Sizing Home Generators
In addition to deciding on the style of generator to purchase, you also need to know how to size a home generator based on your wattage needs.
RUNNING WATTS
To choose the correct home generator size, first consider what appliances you'll need 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 LEFT] 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.
APPLIANCE 
RUNNING WATTS 
Radio 
100 
Computer 
250 
TV 
300 
Lights 
400 
Clothes Dryer (Gas) 
400 
Well Pump 
400 
Electric Blanket 
400 
Freezer 
500 
Furnace 
500 
Garage Door Opener 
600 
Refrigerator 
600 
Sump Pump 
600 
Clothes Dryer (Electric) 
700 
Dish Washer 
800 
Coffee Maker 
1000 
Swamp Cooler 
1000 
Toaster 
1000 
Hair Dryer 
1200 
Space Heater 
1200 
Iron 
1200 
Vacuum 
1400 
Microwave 
1500 
Water Heater 
3000 
Electric Range 
4500 
Central Air Conditioner 
5000 
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 Needs 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 (Stove Top) 
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 
Box Nailer 
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) 
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 
Regrigerator 
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.