verified after the installation is complete. Why? Replacing the central air conditioning system of a 50-floor high-rise might cost $4 million dollars. This could be a good investment if it saves $800,000 per year for the next 25 years, but what if it only saves $400,000? $200,000? For efficiency (and really all renewable energy issues), the devil is in the details. Luckily, the details of accurately measuring efficiency really are not all that complicated.

Here’s how efficiency projects can be reliably measured and verified. (These are not my ideas, by the way, but are common practice by the best efficiency firms and regulatory agencies throughout the country.)


Lighting is the easiest type of efficiency project to verify. Most commercial and manufacturing spaces are full of fluorescent tubes, so I will use fluorescent tubes as an example, (but this method works for any lighting replacement).

The first thing to know about most commercial/industrial lights is that the wattage on the lamp (what most people call a bulb), e.g. 55 watts, is NOT the wattage that is used by the bulb. Fluorescent (and most other) lamps are driven by a “ballast,” a device that sends electricity into the lamp at the exact right voltage and frequency. Ballasts have a “ballast factor,” which allows the lamp to be over or under-driven, drastically changing the amount of electricity the lamp draws. Think of this as a permanently set dimmer switch. To get the actual watts used by the lamp, you simply multiply the lamp wattage by the ballast factor. (Technically, the ballast factor represents the percent of rated light you get out, not the percent of watts actually input, but the relationship in this case is linear so multiplying lamp wattages by ballast factors gets you accurate to within half a watt.) Ballast factors are usually 0.78, 0.88, 1, or 1.2. To figure out your watt savings, multiply the wattage and ballast factors for your old lamps and new lamps, subtract the two sets, and you have your watt savings.

After you have figured out your watt savings, divide by 1,000 to get kW, and then multiply by number of hours you will run the lights each year to get annual kWh savings. Voila. No complicated “measurement” required (except to find out the ballast factor and wattage of the lights you are replacing).

Example with numbers:

You are replacing a 55-watt lamp with a 32-watt lamp. A novice (or sleazy) contractor would tell you the power savings will be 55 – 32 = 23 watts, but reality (what you will be billed on) could be quite different. Go physically look at the old ballasts (your facilities manager will have an extra one lying around, or if not he can pull one out of the ceiling for you in five minutes). Say the old ballast factor was .78, but the new ballast factor is 1. This means the power savings will actually be (55 * 0.78) – (32 * 1) = 10.9 watts, less than half what you would expect! This means this lighting project will take more than twice as long to pay for itself than expected. You will also get substantially more light than you previously had (higher ballast factors mean more light as well as more energy use).

What if you are using a lamp without a ballast (like a Sodium Sulfur lamp)? Well, in the case of a sodium sulfur lamp it actually does have a ballast, it is just called an RF Power Supply, and it has a ballast factor equivalent. If you are using something like an incandescent that really has no ballast, then you can just look at the wattage. But for Pete’s sake stop using incandescents.

If you are really a stickler, ballasts act slightly differently if they are powering one, two, three, or four lamps, so you should look at the “system efficiency” of the new system on the cut sheets. Cut sheets are technical specifications the manufacturer will give you, and you can look up the total wattage draw of lamp/ballast combinations there. System efficiency usually only differs by one or two watts from just multiplying the lamp wattage by the ballast factor.

Final word: do not ever just change lamps, change both lamps and ballasts. A good portion of the energy savings come from replacing your old magnetic (the humming, flickering) ballasts with new quiet, non-flickering electronic ballasts.

Motion Sensors:

Motion sensors are where measurement and verification starts to get fun and require a little equipment. Unlike a lamp/ballast replacement where the energy savings can be calibrated and calculated, motion sensor savings cannot be predicted but must be directly measured.

A current transformer (CT Clip for short) is a ring you can put around a wire that measures the current going through that wire. When hooked up to a power meter that simultaneously measures voltage and power factor, CT clips allow you to tell the power being used by any wire or circuit. Power meters can be easily attached to a recording device that logs their measurements (a logger), and I will now refer to this setup as a “power-logger.” Power-logging is a way to directly meter energy savings from efficiency, much akin to directly metering the output of a solar array.

Back to motion sensors. Ideally, your lights will be on a dedicated circuit (in non-technical speak, this means the wires that connect all your lights are separate from the wires that connect all your computers, etc.). If this is the case, you can simply clamp a power-logger onto your lighting circuit and wait a few weeks.

The power-logger will give you a graph that looks something like this:

Unless those weeks were unrepresentative (e.g. you measured during Christmas), you should be able to figure out the annual hours of operation of your lights on motion sensors. Simply compare this to the annual run hours before you installed motion sensors, and you have your energy savings.

If your lights are not on their own dedicated circuit, measurement and verification is a little tougher but still not too difficult. Put power-loggers on a few representative light fixtures (one in the cubicle area, a couple in private offices, etc.), and do the same thing. You will get a couple different graphs and can back out what all the fixtures’ new run hours will be.

For private offices, expect savings of as much as 25%. For cubicle areas, expect only 10% reductions. [2]

Elevator Motors (or putting Variable Frequency Drivess on any motor):

The Measurement & Verification method is similar to lighting, even though the savings are not from decreased operating hours but are from more efficient mechanical and electrical equipment. The main difference in measurement and verification is that you need to measure usage both after AND before you change your motors or add Variable Frequency Drives too them.

Measure the energy use of your elevator motor circuit (or a couple individual elevator motors if your building does not have a dedicated circuit) with power-loggers for a few weeks. Then, do the motor change, and measure the energy use of the new motors for a few weeks after the upgrade. You can then compare the new motor usage with old and get annualized energy savings.

In my experience, average savings for elevator motors average 0.2 kW per Horsepower of elevator replaced, but the standard deviation was 0.12 kW. In other words, the variation in energy (and hence money) savings is so high with elevator motors that you have to directly measure the equipment before and after the upgrade to have any idea what your elevator motor replacement is saving you.

If you are putting a Variable Frequency Drive (VFD) or Variable Speed Drive (VSD, basically the same thing as a VFD) on your motor, this could save you energy too. It is difficult, although by no means impossible, to predict the energy savings ahead of time, but you need to know some details about your system ahead of time. I will not go into too much detail here, but if you are able to measure the flow of fluid through your system, be it water or air, you can usually figure out savings using either pump or fan curves, motor drive curves, or fan and pump affinity laws. The picture to the right shows how this process starts, but when in doubt (or when in doubt of your engineering firm), make sure to measure the energy use of the motor before the retrofit for a few weeks, and then again after the retrofit for a few weeks with a power-logger. I will save the technical details of how a VFD works for another time, but for now I will just say that if you have any motors with frequently fluctuating loads, VFDs can be an amazing investment. If the load on a motor is constant and the motor is throttled, you can also save energy by using a VFD.


Chillers make the cold water that is used to provide central air conditioning in most large office buildings. Chillers are large pieces of equipment, and measuring the energy savings associated with replacing them can be difficult. Like a car engine, chillers are more or less efficient depending on how hard they are working. The “load” of a chiller is measured in “tons” (one “ton” means the chiller is removing 12,000 btu/hour of heat from the coolant being fed into it), and how much energy it is using is, of course, measured in kW.

Chillers have an efficiency curve, which looks like this:

Source: www.energy.rochester.edu/efficiency/chilleranalysis.pdf

You will note this chiller is most efficient when it is fully loaded. This has several implications which I will talk about in future blog posts, but for now just note that over-sizing your chillers and running them partially loaded will cost you boatloads of money unless you put what is called a VFD on them. (The engineers and chiller manufacturers will know what this means.)

To determine how much energy your chiller replacement is saving, you need to know two things: how much cold water the chillers produced (gallons and temperature), and how many kWh of electricity they took to produce it.

If you are lucky, your facilities engineers will have kept “chiller logs,” where they wrote down the output of the chiller in tons, gallons of chilled water, or some other metric allowing you (or us) to calculate the load on the chiller year round. Then, you can simply measure (again with power-loggers) the efficiency of the chiller at several different loads and calculate how much energy it used per year.

You can then couple the old chillers’ load curve with your new chillers’ efficiency curve to calculate how much energy your new chiller will use and thus determine the energy savings.

If your facilities manager did not keep a chiller log (this is common), then if you really want to figure out what your savings will be, you have to wait till summer and then power-log and load-log your chiller the entire summer before installing any new chillers.

What if you already did the chiller replacement and have no logs? Then it is time for utility bill data. A clever engineer can usually back out something as large as air-conditioning from your utility bills, but this is non-ideal. For one, many factors go into your utility bill, so if you changed tenants, added a data-server to your building, etc., it can be very difficult to determine the energy use of a single piece of equipment. A good statistician is your best bet in this case, but it is really better to measure things before changing equipment.

Windows/Envelope Changes:

Energy savings from window and/or envelope changes are the hardest to measure by far. Multiple models (UAdT, Radiant Time Series, eQuest) can be used to predict the savings garnered from these changes, but verifying them can be difficult, mostly because people rarely do a windows change without doing other efficiency projects at the same time.

Your best bet for actually Measuring and Verifying windows replacements is to get as much utility data (ideally three years, two is okay, one may make things difficult) from the building before the retrofit takes place. Then, use the above methods to precisely measure the energy savings from all the other changes to the building.

Now, wait a few months until you have new utility data and let your statistician out of the basement. Using regression techniques, (specifically, a logit model with panel data for the month, Heating Degree Days, Cooling Degree Days, a Boolean for whether or not the retrofit has taken place, and a variable containing the energy savings from all the other measures), you should be able to back out the savings from just the windows alone.

Again, this is non-ideal. If your building has major tenant changes, is operating the chiller or lights differently than before the retrofit, or makes any other number of behavioral changes, it will be very difficult to calculate the savings from windows or shell improvements.

Thankfully, models like eQuest are very good these days, and if they are relatively close (say, 10%) to your statistically calculated savings, you can be confident that your window replacements are doing close to what you expected. It is best to use a couple models to verify that the measured bill data savings and models are all telling the same story.

[1] http://www.mckinsey.com/clientservice/electricpowernaturalgas/US_energy_efficiency/

[2] https://www1.eere.energy.gov/femp/pdfs/light_controls.pdf

24 Responses to ∫∫ The Right Way to do Measurement and Verification (M&V)

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  7. In your article “The right way to do measurements and verification” is the calculation correct?

    You are replacing a 55-watt lamp with a 32-watt lamp. A novice (or sleazy) contractor would tell you the power savings will be 55 – 32 = 23 watts, but reality (what you will be billed on) could be quite different. Go physically look at the old ballasts (your facilities manager will have an extra one lying around, or if not he can pull one out of the ceiling for you in five minutes). Say the old ballast factor was .78, but the new ballast factor is 1. This means the power savings will actually be (55 * 0.78) – (32 * 1) = 10.9 watts,
    Shouldn’t it be (55*(1/.78)) – (32*(1/1)) = 46?

    • Hi Brian,

      Yes, the calculation is correct. You multiply the Ballast Factor by the rated wattage to find out the actual watts used, not divide.

      In your calculation you’re dividing; the easy way to tell dividing doesn’t work is to look at the savings your calculation yields. You’re showing 46 watts of savings from replacing a 55watt fixture. This would indicate the new fixture is only using 9 watts, and unfortunately, two lamps that are four feet long each use well more than 9 watts.



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