How many solar panels do I need?

I hate this answer but I am going to give it anyway:  It depends.


It depends on how much electricity you want to generate.  It depends on whether or not you want to go totally off of the grid.  It depends on whether you want to generate enough electricity to sell back to the local utility and become an energy entrepreneur.  It depends on the amount of usable roof space that you own.  It depends on whether or not there is a ground-mount option for you.  It depends on whether you are buying or leasing.  The answer is this answer:  It depends!


In order to simplify let’s assume that you want enough solar panels to power your home adequately every day.  Please find below a step-by-step guide that will give you this answer.


Step Number One:  Look at your utility bills for the last twelve months.  Find the month that represents your greatest use of electricity.  If you want to have enough solar panels so that you can provide power to your home every day then you will use enough solar panels so that the month with your greatest use of electricity is covered.   And if you live in a state where net metering is allowed then the excess energy that you produce in some of the other months will be sold back to the utility company.  And you will be an energy entrepreneur!


“But wait,” you say, “I just want to offset some of my usage.  I don’t want to be a full fledged energy entrepreneur.”  So let’s do this another way……..


Step Number One Revisited:  Add together each month’s kilowatt usage and divide by twelve.  This number will give you the average amount of electricity that you use.  Using this analysis, for six months you will generate enough for your own usage, and for another six months you will generate some excess.  When you arrive at your answer divide by 30 to get your daily average electrical usage.


Step Number Two:  Sunshine is measured in units of Solar Insolation. Technically, insolation is described kWh/m2 per day.  This is useful for working out exactly how much energy you can get for your solar panels.  However, it is not a measure of daytime hours. Insolation is a measure of how many usable hours of sunshine you get a day.


Even in very sunny states, the amount of direct sunlight available for making electricity is only about 5-6 hours. This is because the angle of light makes a big difference to how much energy is produced.  There are plenty of websites available that will calculate the solar insolation for your zip code.  Or you can use our map that follows this article.


Step Number Three:  Divide the average daily kilowatts that you use by the number of hours that your location receives in direct sunlight.   Then multiply the results by 1.25 which is a multiplier that takes into account energy losses that result from the wiring and connections.  The result is how many watts of power are needed daily.


Step Number Four:  Assume a household usage of 1800 kWh per month. That’s around 60 kWh per day or 1.25kw per hour.  Now assume six hours of good sunlight will directly hit the solar panels.  This means that you will have one-quarter of a 24 hour day to generate the power that you need.  Thus, each hour you need to generate 5 kW (4 x 1.25). You would therefore need at least 5,000 watts worth of solar panels, plus extras as future replacement panels and to cover inefficiencies such as unanticipated shade, etc.


Step Number Five:  Review the specifications on a solar panel to determine how many watts of power it can provide.  Add up the number of solar panels needed in order to arrive at the number of watts of power that the house requires each day.


Example:  In a previous article we looked at a Grape Solar 390W.  As the name states this is a 390 watt panel.  In order to power our house that uses 5,000 watts per day we will need 13 panels plus 10% for contingencies (2) which equals 15 panels.


Are you ready for solar now?


See You Next Time!  Dr. Stripling

Solar Insolation map of USA