How to Calculate Generator Power Needs
Running an undersized generator may damage the generator and devices connected to it, yet it is a common misconception that small generators are adequate for emergency power, says the Massachusetts Department of Agricultural Resources. Power and protect your appliances during an emergency power outage by calculating the appropriate generator size for your needs. Does this Spark an idea?
Instructions
-
-
1
List all the electrical items you need to provide power to during an outage, such as lights, refrigerator, fans, computers and heating or air conditioning units.
-
2
Read the power requirements on the appliance label for each item. You can usually find the label on the back of the appliance. Look for numbers followed by the words "watts," "volts" or "amps" or the abbreviations "W," "V" or "A." Record the wattage for each item if it is listed; record the voltage and amperage if wattage is not given. For lights, use the wattage of each light bulb.
-
-
3
Multiply volts times amps to get watts for each item that did not have a wattage rating. For a laptop computer with 20 volts and 1.5 amps, for example, multiply 20 by 1.5 to get 30 watts. Record the wattage for each of these items.
-
4
Add the wattages for all the items on your list to get the total running wattage. If you have three 60-watt light bulbs, a computer with 30 watts, a refrigerator with 700 watts, a microwave with 800 watts and an air conditioning unit with 2,000 watts, for example, add 3*60 + 30 + 700 + 800 + 2,000 to get 3,710 watts.
-
5
Determine which appliance has the highest surge wattage at start up -- this will likely be the refrigerator, air conditioner, heating or a water well pump. Find the surge wattage of the appliance in the user's manual or estimate it using a wattage chart. Add the surge wattage to the running wattage. If your highest surge wattage is an air conditioning unit with a 2,200 watt surge and the running wattage is 3,710 watts, for example, add them to get 5,910 watts.
-
6
Divide the running wattage by 1,000 to find the power requirement in kilowatts, the unit typically used for generator power ratings. For example, 5,910/1,000 = 5.91 kilowatts.
-
7
Divide the required wattage by 0.7. For a wattage of 5.91 kilowatts, for example, 5.91/0.7 = 8.44 kilowatts. Purchase a generator with a power output somewhere between your calculated wattage and the wattage divided by 0.7; in this example, the generator should be rated between 5.91 and 8.44 kilowatts. Because generators run most efficiently when the load is around 70% of the maximum power, running a larger generator would reduce efficiency.
-
1
Tips & Warnings
If you cannot find a label on an appliance, look for power requirements in the user's manual. Look up the appliance on a wattage chart to find the typical power consumption if you do not find the power requirements anywhere else.
Check the voltage rating on your air conditioning and heating units. If one or both of them require 240 volts, you need a generator that can generate both 120 volts and 240 volts.
Plan to power up only one appliance that requires surge wattage at a time to avoid overloading the generator. Any appliance with an electric motor has a start-up surge wattage, including most kitchen appliances, heating and cooling units, and clothes washers and dryers. Other items such as lights, computers, televisions and electric ranges do not require a start-up surge and can safely be started simultaneously.
