March 21, 2018 Solar Energy Written by Greentumble Editorial Team
How Many Solar Panels Do I Need?
Solar power is becoming more efficient

and more affordable. Government initiatives, called net metering laws, now require many power companies to buy excess power produced by solar powered homes during sunlight hours by giving credit for power during off-hours when the use [1].

Can you supplement or possibly supplant your fossil-fuel powered electricity use with solar-generated power?

The average home in the United States uses about 900 kilowatt hours (kWh) per month [2].

Obviously, there can be a significant range depending upon the size of your home, your family, your energy consumption habits and whether you are in Alaska and using a lot of power to heat your home or in Florida and using a lot of power to cool your home or whether you live in a more temperate climate.

You can check your electric bill to find your usage. Adding a cushion for those times when your solar panel might not be operating at peak performance, and because it’s easier to do the math, let’s examine how many solar panels you will need to power 1000 kilowatt hours per month.
 

How many kWh does a solar panel produce per month?

First, it is important to note that not all solar panels are created equally.

Presently, the range of efficiency, that is, how much of the sun’s energy hitting the solar cell is converted into electricity averages 20 percent, but can vary depending on the quality of the solar panel. A premium panel, manufactured with more expensive materials will cost more and be more efficient.

Polycrystalline solar panels

Polycrystalline solar panels placed on the roof to achieve the best efficiency

A solar panel operating at 20 percent efficiency produces around 265 watts of power per hour [3].

Manufacturers are required to label the panels with the number of kilowatts they can power per hour during ideal conditions, i.e. direct sunlight on a cloudless and sunny day. This number is called a Standard Test Condition rating (STC) and will be for example 265 if the panel produces 265 watts of power.

If for example, the solar panel has a rating of 250 watts of power, and the panel received a full hour of direct sunlight, and no other factors diminished the power, then you would get 250 watt-hours of electricity.

On average, one such panel would produce one kilowatt hour per day and 30 kWh per month.
 
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How many kWh per month can you expect to generate from sunshine in your area?

Many solar power company websites provide calculators for the average annual kWh for areas across the United States. Combining all of the sunshine that falls on the solar panel over a 24-hour period, the average roof in the United States gets about four hours of “full” or “usable” sun a day.

Again, this number will vary depending whether you live in a cloudless desert or in foggy mountains.

As an example, the U.S. Department of Energy offers a map that shows a solar energy potential in each state of the USA. But goes on to caution that this estimate is based on a south facing array and that you must consider the tilt of your roof, citing that a solar array facing west rather than south can cause a nearly eleven percent output reduction.

Sunny locations are more advantageous for solar power generation.

Sunny locations are more advantageous for solar power generation.

But let’s get back to the example so that we can have a starting point for considering the size of your domestic solar system.

If you have one 250-watt panel receiving four hours of sun, then you will get 1000 watts or one kWh per day from that panel. If you have four panels, you will get 4 kWh per day.

If you have 33 panels, assuming a 30-day month, you will get 1000 kWh per month.
 

Or will you? What can affect panel output efficiency?

The Standard Test Condition rating is based on ideal conditions converting the sun’s energy into power. But the solar system itself is not 100 percent efficient in converting the energy into power.

  • A solar system requires an inverter to convert the Direct Current power the photovoltaic cells receive from the sun to Alternating Current power used in our homes. Power is lost as it goes through the inverter, which can be a single inverter per system, or a single inverter per solar panel. It is estimated that about three percent of electricity is lost passing through the inverter.
  • Different materials used to manufacture the cells can resist the flow of electricity as can resistance passing through the cables.
  • Inevitably, solar panels will pick up grime and dirt from the atmosphere, blocking full sunshine.
  • Temperature can effectively create power losses as well. While it seems that a hot, sunny local would be ideal, the fact is that the STC is based on a temperature of 25 degrees Celsius (77 degrees Fahrenheit). The flow of electrons across the photovoltaic cells is not as efficient at higher temperatures. All in all, solar companies estimate a de-rating factor of around 20 percent.
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    As they are marketed, solar system sizes usually take this into account.

    Clean solar panels will have better efficiency.

    Clean solar panels will have better efficiency.

    Be aware that system sizes are calculated inversely in the United Kingdom and the United States.

    Thus, a typical 1 kWh system in the UK is estimated to produce 850 kWh unit per year, a 2 kWh would create around 1,700 kWh units per year and a 5 kWh system is estimated to create 4,500 kWh [5].

    In the United States, a 5 kWh system is expected to produce 7,161 kWh annually.

    Accordingly, if you are talking with a solar installation company about purchasing a system, then chances are they are already including the 20 percent de-rating factor in their estimate.

    But it is best to do your own preliminary calculations and then talk with them about their energy system offering so that you can be certain the system you purchase provides your anticipated needs.
     
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    So how many solar panels do you really need to get 1000 kWh per month?

    If your goal is to produce 1000 kWh per month, then truly you must produce 1250 kWh per month to allow for loss in output efficiency.

    Remember, if you are receiving an average of four hours of usable sunshine per day and your solar panel is rated at 250 watts of power, then you will need forty panels to reliably generate 1000 kWh per month.

    A typical solar panel today is about five and a half feet by three feet.

    But the number of panels and consequent space needed can vary depending upon whether you select lower-efficiency economy panels or high-efficiency premium panels.

    Consider this table:

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    So go out and take a look at your roof and consider the sunlight falling on it. Think about the size and placement of solar panels. Do a cost analysis of the savings you will experience from creating your own clean, renewable energy.

    Read our article about available solar power incentives and rebates that might make financing your savings attractive now.

     


    References

    [1] https://www.seia.org/initiatives/net-metering
    [2] https://www.eia.gov/tools/faqs
    [3] https://solarpowerrocks.com/solar-basics/how-much-electricity-does-a-solar-panel-produce/
    [4] https://www.theecoexperts.co.uk/which-solar-panels-are-best-how-much-should-i-expect-pay