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Generator Backup Sizing

Picking the right size generator (or UPS system or inverter) to meet your needs is not that complicated.   There are several electrical parameters you need to be concerned with to make a proper selection.  They are:

Additional information is provided in these sections:


For some sample measured data taken on actual electrical loads, see our starting characteristics 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 clamp-on probe type ac current meter is available.  If the building has a traditional style disk type kilowatt-hour 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 non-linear loads can alter this perfect sine wave.  A non-linear 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 non-linear.  Welding generators due to their design and poorly designed generators may also produce a distorted AC wave.  For one generator manufacturer's data click here.  When a significant portion of the load on a generator (or any power source for that matter) is non-linear, 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 misoperate.   A specific example is an uninterruptible power supply (UPS) system 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 non-linear 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 non-linear load portion:

     3 x 2000 W = 6000 W

The generator needs to be at least 6000 watts in accordance with this rule of thumb.

UPS manufacturers usually have specific guidelines for each type of UPS that they sell stating how much to oversize a standby generator.



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.

Noo-load.gif (12700 bytes)
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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.

  1. First locate the electric meter which feeds your building.  To use this method, it must be a traditional style kilowatthour meter with a rotating disk.  The meters shown here are typical for small to medium residential services.

Kwhm1.gif (6323 bytes)
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  1. Next, read the constant on the face of the nameplate shown as Kh.  This value is the number of watthours equivalent to one rotation of the disk.

Kwhm2.gif (5255 bytes)
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  1. Now, start the desired appliances, heating or air conditioning for the condition to be measured.

  2. 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.

  3. Finally, take the three values and use the equation below to calculate the watts seen by the electric meter.

    Meterf1.gif (1266 bytes)

    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:

    Meterf2.gif (1096 bytes)

    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 our Test Reports page.



Measuring Individual Appliance Loads

You can easily measure the electrical requirements for individual appliance loads using our one of the meters that we sell.  Some not only display the wattage used by the appliance but also give total energy consumption over a period of time, electricity cost, and many other parameters.  Click here for a list of these test instruments.