What size of residential standby generator do I need?

Determining which generator to buy for your household can be daunting because it involves terms people are not familiar with and math. Electrical generators are rated in output watts, and they usually have two ratings: "surge watts" and "running Watts." These ratings, compared with the total wattage you need, determine which generator to buy.

How Many Watts Will I Need?

The wattage needed depends upon the size of the house, the type of devices to be powered, and the number of electrical devices power will be delivered to. For example, powering one light bulb and one refrigerator will use fewer watts simultaneously than powering several light bulbs, a refrigerator, a portable heater and a hot plate.

Each electrical device that operates using household electrical current has a plate, sticker or marking showing how many watts that device uses. In many cases, this power rating is given in the form of operating voltage and maximum current ratings.

If the device doesn't identify its watt usage, you can figure it out as long as you know the volts and amps used by it. Watt's Law states that power (in watts) can be calculated by multiplying electrical current (in amps) by voltage. For example, a device with an operating voltage of 110 volts that uses a maximum current of 2 amps will use 220 watts.

To determine which generator to purchase, compile a list of devices you want the generator to power. Add the wattage each uses to produce the total of how many watts are needed simultaneously. For example, a 1000-watt generator can easily power two 100-watt lamps (which consume, collectively, 200 watts of power). However, that generator cannot power 20 100-watt lamps (which collectively consume 2000 watts).

Surge Watts vs. Running Watts

A generator will typically be advertised with the "running watts" rating. This rating represents how many watts the generator can safely output in continuous operation.

Sometimes a generator will be advertised as "2500 surge watts". The "surge watts" rating represents the maximum number of watts the generator can output, typically for a short period of time (such as one minute).

Some electrical loads, such as electric motors, microwave ovens or refrigerators, take more energy to start operating than to operate continuously (this temporary increase is called a reactive load). Therefore, it is best to buy a generator that has a "surge" rating that is higher than the maximum power rating of such devices.

Source:

Common Wattage of Household Appliances; Tim Carter; 2009

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