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Home Generator Sizing

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, 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 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. T-REX 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 home-improvement 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.


  1. Radio 100
  2. Computer 250
  3. TV 300
  4. Lights 400
  5. Clothes Dryer (Gas) 400
  6. Well Pump 400
  7. Electric Blanket 400
  8. Freezer 500
  9. Furnace 500
  10. Garage Door Opener 600
  11. Refrigerator 600
  12. Sump Pump 600
  13. Clothes Dryer (Electric) 700
  14. Dish Washer 800
  15. Coffee Maker 1000
  16. Swamp Cooler 1000
  17. Toaster 1000
  18. Hair Dryer 1200
  19. Space Heater 1200
  20. Iron 1200
  21. Vacuum 1400
  22. Microwave 1500
  23. Water Heater 3000
  24. Electric Range 4500
  25. 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.


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:


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 high-watt tools and appliances possess start-up 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 start-up 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.


T-REX 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 built-in 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 30-second bursts – its lone purpose to provide motor-starting prowess. Generator manufacturers also advise, when planning a load, that your largest appliances be connected first. Operators can employ this trick to enhance the motor-starting capacity of a generator.

Below you will find a list of frequently-used 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 motor-starting 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 swamp-cooler 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 motor-starting 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 central-air conditioning units can be so high that the resulting surge-power requirements dwarf those of any competing tool or appliance. Let’s apply our formula to a 5,000-running watt central air conditioner. Multiplying 5000(running watts) x 1.50(equivalent of 150%) = a 7,500-watt 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 5,000-watt central air conditioner we described, you will need 7,500 free watts going to the transfer-switch circuit running it.

Remember too that other devices being run by the same manual-transfer switch circuit can sap part of the motor-starting 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 2,000-watt motor-starting 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

  1. Electric Chain Saw None
  2. Coffee Maker None
  3. TV None
  4. Dessicator None
  5. Dish Washer (hot dry) None
  6. Electric Blanket None
  7. Electric Grill None
  8. Electric Lawn Mower None
  9. Electric Range (Stove Top) None
  10. Electric Pot None
  11. Electric Skillet None
  12. Fret Saw None
  13. Hair Dryer None
  14. Induction Cooker None
  15. Iron None
  16. Radio None
  17. Soldering Iron None
  18. Toaster None
  19. Vacuum Cleaner None
  20. Electric Drill 120% Of Running Watts Multiply running watts by 1.20
  21. Microwave Oven 120% Of Running Watts Multiply running watts by 1.20
  22. Electric Cutter 120% Of Running Watts Multiply running watts by 1.20
  23. Electric Hammer 120% Of Running Watts Multiply running watts by 1.20
  24. Box Nailer 130% Of Running Watts Multiply running watts by 1.30
  25. Router 130% Of Running Watts Multiply running watts by 1.30
  26. Central Air Conditioner 150% Of Running Watts Multiply running watts by 1.50
  27. Swamp Cooler 170% Of Running Watts Multiply running watts by 1.70
  28. Electric Range (oven) 200% Of Running Watts Multiply running watts by 2.00
  29. Electric Sander (disc/belt) 200% Of Running Watts Multiply running watts by 2.00
  30. Dish Washer (cool dry) 200% Of Running Watts Multiply running watts by 2.00
  31. Toaster Oven 200% Of Running Watts Multiply running watts by 2.00
  32. Well Pump 200% Of Running Watts Multiply running watts by 2.00
  33. Mercury Lamp 250% Of Running Watts Multiply running watts by 2.50
  34. Clothes Dryer (gas) 250% Of Running Watts Multiply running watts by 2.50
  35. Furnace 250% Of Running Watts Multiply running watts by 2.50
  36. Freezer 300% Of Running Watts Multiply running watts by 3.00
  37. Drill Press 300% Of Running Watts Multiply running watts by 3.00
  38. Sump Pump 300% Of Running Watts Multiply running watts by 3.00
  39. Washing Machine 300% Of Running Watts Multiply running watts by 3.00
  40. Deep Well Pump 350% Of Running Watts Multiply running watts by 3.50
  41. Residential Under-Water Pump 350% Of Running Watts Multiply running watts by 3.50
  42. Electric Tap 350% Of Running Watts Multiply running watts by 3.50
  43. Regrigerator 350% Of Running Watts Multiply running watts by 3.50
  44. Air Compressor 400% Of Running Watts Multiply running watts by 4.00
  45. Recorder 500% Of Running Watts Multiply running watts by 5.00


If you notice 100% copper windings listed as a prominent feature of certain generator brands, it is for good reason. Lower-priced 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 copper-coated 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 cost-cutting measure – copper is a precious metal; aluminum and steel aren’t. Copper-coated windings are impossible to distinguish by sight – they look no different from pure-copper windings – nor will generator wear or reliability suffer. What copper-coated 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 pure-copper 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. T-REX 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).


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 motor-starting needs. Say you have identified 5,000-watts’ 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.

T-REX 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 running-watt capacity, you may wish to purchase a larger, more-powerful 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 running-capacity increases.

Summary Recommendations – after you have arrived at a final running-watts total, return to the lists above, and perform a few quick calculations – to insure that motor-starting 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 hard-earned wages on our biggest, highest-priced 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.

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