Generator Backup Sizing
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Additional information is provided in these sections:
For some sample measured data taken on actual electrical loads, see our starting characteristics page.
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 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:
An estimate can also be obtained by using the bar chart further below.
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.
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. 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.
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.
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 using Watts Up? Meter You can easily measure the electrical requirements for individual appliance loads using our Watts Up? meter. This device not only displays the wattage used by the appliance but also gives total energy consumption over a period of time, electricity cost, and many other parameters. Simply plug the appliance into the outlet on the face of the meter to read the measurements. Click here for further details. |
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