| This page explains how to
determine loads so that you can
size your generator for your needs. Normally you want to power
the "essential" things and leave some room for other uses.
You must always remember that
the generator cannot run at full load for long periods. Always assume
that you will need more items powered rather then less and leave a margin
of safety above and beyond the continuous load of the generator you select.
It is less expensive to buy
more power (higher wattage) in one unit then to buy a second unit or "upgrade"
by trading in your old generator for a new one. Often installation costs
for a larger unit is a fraction more initially but if you have to upsize
your connection afterwards its very expensive.
All these require common
sense judgment and some experience. Ask your electrician for advise, he
can help you decide what you need.
Below is a short tutorial
which will help you understand the basics. The
Wattage Guide
will provide you with values for the most common items at home and on the
job. If you can get nameplate data your calculations will be more accurate,
however using the Guide will be sufficient in most cases, especially if
you leave yourself plenty of growing room. Make sure you have plenty of
size by getting generator
big enough for now and the future.
Go to
Calculations
to use your new understanding and apply the numbers you gathered. You should
be able to determine the right size generator for you.
GENERATOR
USAGE
Generators are used to perform a wide variety
of chores. The wide variety of generators meet the demands of the variety
of almost all potential users. Generators offered by GeneratorJoe provide
a high quality power source that is reliable and convenient to use.
GENERATOR POWER
Most generators produce
AC voltage, very similar to the voltage available in your home.
The amount of power
that a generator can produce is rated in watts (power).
For Example, an EM2500
generator produces a
MAXIMUM
2500
watts of power. This means the EM2500 could provide power to 25
one hundred watt light bulbs at the same time. The generator would
then be at its
MAXIMUM
power output.
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Generator Power
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Model
= Power |
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Power
= Watts |
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MAXIMUM AND RATED POWER
A generator should
never be operated at its
MAXIMUM
power output for more than 30 minutes.
RATED
power is a more reliable measure of generator power. It is
the power that a generator can produce for long periods of time.
Typically the
RATED
power is 90% of the
MAXIMUM
power.
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Rated and Maximum
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Model
number = maximum
.
output in watts. |
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Maximum
power for 1/2 hour. |
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Rated
is usually 10% less. |
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LOADS
In the previous example, the light bulbs are the
LOAD
of the generator. The EM2500 generator can handle a
LOAD
of no more than 2500 watts maximum.
The light bulb example
is called a
RESISTIVE
type load and the POWER it requires is pretty easy to understand.
Other RESISTIVE
types of LOAD
are things like toasters, convection ovens, hot plates, curling
irons, coffee makers, stereos and TV's.
RESISTIVE
LOADS
are usually those that do not have electric motors.
Another load is the
REACTIVE
type and is a little more confusing. Typically, a
REACTIVE
load contains an electric motor. This type of load may require up
to three times as much power (wattage) to
START
as it does to keep it running. Examples of
REACTIVE
type loads are air conditioners, refrigerators / freezers, furnace
fans, well pumps, bench grinders and air compressors.
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Loads
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Resistive |
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Reactive |
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RESISTIVE LOADS
The equation shows
the relationship between watts, volts and amps in a
PURELY RESISTIVE
load. If you know any of the two variables, the third can be calculated.
Example: You want
a generator to power a 1000 watt flood light. The light is 120V
and requires 1000 watts of power. Using the equation, we can calculate
that the floodlight will draw 8.3 amps of electrical current.
For
REACTIVE
loads, the equation shows only a general relationship between watts,
volts and amps. That's because the power requirements for
REACTIVE
loads changes with operating conditions.
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Resistive Loads
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Watts = Volts x Amps |
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REACTIVE LOADS
When determining the proper generator
for REACTIVE type loads, you must consider three modes of operation:
STARTING - The
electric motor requires more power to start. The starting power
required can
be THREE times the running amount.
RUNNING -
The power required to
run the electric motor after it has been started.
LOADED -
When the electric motor
begins to work (saw begins cutting wood), its power requirement
will increase. This is not applicable for most household appliances.
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Reactive Loads
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LOAD POWER REQUIREMENTS
Here are a few ways
to determine power requirements for various loads that are expected
to be powered by a typical generator.
Method 1, using estimating
charts, can be used to get a general idea of the generator size.
Method 2, reading
the motor data tag, is more accurate since the data tag information
is provided by the motor manufacturer. Data tag information does
not always show
STARTING
power requirements for
REACTIVE
type loads. See "CODE
CHART"
at the end of this page.
*Note: Data tag information
can usually be found in the owners / operating manual as well.
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Determine Load
Requirements
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Estimating
Chart
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Read
data tags |
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ESTIMATING CHART
Method 1 requires
using the load
wattage
estimating
chart. It gives two figures
of watts needed to power various loads; Running and starting watts.
Add up all of the
watts estimated for the loads / appliances / tools that you want
to operate at the same time.
If the load is a
reactive type, use the STARTING WATTS for your estimation.
Select a generator
that is as large or larger than the total number estimated. Example:
If you only want to run a refrigerator (2200 starting watts) and
(2) 100 watt light bulbs (200 watts) - You would need a 2500 watt
generator.
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Wattage Estimating Chart
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What are the loads? |
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Add up the total wattage of loads |
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Use the starting watts if the load is a reactive type |
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Select a generator |
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DATA TAG
Method 2 requires
a visual inspection of the data tag supplied by the electric motor
manufacturer. All electrical motors have a data tag attached to
their bodies that give volts, amps, phase, cycles, hp, and sometimes
a code.
Volts (V) - The volts
must be either 120 (110-120) or 120/240. 120/240 means that the
motor can be wired to operate on 120V or 240V. Typical generators
are either 120V or 120/240V.
Amps (A) - Indicates
the amps required to RUN the electric motor but
doesn't consider STARTING or LOADED power requirements.
Phase (PH) - The
most commonly used generators can power only single phase motors
only.
Horsepower (HP) -
Rating of how much work an electric motor can perform.
Code - This isn't
always supplied on the data tag. It represents the maximum STARTING
power required of the electric motor.
Cycles (Hz) - All
of U.S. electric appliances run at 60 cycles per second.
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Read the Motor
Mfg. Tag
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Volts |
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Amps |
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Phase |
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Hp |
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Code |
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Cycles |
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