Generators should be installed by qualified electricians. Most generators are rated in watts. The formula for watts is Watts=Volts x Amps. If you you have a 120 watt bulb at 120 volts you would have one amp. If you were to run 25 100 watt bulbs you would need a 2,500 watt generator. Motor circuits such as circulating motors for heat and compressors for refrigerators must be calculated differently. It takes about three times the current to start a motor. Most motors have the amperage rating on the name tag. If the motor has a nameplate rating of 3 amps you might want to add 9 amps on your generator calculations. Also because most generators may run over a period of time you may not want to run at full capacity. About 80% of the wattage rating of your generator would be the way to go.
To view a wattage table for electrical devices Click Here
Click Here for Ohms Law Calculator.

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 There are a few things to consider when getting a generator.
The cost involved.
The addition of a transfer switch.
A room outside to put the generator.
You need to maintain a generator regularly, including running it every so often so that it will work when the power goes out. Also check while it is running.
Adding fuel. The generator will need to be shut off when adding fuel. Anytime you have fuel and electricity together there is a danger.
If you are getting a generator just to run your computer , you might be better off with a laptop or a UPS. system.