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How Emergency Power Systems Work

Understanding Your Power Needs
In order to choose the right emergency power source and to size it properly, you need to understand something about the power requirements of the devices you plan to operate.

The basic unit of power measurement is the watt, and with an emergency power source there are two wattage ratings that are important: steady-state wattage and surge wattage. A normal 60-watt incandescent light bulb requires, as you would expect, 60 watts, and it requires that wattage both when you turn it on and while it is running. A ceiling fan motor, on the other hand, might require 150 watts to get it started and 75 watts while it is running. That extra wattage to start the motor is called the surge wattage and is typical of anything that contains an electric motor. Here are the usual wattages of some of the devices found in a typical household:

Typical wattage
Surge Wattage
Light bulb
60 watts
60 watts surge
75 watts
150 watts surge
Small black/white television
100 watts
150 watts surge
Color television
300 watts
400 watts surge
Home computer and monitor
400 watts
600 watts surge
Electric blanket
400 watts
400 watts surge
Microwave oven
750 watts
1,000 watts surge
Furnace fan
750 watts
1,500 watts surge
1,200 watts
2,400 watts surge
Well pump
2,400 watts
3,600 watts surge
Electric water heater
4,500 watts
4,500 watts surge
Whole-house A/C or heat pump
15,000 watts
30,000 watts surge

One thing you can see from this chart is that the heat pump or air conditioner for an entire house has a huge appetite for power. If your house has a heat pump and you want to be able to keep the house warm during a power failure in the winter, then you will either need to purchase a very large generator (which costs a whole lot) or you will need a backup heat source, such as wood or propane.

One other thing to note is that if you plan to operate sensitive equipment like TVs and computers from an emergency power supply, you will want to have in place excellent surge protection equipment and, in the case of a computer, an uninterruptible power supply (UPS). When a large device like a refrigerator turns on, there is no way that a small generator will be able to keep power stable during the surge. A UPS will prevent your computer from crashing during the blip.

To calculate your power needs, you need to add up the normal and surge wattage figures for all of the devices you want to operate simultaneously. Here are two examples:

  1. If you plan to operate a small TV and two 60-watt light bulbs, then you need an emergency power supply that has a capacity of at least 220 watts and a surge capacity of 270 watts. Rounding up, that's 250 watts continuous and 300 watts surge.

  2. If you plan to operate your refrigerator, a color TV and a microwave simultaneously, you need 2,250 watts continuous and 3,800 watts surge in the worst case (if all three happen to turn on at exactly the same moment). If you are willing to manage your power a bit and make sure they do not all turn on at once, then your surge power requirement is only 2,400 watts. If you are willing to operate only one of these devices at a time, then, because your refrigerator is the largest power user, you need to size your emergency power system so it is large enough to handle the refrigerator.
The point made in the second example about staggering your power consumption is important. Generators tend to get very expensive as you move above 5,000 watts. For example, an inexpensive 5,000-watt generator might cost you $600. A 10,000-watt generator, on the other hand, will normally cost over $2,000. If you are willing to stagger your usage -- for example, running the refrigerator for an hour and then running the well pump, but never operating them together -- you can get by with a much smaller generator.