Instead
of using this precise method of calculating loads you can use values "off
of face plates" on equipment or the values shown on the
Wattage Guide.
If you have
no name plate data or the device is not listed on the Wattage Guide the
follow the instructions here to determine the full load or device load.
For calculating
full loads follow the procedures shown below. If you have an amp meter and
you know how to use it, you can run the load and use a meter to determine
the Running Load Requirements.
Picking the
right size generator to meet your needs is not that complicated. There are
several electrical parameters you need to be concerned with to make a proper
selection.
Additional
references:
Formulas
These formulas convert
data from one value to another.
For some
sample measured data taken on actual electrical loads, see our
Examples
page.
Voltage Requirements
Most appliances
operate on 120 volts. In spite of this, most electrical services for homes
and small businesses are made up of a dual 120 volt feed, more commonly
referred to as 120/240 volt. This type of feed allows the connection of
those larger 240 volt electrical loads such as kitchen ranges, clothes dryers,
well pumps, water heaters, heat pumps and central air conditioners. If you
ever expect to have to power any 240 volt load, you should select a generator
with 120 and 240 volt output. Also, if you intend on connecting the generator
to the home or building at the 120/240 volt electric panel you should select
a 120/240 volt model. Only in the case of separately powering individual
120 volt appliances should you consider a generator without 240 volt capability.
Running Load Requirements
Running
load requirements are simply a total of all the loads to be operated simultaneously.
This can be based on actual measurements if a clampon probe type ac current
meter is available. If the building has a traditional style disk type kilowatthour
meter an alternative is to use a stop watch and count rotations of the disk
when the loads of interest are operating. See the procedure outlined below.
Otherwise, an estimate of the total running load can be made by adding up
the running wattages of all items to be powered at one time. Some appliances
may not list the wattage on the nameplate, but may show the rated current
in amps and voltage. The wattage can estimated from these two figures
by multiplying them together to get the watts. See formula below:
watts = volts
x amps
(for reactive loads this
actually over estimates the watts slightly)
An estimate can also be obtained
by using the bar chart further below.
Starting Load Requirements
Determining
the starting requirements can be a bit more complicated. Certain electrical
devices require additional power and current when initially turned on.
This is true for motors because the rotor of the motor and the shaft driven
load (fan, pump, compressor, saw, etc.) is initially at a standstill. It
requires more energy to accelerate these rotating parts to operating speed
than it does to keep them rotating. Therefore, during the period of acceleration,
the demand on the power supply is greater. To precisely evaluate the motor
starting capability the detailed motor characteristics need to be known.
However, a rule of thumb is usually sufficient. Most engine driven generators
will start a motor with up to 1/5th the horsepower of the engine, if it
is the first load connected. For example, a 2500 watt generator driven by
a 5 horsepower engine will usually start up to a 1 horsepower motor. This
assumes a common type of motor design with NEMA code G starting characteristics.
This data can be found on the motor nameplate.
Power Quality & Distortion
Perfectly
pure AC power is a sine wave for both the voltage and current. Resistive
loads such as incandescent lights and heaters are linear loads since the
current is always proportional to the voltage applied. Some types of generators
and nonlinear loads can alter this perfect sine wave. A nonlinear electrical
load does not have a linear relationship between the voltage applied and
the current that flows into the load. Certain types of electronics, lighting
ballasts, arc welders and other devices are nonlinear. Welding generators
due to their design and poorly designed generators may also produce a distorted
AC wave. When a significant portion of the load on a generator (or any power
source for that matter) is nonlinear, all the loads fed by the source will
see this distortion. A measure of this distortion is called THD, or total
harmonic distortion. If the distortion is severe enough, motors and transformers
will operate hotter. Over a long period of time this can cause a reduction
in life. And some other sensitive electronic equipment may not operate.
An specific example is uninterruptible power supply (UPS) systems powering
computers or communications equipment. These types of devices cause some
distortion of the AC wave and at the same time can be negatively affected
by it. A UPS system powered by an inadequately sized backup generator may
continue draining the internal battery rather than switching over to generator
power and charging the battery.
To reduce chances for THD problems, the rule of thumb is to select a backup
generator kW size at least three times the kW of nonlinear loads to be
powered. For example, if you have 2000 watts of computers fed by UPS systems
and 1000 watts of incandescent lighting to be fed by a generator first total
the power:
2000 W + 1000 W = 3000 W
Then compare the total with three times the nonlinear load portion:
3 x 2000 W = 6000 W
The generator needs to be at least 6000 watts in accordance with this rule
of thumb.
Typical Running and Starting Loads
The graph below shows
typical values for common residential loads. For applications that are approaching
generator ratings, the actual nameplate load data, or better yet measured
data, should be used to ensure an adequately sized unit.
Measuring Building Electrical Load Using
a Stopwatch
If you intend
on powering most of the items fed by your utility electric meter you can
measure your total building load at any time using simply a stop watch while
observing your meter. Follow the steps below to make this measurement.

Now, start
the desired appliances, heating or air conditioning for the condition
to be measured.

Using a
stopwatch while watching for the black mark on the meter's disk, measure
the time it takes for one or more disk rotations. If the disk is rotating
rapidly, better accuracy will be attained if you time more than one
rotation.

Finally,
take the three values and use the equation below to calculate the watts
seen by the electric meter.
Here is an example
calculation for the first meter above. From the meter's face, Kh = 7.2.
The time measured for 5 rotations of the disk was 24 seconds. Thus, Rev
= 5 and T = 24 seconds. Solving for the electrical demand we have:
This is within
the capability of common portable generators.
Note that this
method is not practical for measuring the peak inrush watts needed to start
most loads because the transient happens so fast. Typical acceleration times
for most motor loads are less than 1 second. Unless the response time of
your eye and your thumb is extremely fast, you won't be able to measure
these starting wattages without special recording instrumentation. For a
sample of these measurements, see Test Reports below. .
Test Reports
This section contains test
results and measurements for various electrical loads. Note that these measurements
are representative only for the specific models shown and under the particular
test conditions. Different conditions can alter both the current values
and starting times.
Slightly different line voltages
will affect results as will the wire size and length feeding the load. Ambient
temperatures and thermostat settings will affect refrigeration compressor
current demands. For well pumps, the pressure settings and the depth that
the pump is positioned will affect measured results.
Additional test
measurements are being added to this page on a regular basis. If you have
some tests results that would be helpful to others, send them to us for
posting here.
Refrigerator Starting
Current
This test measured
the starting current for a refrigerator with the following nameplate data.
Voltage 
115 V AC 
Frequency 
60 Hz 
Amps 
5.0 
Mfr. Date 
3 / 86 
Results of one measurement
are shown in the plot below. It shows a maximum inrush current of about
13 amps which lasts only about onehalf second. Also, the running current
is significantly less than the nameplate value. It should be noted that
most refrigerators (including this one) are "frost free." This means that
on a regular basis a timer shuts off the compressor and turns on resistance
heaters to clear frost buildup in the freezer section. This defrost current
was not measured and is probably greater than the compressor running current.
This may explain the large difference between the nameplate current value
and the measured value.
Freezer Starting Current
This test measured
the starting current for a freezer with the following nameplate data.
Voltage 
115 V AC 
Frequency 
60 Hz 
Amps 
5.0 
Mfr. Date 
early to mid 1970s 
Results of one measurement
are shown in the plot below. It shows a maximum inrush current of less than
5 amps which lasts only about 0.3 seconds. Also, the running current is
significantly less than the nameplate value. It should be noted that most
freezers are "frost free." However, this one is not.
Well Pump Starting Current
This test measured
the starting current for a well pump motor. The motor is rated 3/4 hp and
240V. Results of one measurement are shown in the plot below. It shows a
maximum inrush current of about 18 amps which lasts only about 0.2 seconds.
