How to determine generator size
When
you have decided on the specific purpose your generator will fulfill(for
hints, see our home page), you are ready to decide on its size and wattage.
RUNNING
WATTS
To
choose the size and power of your generator, first consider what appliances
you'll need for it to run. When these have been identified, take note of their
running watts. Using estimates, such as those provided in our Running Watts of Typical Residential Devices
list[SEE BELOW] is convenient. As you plan for a
generator purchase, lists like ours can offer a quick ballpark idea of the
wattage required. We recommend, before you select a generator model, checking
the exact wattage demands of at least the major appliances you'll be running.
Exact running watts can normally be found on a data plate[SEE GRAPHIC], on an appliance's back or underside. Occasionally
a data plate will provide amps and volts rather than watts. If this is true,
simply multiply the amps by the volts(Amps x Volts = Watts) to obtain the running watts of an appliance.
If a data plate can't be located, a wattage meter[PICTURED
AT RIGHT] is a convenient alternative for determining the precise wattage needs
of an appliance. A wattage meter is inserted between an appliance and wall
outlet; your appliance is plugged into the wattage meter  the meter is then
plugged into the wall. TREX offers a Reliance wattage meter, considered by us among
the best available. For information on this and other generator accessories you
might find useful, click on the Accessories
tab above. Quality wattage meters are also available at most hardware stores, and
homeimprovement warehouses.
When you have identified the running watts of the appliances
you'd like to power, add these together. Since generator manufacturers
recommend against maxing out a model's available watts, ideally you'll need a
generator with available wattage slightly above your total. In our experience,
if your full load occupies 80  90% of generator capacity, that provides plenty
of excess margin.
Running Watts of Typical Residential Devices
APPLIANCE

RUNNING
WATTS (approximate)

Radio

100

Computer

250

TV

300

Lights

400

Clothes
dryer(gas)

400

Well
pump

400

Electric
blanket

400

Freezer

500

Furnace

500

Garagedoor
opener

600

Refrigerator

600

Sump
pump

600

Clothes
dryer(electric)

700

Dish
washer

800

Coffee
maker

1,000

Swamp
cooler

1,000

Toaster

1,000

Hair
dryer

1,200

Space
heater

1,200

Iron

1,200

Vacuum

1,400

Microwave

1,500

Water
heater

3,000

Electric
range

4,500

Central
air conditioner

5,000

This list is intended to provide
a ballpark number, based on typical wattage of devices named. Since models
vary, it is always best to check the data plate[SEE EXPLANATION
ABOVE] on the rear or bottom of your appliance for precise wattage
requirements.
CONSIDERING SURGE WATTS
Most generator manufacturers and
some retailers suggest that consumers disregard the running watts of an appliance(the number you arrived at above), and instead
select a generator model based on the higher surge watts of appliances you'll
wish to run. In most instances, we don't believe in following this strategy. To
decide if it's an appropriate one for you, consider the following:
WHICH APPLIANCES POSSESS SURGE WATTS?
Surge watts are extra watts
demanded by electric motors in certain power tools  air compressors for
instance  or major home appliances  like refrigerators, furnaces, and air
conditioners  when they first click on. You can witness the effect of surge
watts in home appliances when lights dim for a split second as certain brawny devices
are started. Not all highwatt tools and appliances possess startup motors.
Even though a vacuum and electric hair dryer can cause lights to flutter or dim
when they are being used, the number of watts needed at startup is the number
of watts these appliances will need after five minutes of operation, or thirty
minutes of operation. In other words, the wattage requirements never vary.
THE SURGEWATTS DEBATE: WHAT YOU
NEED TO KNOW
TREX believes that purchasing a
generator based on running watts of given appliances is usually sufficient. Any
generator powerful enough to run an appliance should start its motor, if the load
is effectively managed.
This is because generators possess
a builtin maximum power or surge watts rating. Maximum or surge
power is typically from 10  20% above a model's running watts. It is not
constant, not designed to actually run tools or appliances for an extended time(like your continuous or running watts). Figure on surge
power being available in 30second bursts  its lone purpose to provide motorstarting
prowess. Generator manufacturers also advise, when planning a load, that your
largest appliances be connected first. Operators can employ this trick to enhance
the motorstarting capacity of a generator.
Below you will find a list of
frequentlyused tools and appliances, and the surge watts needed to start them(listed as a percentage of their running watts). Don't panic if this sounds complicated; it is actually quite
simple. Here's the way to use our list:
Figuring Approximate Surge Watts of a Swamp
Cooler
Say you own a swamp cooler(window air conditioner) which requires 1,000 watts to
run continuously. You need to determine roughly what its motorstarting watts
will be. The list below notes that, in general, you can expect the surge watts
required by a swamp cooler to be about 170% of its running watts(that
is, running watts plus an additional 70%). Multiply your swampcooler running watts(1,000) by 1.70(the mathematical equivalent of 170%), using
the formula (1,000 x 1.70 = Surge Watts), and you obtain a motorstarting
demand of roughly 1,700 watts. As long as your generator has 1,700 available watts(which can be in unused running watts, surge watts, or
a combination of running and surge watts), it will always power on your swamp
cooler. If you connect enough tools or appliances to a generator so that 1,700
watts aren't available, it doesn't matter what you do, or how big your
generator is, the swamp cooler motor won't budge.
Figuring Approximate Surge Watts of a
Central Air Conditioner
The home appliance which
requires the most starting watts is often a central air conditioner. While central
air conditioners need just 150% of their running capacity to start, a lower
percentage of running capacity to start up than other tools and appliances, the
running watts of centralair conditioning units can be so high that the
resulting surgepower requirements dwarf those of any competing tool or appliance.
Let's apply our formula to a 5,000running watt central air conditioner.
Multiplying 5000(running watts) x 1.50(equivalent of 150%) = a 7,500watt surge
requirement. This means that to start your central air conditioner, the
generator model must have 7,500 watts(running, surge,
or a combination thereof) available to it.
Central air conditioners can't
be plugged directly into a generator outlet panel. To run a central air
conditioner, you will need a manual
transfer switch(for
more information regarding transfer equipment, click on Power Transfer With Transfer Switches above). However the inclusion
of a manual transfer switch doesn't alter the results of our formula. To start
the 5,000watt central air conditioner we described, you will need 7,500 free
watts going to the transferswitch circuit running it.
Remember too that other devices
being run by the same manualtransfer switch circuit can sap part of the
motorstarting wattage you'll need!
How About an Air Compressor?
One more
demonstration. An air compressor may only require 500 running watts, however its stout motor needs a whopping 400% of
running watts to start. Applying this information to our formula, 500(running
watts) x 4.00(400%) = a 2,000watt motorstarting demand. In other words, if
2,000 watts(running, surge, or a combination thereof)
are available to your generator, it will always power on the air compressor.
APPLIANCE

SURGE
WATTS NEEDED TO START (by % of running watts)

HOW
TO CALCULATE

Electric
chain saw

None


Coffee
maker

None


TV

None


Dessicator

None


Dish
washer(hot dry)

None


Electric
blanket

None


Electric
grill

None


Electric
lawn mower

None


Electric
range(stovetop burners only)

None


Electric
pot

None


Electric
skillet

None


Fret
saw

None


Hair
dryer

None


Induction
cooker

None


Iron

None


Radio

None


Soldering
iron

None


Toaster

None


Vacuum
cleaner

None


Electric
drill

120% of running watts

Multiply
running watts by 1.20

Microwave
oven

120% of running watts

Multiply
running watts by 1.20

Electric
cutter

120% of running watts

Multiply
running watts by 1.20

Electric
hammer

120% of running watts

Multiply
running watts by 1.20

Boxnailer

130% of running watts

Multiply
running watts by 1.30

Router

130% of running watts

Multiply
running watts by 1.30

Central
air conditioner

150% of running watts

Multiply
running watts by 1.50

Swamp
cooler

170% of running watts

Multiply
running watts by 1.70

Electric
range(oven part only)

200% of running watts

Multiply
running watts by 2.00

Electric
sander(disc/belt)

200% of running watts

Multiply
running watts by 2.00

Dish
washer(cool dry)

200% of running watts

Multiply
running watts by 2.00

Toaster
oven

200% of running watts

Multiply
running watts by 2.00

Well
pump

200% of running watts

Multiply
running watts by 2.00

Mercury
lamp

250% of running watts

Multiply
running watts by 2.50

Clothes
dryer(gas)

250% of running watts

Multiply
running watts by 2.50

Furnace

250% of running watts

Multiply
running watts by 2.50

Freezer

300% of running watts

Multiply
running watts by 3.00

Drill
press

300% of running watts

Multiply
running watts by 3.00

Sump
pump

300% of running watts

Multiply
running watts by 3.00

Washing
machine

300% of running watts

Multiply
running watts by 3.00

Deep
well pump

350% of running watts

Multiply
running watts by 3.50

Residential
underwater pump

350% of running watts

Multiply
running watts by 3.50

Electric
tap

350% of running watts

Multiply
running watts by 3.50

Refrigerator

350% of running watts

Multiply
running watts by 3.50

Air
compressor

400% of running watts

Multiply
running watts by 4.00

Recorder

500% of running watts

Multiply
running watts by 5.00

100% COPPER WINDINGS  WHY THEY'RE SIGNIFICANT
If you notice 100% copper windings listed as a prominent
feature of certain generator brands, it is for good reason. Lowerpriced or
bargain models normally don't possess this feature(if
it isn't specifically advertised, a good rule is to assume a model doesn't have
it). Instead of pure copper, alternators of cheaper brands will be wound with
coppercoated steel or aluminum.
Coated windings are sometimes employed to reduce a model's
overall weight, critical to lightweight portables especially. Most of the time
they are used as a costcutting measure  copper is a precious metal; aluminum
and steel aren't. Coppercoated windings are impossible to distinguish by sight
 they look no different from purecopper windings  nor will generator wear or
reliability suffer. What coppercoated generator windings mean for a consumer
is that the manufacturer's listed running and surge watts will be overstated.
For better or worse, it is standard practice in the industry
for generator manufacturers to rate products as if they contained purecopper
wound alternators  even if they do not. If products lack this feature, it
doesn't mean they should be avoided, or that they are of inferior quality or
unreliable. TREX personnel have used many such models, and members of our
staff currently own several. We'd have no problem recommending these for
purchase. Simply be aware that, at the generator outlet panel, you'll get about
80% of a model's stated watts, running and surge. You may end up doing a little
better or a little worse than that, but it's a good ballpark number to figure
on, in particular if you intend for your portable or home generator to power
major appliances(i.e. a water heater, electric range,
or central air conditioner).
CONCLUSIONS  WHAT WE AT TREX RECOMMEND
Certain generator sellers recommend that you figure out
running watts of the appliances you'll wish to power  the way we did above 
then select a generator model with twice or even three times that wattage to
account for surge or motorstarting needs. Say you have identified 5,000watts'
worth of appliances to power, if you followed these guidelines you would end up
with a generator of 10,000, perhaps of 15,000 watts. In our opinion, this waste
is ludicrous. Anyone purchasing a generator by these guidelines will end up
with a far bigger model than needed. Bigger models are harder to transport and
store, they require more fuel reserves, and they also cost more to operate,
running either a full or partial load. If you buy a generator two or three
times the size you need, count on it benefiting at least one party  the person
hawking the generator.
TREX recommends that you focus first on running watts of appliances
you'll wish to power. When you have that number identified, consider the
following three things and add to your wattage totals accordingly:
A]Remember that generator
manufacturers advise against maxing out a model's continuous or running watts.
They recommend a power reserve of 20%. In other words, they advise that your
load not exceed 80% of generator capacity. Observing this rule isn't a bad
idea. However in our experience planning for an extra 10% will probably be
sufficient.
So add from 10  20% to the total running watts you figured
above.
B] If you are selecting a model without 100% copper windings, and you consider that 20% of running
and surge capacity you won't receive especially critical, you may want to make
up for it yourself.
Add up to 20% as a precaution against the power loss of coated
windings.
C]If you believe that your
generator needs will expand, that within the foreseeable future you will be
requiring a higher runningwatt capacity, you may wish to purchase a larger, morepowerful
model than you need at present. Something to keep in mind: generators are like
power mowers and chain saws, their motors operate at a high RPM and therefore
can't be expected to last forever. If you buy a generator for frequent rather
than occasional(emergency) use, you may be better off with a model which
handles current needs, then upgrading to a larger model down the road if and
when necessary.
Add discretionary percentage based on projected
runningcapacity increases.
Summary Recommendations
 after you have arrived at a final runningwatts total, return to the lists
above, and perform a few quick calculations  to insure that motorstarting
needs are satisfied for all tools and appliances you wish to run. If you're not
satisfied, if you'd feel safer with more excess capacity, if you don't mind
spending the extra money, there's nothing criminal in buying a larger and
costlier model than you absolutely need.
On the other hand, we can't see recommending that our customers
spend hardearned wages on our biggest, highestpriced units if modest generators
will perform a function just as well, and be easier, in many cases, to
transport and store. Nor will we recommend that a customer buys an extravagant
generator when, under the same circumstances, we'd choose a model less expensive,
and expect it to deliver the same wear and serviceability. The most important
thing in making any purchase, a generator purchase included, is to take your
time, and select a product which makes you
happy and comfortable, which best satisfies your
needs.
For any questions or concerns, or for help selecting your
ideal generator, call our tollfree hotline at 8666139115. 