Nonintrusive Appliance Load Monitoring
This page is was created by George Hart
many years ago, and is now quite out of date. I am no longer
research in this area and can provide no more recent information,
but here are links to two recent conferences, which should lead you
to current information:
People to Contact
References and Related Links
A Nonintrusive Appliance Load Monitor (NALM) is designed to monitor
electrical circuit that contains a number of devices (appliances)
switch on and off independently. By a sophisticated analysis of the
and voltage waveforms of the total load, the NALM estimates the
and nature of the individual loads, their individual energy
and other relevant statistics such as time-of-day variations. No
to the individual components is necessary for installing sensors or
measurements. This can provide a very convenient and effective
gathering load data compared to traditional means of placing sensors
each of the individual components of the load. The resulting end-use
data is extremely valuable to consumers, energy auditors, utilities,
policy makers, and appliance manufacturers, for a broad range of
For example, a monitor placed outside a home can determine how much
goes into each of the major appliances within the home.
In a utility application, a NALM connects with the total load
the standard revenue meter socket interface, as shown in the
This permits very easy installation, removal, and maintenance
with traditional intrusive load monitoring techniques that require
and interior wiring. The NALM monitors the total load, checking
``signatures" which provide information about the activity of the
which constitute the load. For example, if the residence contains
which consumes 250 W and 200 VAR, then a step increase of that
size indicates that the refrigerator turned on, and a decrease of
size indicates the turn-off events. Other appliances have other
signatures. After determining the exact on and off times from the
events, any desired statistics, such as energy consumption vs.
day or temperature, can be tabulated.
To appreciate how this works, consider this figure, which plots
(real) power consumption vs. time for a single-family home over a
period. During this interval, the total load shows activity due to
and a heater. Two different-sized step changes are clearly
characteristic signatures of the refrigerator and the heater. The
cycles on and off three times, the heater six times. By measuring
load ouside the home, it is not difficult to find these step
measure their size. Knowing the time of each on and off event, the
energy consumption of the refrigerator and the heater are easily
By also considering measurements of the total reactive power or
current, along with the real power shown, changes in the resulting
function of time would reveal even more information about the
Traditional load research instrumentation involves complex
hardware but simple software. A monitoring point at each appliance
and wires (or sometimes power-line carrier techniques) connecting
to a central data-gathering location provide separate data paths,
software merely has to tabulate the data arriving over these
channels. The NALM approach reverses this balance, with simple
but complex software for signal processing and analysis. Only a
point in the circuit is instrumented, but mathematical algorithms
separate the measured load into separate components. In many
applications, this is a very cost-effective tradeoff, which is a
advantage of the NALM.
In order to accurately decompose the aggregate load into its
a model-based approach for describing individual appliances and
is used. These models suggest certain signatures which can be
in the total load to indicate the activities of the separate
This leads naturally to practical architectures and algorithms for
NALM. For full details, see the references
We have implemented these ideas and carried out a number of
tests on residential loads to compare the NALM to traditional load
techniques employed by electric utilities. Based on these tests, a
version of the NALM is being developed for widespread utility use.
My recent research has focused on appliances which can be understood
multistate devices, using finite-state machine (FSM) models. There
three classes of appliance models from the NALM perspective:
Appliances such as light bulbs or toasters, which are either on
at any given moment. Early research focused on techniques for
Appliances such as washing machines or dishwashers, with
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
they do not generate step changes in power.
To learn the FSM control structure of different multistate
we have developed the portable instrumentation illustrated here.
a new tool, which analyzes the behavior of an operating electrical
in a novel manner by automatically describing it with a
of its control structure. A personal-computer-based system
of real and reactive power consumption over time, and
the control structure of the load in real time, drawing its
diagram. The system also reports the load's state at each point in
the total time spent in each state, and total energy consumption
of the states.
One use of this tool is to provide a database of common appliance
structures for the NALM project. This research also has
behavioral analysis, energy monitoring, fault monitoring, fault
and power quality analysis of many types of electrical loads,
and power sources. Although only tested on residential loads and
appliances so far, the underlying methods should also work on
and industrial loads, e.g., HVAC control systems.
As an example, if a three-way lamp is operated, a plot of power
time shows plateaus at the low, medium, and high power levels. The
algorithm in the instrument detects these, and constructs the FSM
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
plots of real and reactive power, the six-state FSM shown is
The inner three states correspond to the light being off (the door
and the outer three occur when the light is on. The power plot shows
on/off cycles of the compressor are followed by a single defrost
in which the motor is off but a heater is on (so there is a large
power, but no reactive power). The FSM generated captures all this
For details of the algorithm, discussion, and many more examples,
my paper ``Automatic Construction of Finite-State Load Behavior
listed in the references below.
NALM Development Status
The Electric Power Research
(EPRI) has sponsored NALM research since its conception in the early
EPRI has chosen Telog Instruments to commercialize the NALM into a
tool available to electric utilities. A beta-test program of the
version of the NALM is underway, and units are expected to be
to electric utilities in 1997. For exact availability information,
830 Canning Parkway
Victor, NY 14564-8940
People to Contact
A number of people at a number of organizations are involved in
and development of NALM techniques.
George W. Hart
Originator and developer of the NALM. Originally at MIT, then at
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.
Mr. Malmendier is the product manager of Telog Instruments, Inc.
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
information in commercial buildings.
Jackie Lemmerhirt and Ralph Abbott
Plexus Research (in Acton, Massachusettes) is involved in
the electric utility community to the NALM development efforts.
is carrying out a project comparing NALM output with independent
in a number of test houses. This should provide solid data on
of the NALM. Mr. Abbott is president of Plexus.
A good introductory tutorial survey is the following article. (The
overview is excerpted from it.)
Hart, G.W., ``Nonintrusive Appliance Load Monitoring," Proceedings
the IEEE, December 1992, pp. 1870-1891.
And this earlier technical report is now available as a pdf.
A complete bibliography of published papers
concerning nonintrusive load monitoring is also available.
- Hart, G.W., Prototype Nonintrusive Appliance Load Monitor,
MIT Energy Laboratory Technical Report, and Electric Power
Research Institute Technical Report, September 1985, online copy.