One of the first questions a homeowner faces when deciding on whether to install a home solar power system is, “How many solar panels do I need?” If you are looking to replace only some of the power that you currently buy from your local utility and generate enough power to offset some of the cost, then any number of panels can help them accomplish this. But if you are looking to eliminate your electricity bill completely, knowing how large a solar panel system you need to accomplish this takes a little work.

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The first thing you’ll need to figure out is how much electricity you typically use for your home each year. This is a very easy thing to do and requires you to gather up as many of your monthly electricity bills as you can to help you calculate this number. This has gotten easier in recent years since many utility companies will post your bills online and give you at least a years worth of access to them. In some cases they will even calculate your monthly average for the year and this can save you some time. Once you have your bills, you need to figure out the total kWh (kilowatt-hours) you used each month for your home over the last year. If you can’t find a year’s worth of electric bills, you can use the value for one month, which is easily found on any electric bill. It is best to use the value from the month in which you use the most electricity, but for a rough estimate, you can use the value from any month. Next, you need to divide the value for one month by 30 to get the average daily kWh of electricity usage. A kilowatt-hour is basically a measure of how much electricity is used for a given period of time. It is 1,000 watts of power used for 1 hour. For example, if you have a 100 watt bulb burning for 10 hours, you have used 1 kWh (100 x 10 = 1000 watt-hours or 1 kWh). This is an easy standard for the power company to use for billing and is how solar power systems are rated as well. This makes calculating the amount of solar panels you need easier since the ratings are in the same units of measure.

There is one other factor that needs to be considered and this is called insolation. Insolation has to do with how much of the average day’s sun will actually be useful to the solar panel for producing electricity. Solar panels turn sunlight into electricity based on that sunlight hitting the solar cells directly. They are also expecting a certain intensity of that sunlight for their calculations on how much output power they can generate. If there is a less intense sunlight, they will produce less power. The angle and resulting intensity with which sunlight hits the solar panels will vary during a given day, and over the course of a year. Sunlight is less intense in the early morning and late afternoon and these variations affect the amount of power produced by a solar panel. Every location on earth has an average insolation value and you need to know what your location is rated in insolation hours. There are maps of these values published on the internet for easy reference. Once you have this number, you can simply divide the kWh of electricity you need to produce in a given day by the insolation value to get the real value you need for your panels.

The last factor you need to consider is the loss in your solar power system. Even though these newer systems are much more efficient than older systems, there is always a loss in conversion. A good rule of thumb to use is 25%, so be sure to multiply whatever your final figure turns out to be by 1.25 to make sure you account for any loss in your solar panel system. Now that you have your daily kWh requirement that has been adjusted for insolation and efficiency, you know how much total power your solar panels need to produce.

To get the final answer to the question from the top, “*How many solar panels do I need*,” divide the desired total power output that you just calculated by the power output of a single solar panel and round up. This is the number of solar panels you will need to produce the desired amount of electricity.

Click here for the best guide to building your own solar panel, complete with step-by-step videos.

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