Look at your electric bill for your average daily consumption of electricity. If your bill doesn't offer your daily usage, divide your monthly usage by the number of days in the month. An even better average is to divide your yearly use, which is sometimes offered on your bill, by 365.
Reference the average daily peak sunlight hours for your area. The Renewable Resource Data Center's Solar Radiation Data Manual (see Resources) is one place to start.
Divide your daily electricity usage by the peak sunlight hours for your area. As an example, if you used 30 kilowatt hours per day, and the peak sunlight hours averaged five hours per day, you would need to produce 6 kilowatts per hour from your solar panels, or 6,000 watts per hour.
Divide wattage requirement by the wattage rating of the solar panel to calculate the number of solar panels required. In the example, if the solar panels you were looking at buying were 200-watt models, then you would need 30 of them to run your household.