THE GENERATOR SET SIZING GUIDE
HOW MUCH STANDBY POWER DO I NEED?
The single most important thing to determine when choosing backup power
system for your home or business is how much electrical power you will
need in emergency. There are three basic sizing requirements: continuous
(running) power, peak (startup) power and surge current capabilities.
There are plenty of generator buying guides and load calculators on the
web. However, in my view, most of them are either useless or misleading.
For example, many reference guides just give you wattage charts for
various categories of appliances. Since within each category actual
wattage of different models often varies within 2:1 range, this info may
be good only for ball park estimates, but not for real sizing.
Other guides tell you to find the rated
power of your devices from their
name plates and then to add up the watts of all your devices. The problem
is that with such method you will wind up with an
significantly oversized generator: the nameplate power (or current)
rating is just the worst-case number required by UL.
In almost all cases, it is well
above the actual operating level, often
by up to 30%. In addition to this, compressors of many appliances (such
as refrigerators, a/c, etc.) are continuously cycling on and off. It is
highly unlikely that all of them would
operate at the same time.
The U.S. National Electrical Code (NEC) recognizes this fact and allows electrical system designers to use
a diversity factor (0.67 or so), anticipating that not
all devices are running at full load all of
DETERMINING RUNNING WATTS.
In my view the best way to determine your required power is to actually
measure it. First, decide what lights and equipment you will need to
operate simultaneously during power outages. Then have your electrician
measure your power consumption when all these loads are running. If you
are of a legal age, have a proper electrical training and know how to
safely work with electricity, you can measure it by yourself: all you
will need is a hand-held clamp-on amp meter or a power analyzer with
"Peak" function to capture inrush currents.
To do the measurement, remove front
panel of the main breaker box and enclose the power line wire within
the clampmeter device. If you have a typical single-phase 220
volt service, take current reading on each power line
For a 2-wire single-phase service, such
as 220-230VAC , measure the current in one AC wire and multiply it by the
working voltage. Make several
measurements with 10-minute intervals and take the largest reading.
Finally, add 10 to 20 percent safety margin to the obtained wattage
value for system derating and to prevent false tripping of your
generator circuit breaker: this will be your required continuous power
Again, do not attempt to do these measurements by
yourself unless you have a proper electrical qualifications: you
are dealing with lethal voltage levels.
Note that single phase residential and commercial generator sets are
normally rated for the loads with power factor PF=1.0. This means that
not only your total load's wattage, but also total volt-amps can't
exceed genset's rated wattage even if active (average) power of your
load is less. Also note that if for a cost
reason you bought a generator that can't power your entire
home,you can rotate usage among essential appliances as you need to.
For example, run the refrigerator for an hour, then run the freezer,
then run the water pump, etc.
HOW TO SIZE A HOME GENERATOR TO START an A/C OR
Motor driven appliances (such as refrigerator, furnace fan, air
conditioner) require larger amounts of current for initial start-up than
when they are running. This is because induction motors initially act
like a short-circuited transformer. When motors start, they draw a
current surge causing a voltage dip of the generator. The maximum start
up current is referred to as "Locked Rotor Amps" (LRA). The current of a
motor will drop significantly when it accelerates to about 80% full
speed. The LRA of a typical motor is 5 to 7.5 times its continuous
operating current. Note that does not equate to 5 to 7.5 times power.
The power factor of a starting motor is low (<0.5). Depending on the
load type, the starting real power could be below the generator rated
power even though the motor starting current may be well above the
generator's rated current. That's why when sizing home system for
starting requirements, you must consider not just starting kW, but also
the current surge capability.
Although LRA of a motor is often indicated on its nameplate, the most
accurate way to determine its starting current is to likewise actually
measure it. Wear line-worker's rubber gloves, remove front panel of your
distribution box, set amp meter to Peak (surge) reading and enclose the
wire that feeds your load within the clamp-on device. Then have someone
turn on the appliance and take the surge current reading. If you have
several motor driven loads, find the device with the greatest difference
between surge and running current. Add that difference to the total
current of all appliances assuming multiple devices rarely start up at
exactly the same time. This will give you surge current requirements of
your backup system. If you are looking for a genset that supplements
your solar system, you need to take into account a recommended charging
current of the storage battery bank.
Due to low power factor, starting power may not be an issue (high surge
power often shown in various sizing guides are actually starting
volt-amps rather then real power). The main issue is usually with the
starting current. For reference, average or real power is volt-amps
times power factor: P=V×I×PF, where PF is always less then 1.
Generator manufacturers often specify their models' surge wattage
capability, but unfortunately, they rarely state LRA capability. The
chart below shows surge current capability of Generac's single phase
standby generators at 30% voltage dip based on their Sizing Guide
If you plan to start several motors simultaneously, you'll need a
generator with the capacity to provide the necessary total starting
current for these loads. Without sufficient starting current, motors can
overheat, burn out, or trip the genset's circuit breaker. Alternatively,
you may set your backup system to manual mode, and in emergency first
turn on the a/c and then all other loads sequentially.
||(To operate appliance)
||( To start appliance)
|Air Conditioner (Evaporative model)
|Air Conditioner (Reverse cycle)
|Domestic Water Pumps
|Kettle or jug
DOWNLOAD YOUR FREE HERCULES KVA CALCULATOR HERE ( Requires Microsoft Excel )
Inductive loads (typically electric motors, electric pumps, electric compressors and air conditioners) require
6-8 times more current to start than to run. Most generators can provide 100% overload for start up purposes. A 1
Hp electric motor therefore will require 3-4kVA for start up. Once the 1 Hp electric motor has started, it will
only consume 1 kVA leaving a extra 2-3kVA available for other appliances.
However, the newer 'Inverter' style generators may be more efficient and capable of starting higher loads.
The generator size must be equal or greater than the total consumption of all applications. The higher starting
requirement must he taken into account accordingly. To maximise the generator potential, the largest electrical
motor should be started on its own and further appliances should only be switched on thereafter.