Once you've decided to add a system of solar photovoltaic power panels to your house or business or determined that it's time to expand an existing system, the next question is: how much wattage do I need?
To find this out, you will need to know how much power you consume (or how much you plan to consume, for new buildings or for additions that will require lots of power - like a woodworking shop or a hot tub) and how much sunlight you receive throughout the year.
Assuming that you've been at your location for at least a year (so that you have a good feel for your electric needs for each season), take each electricity bill and write down the number of kilowatt-hours (kw-h) that were billed and the number of days in the billing cycle. Then, divide the number of kw-h by the number of billing days to get the average kw-h per day. To make things simpler, some people just assume 30 days for each billing cycle (and not all power companies report the billing days). That's how many kw-h your system will need to generate each day to meet your power needs. Once you've done this for the entire year's worth of bills, you'll know both your average daily consumption and your peak consumption. You may choose to size your system based on either value. If you use the average value (and assuming your power company allows you to sell power back to them, you should definitely check on this first!), then some days you'll make extra power and sell it back to the power company and some days you'll be buying power, but over the course of a year, your net electric bill should be close to $0. If you use the peak number, then you should be selling power back to the power company almost every day, but your system will cost more than if you sized the system based on the average power consumption.
Now, to get the wattage of solar panels that you need, you'll need to know how many hours of direct sunlight that your system will receive. If you've been at your location for several years already, you might have a good idea how many hours of direct sunlight your building receives, either seasonally or on average throughout the year. One way to determine how many hours of sunlight you receive is to take observations several times a day as to whether the portion of your roof where the solar panels are to be installed is sunlit. If you do this every day for a year, you'll have a good idea of how many hours of sunlight your solar panel will receive on a seasonal basis. There are a few problems with this approach: it's very time consuming as it requires you to take measurements throughout the day nearly every day; there will be some days when you just can't take the measurements (you're out of town, working and cannot get to the house to observe the roof, clouds are obscuring the sun, etc.); and, unless you have a solar panel system that tracks the sun, the number of hours of sunlight your roof receives is not the same as the hours of direct sunlight, since much of that light will probably not be coming in at right angles to your roof throughout the year.
Another way to estimate the number of hours of direct sunlight you receive is to assume a value or to use a rule-of-thumb. Often, 5 hours direct sunlight on average throughout the year is assumed. Although this is a fairly common assumption, you might end up disappointed if your solar system generates less power than you anticipated because you get fewer average hours of sunlight than this.
A third method is to look up an average regional value. The Florida Solar Energy Center presented a study in August 2004 at the ACEEE 2004 Summer Study on Energy Efficiency in Buildings titled "Geographic Variation in Potential of Rooftop Residential Photovoltaic Electric Power Production in the United States." This study simulated the power generation of a 2 kilowatt (kw) photovoltaic rooftop system for 236 locations throughout the US. Depending on the location, they found that this 2 kw rooftop system would generate between 5 and 8.5 kw-h per day on average throughout the year.
Using the rule-of-thumb, a 2-kw system receiving direct sunlight for 5 hours per day would expect to generate 10 kw-h per day on average throughout the year. From the Florida study, the same system would be expected to generate only 5 - 8.5 kw-h per day throughout the year.
How you proceed depends on your goals. If your goals are to offset your electricity cost, then any of the approaches will provide an estimate of the number of hours of direct sunlight that you receive. However, if your goal is to become energy independent, then using the results of the Florida simulation would provide a more conservative value that would be more likely to provide a system sized for your needs. Take the results of the simulation and a few of your own observations (whether you have a lot of tall evergreen trees on the south side of your house that block a majority of the winter sun, etc.) and you should be able to estimate the number of hours of direct sunlight that you receive.
For example, if you've decided to use the average power consumption value and you use 600 kw-h per month and your billing company doesn't report the number of days per billing cycle (assume 30 days per month), then you consume 20 kw-h per day of electricity (600 kw-h divided by 30 days per month). Using the Florida simulation data and assuming you live in Western Kentucky, you receive approximately 3.5 hours of direct sunlight per day (from the graphic, estimate a value of 7.0 kw-h divided by 2 kw - the study is based on a 2 kw system). Then, your system would need to be sized for approximately 5,700 watts.
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