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 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 75 watts while it is running and 150 watts to get it started. That extra wattage to start the motor is called the surge wattage, and it is typical of anything that contains an electric motor. Here are some of the devices found in a normal household and their typical wattages:

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 is unlikely because of the cost), 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 Uninterruptable 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:

Example 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 continous/300 watt surge.

Example 2 - If you plan to operate your refrigerator, a color TV and a microwave simultaneously, you need 2,250 watts continuous and 3,800 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 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 example 2 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.