How to Advice

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. 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.

Running Watts of Typical Residential Devices

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. 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).

CONCLUSIONS - WHAT WE AT T-REX 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 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.

For any questions or concerns, or for help selecting your ideal generator, call our toll-free hotline at 866-613-9115.