Nonintrusive Appliance Load Monitoring
This page is was created by George W. Hart
some 5 years ago, and is now fairly out of date. I am no longer doing
research in this area and can provide no more recent information.
People to Contact
References and Related Links
A Nonintrusive Appliance Load Monitor (NALM) is designed to monitor an
electrical circuit that contains a number of devices (appliances) which
switch on and off independently. By a sophisticated analysis of the current
and voltage waveforms of the total load, the NALM estimates the number
and nature of the individual loads, their individual energy consumption,
and other relevant statistics such as time-of-day variations. No access
to the individual components is necessary for installing sensors or making
measurements. This can provide a very convenient and effective method of
gathering load data compared to traditional means of placing sensors on
each of the individual components of the load. The resulting end-use load
data is extremely valuable to consumers, energy auditors, utilities, public
policy makers, and appliance manufacturers, for a broad range of purposes.
For example, a monitor placed outside a home can determine how much energy
goes into each of the major appliances within the home.
In a utility application, a NALM connects with the total load using
the standard revenue meter socket interface, as shown in the figure above.
This permits very easy installation, removal, and maintenance compared
with traditional intrusive load monitoring techniques that require ``submetering''
and interior wiring. The NALM monitors the total load, checking for certain
``signatures" which provide information about the activity of the appliances
which constitute the load. For example, if the residence contains a refrigerator
which consumes 250 W and 200 VAR, then a step increase of that characteristic
size indicates that the refrigerator turned on, and a decrease of that
size indicates the turn-off events. Other appliances have other characteristic
signatures. After determining the exact on and off times from the signature
events, any desired statistics, such as energy consumption vs. time of
day or temperature, can be tabulated.
To appreciate how this works, consider this figure, which plots total
(real) power consumption vs. time for a single-family home over a two-hour
period. During this interval, the total load shows activity due to a refrigerator
and a heater. Two different-sized step changes are clearly present, providing
characteristic signatures of the refrigerator and the heater. The refrigerator
cycles on and off three times, the heater six times. By measuring the total
load ouside the home, it is not difficult to find these step changes and
measure their size. Knowing the time of each on and off event, the total
energy consumption of the refrigerator and the heater are easily determined.
By also considering measurements of the total reactive power or harmonic
current, along with the real power shown, changes in the resulting vector
function of time would reveal even more information about the particular
Traditional load research instrumentation involves complex data-gathering
hardware but simple software. A monitoring point at each appliance of interest
and wires (or sometimes power-line carrier techniques) connecting each
to a central data-gathering location provide separate data paths, so the
software merely has to tabulate the data arriving over these separate hardware
channels. The NALM approach reverses this balance, with simple hardware
but complex software for signal processing and analysis. Only a single
point in the circuit is instrumented, but mathematical algorithms must
separate the measured load into separate components. In many load-monitoring
applications, this is a very cost-effective tradeoff, which is a major
advantage of the NALM.
In order to accurately decompose the aggregate load into its components,
a model-based approach for describing individual appliances and their combination
is used. These models suggest certain signatures which can be detected
in the total load to indicate the activities of the separate components.
This leads naturally to practical architectures and algorithms for the
NALM. For full details, see the references below.
We have implemented these ideas and carried out a number of initial field
tests on residential loads to compare the NALM to traditional load monitoring
techniques employed by electric utilities. Based on these tests, a commercial
version of the NALM is being developed for widespread utility use.
My recent research has focused on appliances which can be understood as
multistate devices, using finite-state machine (FSM) models. There are
three classes of appliance models from the NALM perspective:
Appliances such as light bulbs or toasters, which are either on or off
at any given moment. Early research focused on techniques for monitoring
Appliances such as washing machines or dishwashers, with distinct types
of ON states, e.g., fill, rinse, spin, pump, etc. Recent research has extended
the methods to apply to the multi-state case.
Appliances like light dimmers and variable-speed hand tools, with a continuous
range of ON states. These are difficult to monitor nonintrusively, because
they do not generate step changes in power.
To learn the FSM control structure of different multistate appliances,
we have developed the portable instrumentation illustrated here. This is
a new tool, which analyzes the behavior of an operating electrical load
in a novel manner by automatically describing it with a finite-state model
of its control structure. A personal-computer-based system collects samples
of real and reactive power consumption over time, and automatically learns
the control structure of the load in real time, drawing its finite-state
diagram. The system also reports the load's state at each point in time,
the total time spent in each state, and total energy consumption for each
of the states.
One use of this tool is to provide a database of common appliance FSM
structures for the NALM project. This research also has applications to
behavioral analysis, energy monitoring, fault monitoring, fault analysis,
and power quality analysis of many types of electrical loads, controllers,
and power sources. Although only tested on residential loads and consumer
appliances so far, the underlying methods should also work on commercial
and industrial loads, e.g., HVAC control systems.
As an example, if a three-way lamp is operated, a plot of power versus
time shows plateaus at the low, medium, and high power levels. The patern-recognition
algorithm in the instrument detects these, and constructs the FSM disaram
shown, illustrating how the four states are cyclically connected.
Frost-free Refrigerator with Interior Lamp
A more complex example is this frost-free refrigerator. From the measured
plots of real and reactive power, the six-state FSM shown is generated.
The inner three states correspond to the light being off (the door closed)
and the outer three occur when the light is on. The power plot shows how
on/off cycles of the compressor are followed by a single defrost cycle
in which the motor is off but a heater is on (so there is a large real
power, but no reactive power). The FSM generated captures all this behavior
For details of the algorithm, discussion, and many more examples, see
my paper ``Automatic Construction of Finite-State Load Behavior Models,"
listed in the references below.
NALM Development Status
The Electric Power Research Institute
(EPRI) has sponsored NALM research since its conception in the early 1980's.
EPRI has chosen Telog Instruments to commercialize the NALM into a research
tool available to electric utilities. A beta-test program of the commercial
version of the NALM is underway, and units are expected to be available
to electric utilities in 1997. For exact availability information, contact:
830 Canning Parkway
Victor, NY 14564-8940
People to Contact
A number of people at a number of organizations are involved in research
and development of NALM techniques.
George W. Hart
Originator and developer of the NALM. Originally at MIT, then at Columbia
University, I was briefly at Hofstra University. Contact information and additional
information regarding my research is available on my
Mr. Carmichael is the project supervisor at the Electric
Power Research Institute (EPRI) in charge of the NALM. (415) 855-7982
Mr. Malmendier is the product manager of Telog Instruments, Inc. in Rochester,
New York. (716) 742-3000, email: TelogSales@telog.com
Leslie Norford and Steven Leeb
Profs. Norford and Leeb are engaged in research at MIT
to explore the possibilities of extending NALM techniques to transient
information in commercial buildings.
Jackie Lemmerhirt and Ralph Abbott
Plexus Research (in Acton, Massachusettes) is involved in coordinating
the electric utility community to the NALM development efforts. Ms. Lemmerhirt
is carrying out a project comparing NALM output with independent instrumentation
in a number of test houses. This should provide solid data on the accuracy
of the NALM. Mr. Abbott is president of Plexus.
A good introductory tutorial survey is the following article. (The above
overview is excerpted from it.)
A complete bibliography of published papers
concerning nonintrusive load monitoring is also available.
Hart, G.W., ``Nonintrusive Appliance Load Monitoring," Proceedings of
the IEEE, December 1992, pp. 1870-1891.