Every year, the light we get from the sun changes depending on the month we are in. This doesn’t seem like it will be changing anytime soon. So, we might as well understand what this means to our solar systems and the electric production we can expect out of them during the winter months.

In the winter, most places in the United States experience a 30-50% decrease in usable sunlight hours. This is going to equate to an approximate 30-50% decrease in electric production from solar panels as well. It is just that simple on the math side. Since sunlight is an essential part of making the solar systems work, when the sunlight hours go down, so does the energy production.

For example here are some average sunlight hour comparisons for some winter months compared to some spring/summer months.

• Los Angeles California  sunlight hours can change from about 6.1 sunlight hours per day in May to 3.8 hours in October.
• Jacksonville Florida sunlight hours can change from about 4.3 sunlight hours in February to 6.1 sunlight hours in May.
• Boise Idaho can have about 3.5 sunlight hours in February and shoots way up to 7.6 average sunlight hours in June
• Indianapolis Indiana can have 2.8 sunlight hours in January and a total of 6.7 sunlight hours on average in May
• The state of Kansas is usually around 3.9 sunlight hours in February and goes to about 6.5 hours of sun in May
• Billings Montana has a massive change from 2.9 hours of sunlight in January up to 7.2 hours of sunlight in June
• Las Vegas Nevada goes from 4.4 hours of sunlight in February to 8.1 hours of sunlight in June

It is really obvious to see that sunlight hours are always changing just as we rotate around the sun.  And as sunlight hours are essential to making the solar panels operate, the production of the solar systems is going to change as the year moves along.

solar panels covering a parking garage

One popular place for solar panels is starting to be the parking garage, or at least the roofs of parking garages. In fact, it is estimated that parking garages will become the most popular place for solar systems within the next 20 years for several different reasons.

One of the reasons that a parking garage makes such a good location for PV solar panels is due to the easiness of accessing the area for the actual installation. Many parking garages have open top levels for parking which also usually have unobstructed views towards the sun. Since shade is one of the worst things for a PV solar system, having no shade at all makes the solar electric systems much more efficient.  It will never change that maximum sunlight hours is key to the output of a solar system.

Another reason that businesses and universities are installing PV solar systems on top of parking garages is because it gives off a great show of interest in enviornmental awareness or “Going Greenness” to all of the people who use the parking garage. Having a PV solar system on top of a parking garage is a great way to prove you are going green.

A solar system mounted on a parking garage on a rack system

A third reason why the parking garage is becoming such a popular location for PV solar systems to be located is for cost reasons. When working on top of roofs, there is often the need to re-roof prior to installing the PV solar system. This is obviously very costly. But parking garages are usually built for the long haul, and often a Rack mounted solar system is installed, so the actual PV system will sit on a custom fabricated set of stands, which will last a lifetime when done properly.

One of the first questions that you will ask is: how many kilowatt hours (kwh) of electricity will my solar system produce where I live? That’s a great question, and one which everyone should ask themselves when you buy a solar system.

Las Vegas Solar Cells

There are several ways to figure this out. This post will explain the easiest way to determine the electric production for a solar system.

-The size of the solar system that we will evaluate is going to be for a 2.0 kw PV solar system. That is equal to a 2000 watt solar system because there are 1000 watts in a kw.
-The location that we will use to come up with an average sunlight hour number is Las Vegas Nevada. The average number of sunlight hours there is approximately 6.4 hours per day.
-We will use an energy loss amount of approximately 15% when the current goes from DC to AC

So, here is the equation:
2000 wattsDC X 6.4 hours/day = 12,800 watt hoursDC/day = 12.8 kwh/day on average in DC

Next, we will factor in the energy loss of 15%:
12.8kwh X .15= 1.92kwh of lost energy in the conversion to Alternating Current

Next we will figure out how much AC current is left to use:
12.8kwhDC-1.92kwh(lost)= 10.88kwh AC produced per day

Next, we will figure out how many kilo watt hours are produced each month on average:
10.88kwhAC/day X 30(days in the average month) = 326 kwhAC produced on average per month

Next, we will figure out how many kilo watt hours are produced on average per year:
326 kwh AC per month X 12(moths in the year) = 3,912 kwh per year

SO, in Las Vegas, Nevada, you could expect a 2.0 kwh solar system to produce approximately 3,912 kilo watt hours of electricity per year.

One of the assumptions is that there would be no shade hitting the solar panels as that would adversely affect the electricity production.

When you are buying a solar system in Las Vegas, be sure to ask the solar professional to explain the expected sunlight hours to you.  This is quite possibly the most important bit of data you need to know before writting any checks.

How many hours of sunlight where you live?

Here is some data on US Cities Sunlight Hours.  This data is the average number of sunlight radiation hours per year:

• Birmingham, AL  has average sunlight hours of  5.2  per day
• Phoenix, AZ          has average sunlight hours of 6.6  per day
• Tucson, AZ            has average sunlight hour of  6.7 per day
• Little Rock, AR    has average sunlight hours of 5.0 per day
• Los Angeles, CA  has average sunlight hours of 5.5 per day
• San Francisco, CA has average sunlight hours of 5.3 per day
• Sacramento, CA  has average sunlight hours of 5.5 per day
• Bakersfield, CA has average sunlight hours of 5.7 per day
• Denver, CO has average sunlight hours of 5.5 per day

• Grand Junction, CO has average sunlight hours of 5.7 per day
• Colorado Springs, CO has average sunlight hours of 5.5 per day
• Hartford, CT has average sunlight hours of 4.4 per day
• Bridgeport, CT has average sunlight hours of 4.4 per day
• Wilmington, DE has average sunlight hours of 4.5 per day
• Jacksonville, FL has average sunlight hours of 4.9 per day
• Miami, FL has average sunlight hours of 5.1 per day
• Tampla, FL has average sunlight hours of 5.2 per day
• Atlanta, GA has average sunlight hours of 5.0 per day
• Savannah, GA has average sunlight hours of 5.0 per day
• Augusta, GA has average sunlight hours of 5.0 per day
• Honolulu, HI has average sunlight hour of 5.5 per day
• Boise, ID has average sunlight hours of 5.0 per day
• Chicago, IL has average sunlight hours of 4.4 per day
• Rockford, IL has average sunlight hours of 4.5 per day
• Indianapolis, IN has average sunlight hours of 4.6 per day
• South Bend, IN has average sunlight hours of 4.4 per day
• Des Moines, IA has average sunlight hours of 4.8 per day
• Waterloo, IA has average sunlight hours of 4.6 per day
• Topeka, KS has average sunlight hours of 4.9 per day
• Wichita, KS has average sunlight hours of 5.2 per day
• Lexington, KY has average sunlight hours of 4.8 per day
• New Orleans, LA has average sunlight hours of 4.9 per day
• Baton Rouge, LA has average sunlight hours of 4.8 per day
• Portland, ME has average sunlight hours of 4.6 per day
• Baltimore, MD has average sunlight hours of 4.6 per day
• Boston, MA has average sunlight hours of 4.5 per day
• Worcester, MA has average sunlight hours of 4.5 per day
• Detroit, MI has average sunlight hours of 4.3 per day
• Grand Rapids, MI has average sunlight hours of 4.2 per day
• Minneapolis, MN has average sunlight hours of 4.6 per day
• Deluth, MN has average sunlight hours of 4.5 per day
• Rochester, MN has average sunlight hours of 4.5 per day
• Jackson, MS has average sunlight hours of 5.0 per day
• St. Louis, MO has average sunlight hours of 4.8 per day
• Kansas City, MO has average sunlight hours of 4.9 per day
• Billings, MT has average sunlight hours of 5.0 per day
• Missoula, MT has average sunlight hours of 4.4 per day
• Helena, MT has average sunlight hours of 4.7 per day
• Omaha, NE has average sunlight hours of 4.9 per day
• Las Vegas, NV has average sunlight hours of 6.4 per day
• Reno, NV has average sunlight hours of 5.8 per day
• Concord, NH has average sunlight hours per day of 4.6
• Newark, NJ has average sunlight hours per day of 4.4
• Atlantic City, NJ has average sunlight hours per day of 4.6
• Albaquerque, NM has average sunlight hours per day of 6.3
• New York, NY has average sunlight hours per day of 4.5
• Syracuse, NY has average sunlight hours per day of 4.2
• Albany, NY has average sunlight hours per day of 4.3
• Buffalo, NY has average sunlight hours per day of 4.1
• Charlotte, NC has average sunlight hours per day of 4.9
• Fargo, ND has average sunlight hours per day of 4.6
• Oklahoma City, OK has average sunlight hours per day of 5.3
• Portland, OR has average sunlight hours per day of 4.0
• Harrisburg, PA has average sunlight hours per day of 4.6
• Pittsburgh, PA has average sunlight hours per day of 4.2
• Bradford, PA has average sunlight hours per day of 4.2
• Philadelphia, PA has average sunlight hours per day of 4.3
• Providence, RI has average sunlight hours per day of 4.5
• Charleston, SC has average sunlight hours per day of 5.0
• Rapid City, SD has average sunlight hours per day of 5.1
• Memphis, TN has average sunlight hours per day of 5.1
• Chattanooga, TN has average sunlight hours per day of 4.7
• El Paso, TX has average sunlight hours per day of 6.3
• Austin, TX has average sunlight hours per day of 5.2
• Houston, TX has average sunlight hours per day of 4.7
• San Antonio, TX has average sunlight hours per day of 5.3
• Waco, TX has average sunlight hours per day of 5.3
• Dallas, TX has average sunlight hours per day of 5.3
• Salt Lake City, UT has average sunlight hours per day of 5.2
• Burlington, VT has average sunlight hours per day of 4.3
• Richmond, VA has average sunlight hours per day of 4.7
• Lynchburg, VA has average sunlight hours per day of 4.9
• Seattle, WA has average sunlight hours per day of 3.8
• Charleston, WV has average sunlight hours per day of 4.3
• Madison, WI has average sunlight hours of 4.6 per day
• Cheyenne, WY has average sunlight hours of 5.3 per day

Sunlight and solar panels go hand in hand.

It’s not a big mystery.  You could probably ask any homeowner who has a solar system what the one thing is they would wish to have more of, and everyone would say more hours of sunlight.  The reason is simple: Solar panels work when the sun is shining on them, period.  This is why solar sunlight hours are key to the overall success of a PV installation.

Since sunlight is such an important part to increasing the energy output of solar panels, use this knowledge to your advantage.  Don’t make the biggest mistake others have made when you are determining where to put your solar system, make sure to put the panels in a spot either on the roof or on the ground which gets the most sunlight hours.

For many people, there might be several good positions for a solar system on the property or roof that is being considered.  And for all of these people, there is going to be one spot that is going to allow the solar system to produce the most energy, which should be the goal.

There are a few different ways that you can calculate the sunlight hours for different spots on your property, one way would be to sit outside and keep track by viewing the sun over the course of several days, and preferably over the course of different months of the year.  Sunlight hours will always vary depending on what time of year(more sun in summer, less in winter, etc…)

But I don’t know about you, this is method is just not efficient and usually ends up being less than accurate.  So here is another more scientific approach.  Use a device that is called a Solar Pathfinder, or a Sun Eye device.  This is a small remote control looking thing that will allow the person using it to take an accurate reading of a certain point on your property to determine exactly how many sunlight hours exist for each month of the year.  There are lasers and other metrics involved that are a bit complicated to explain, but the device can read the surroundings(like trees and shadows) and give you the numbers you want.

Now of course, most people don’t have one of these devices since they can cost a couple hundred dollars, but for sure a local solar professional will have one, and for a small fee, should let you use it for a few minutes.  The cost of getting this reading will pay off 1000 times, so just do it.

Some other great advice would be to make sure that future sunlight will not be obstructed from things like new buildings, new garages, growing trees, or other objects which might get in the way of the sunlight hitting the solar panels.  It is very sad to know that right this minute people are losing hundreds of dollars across the country in energy they could be producing simply because a tree has grown and is now blocking the sunlight that could be hitting the solar panels they installed a few years ago.(but its very true) Don’t fall into that trap, and make sure to clear all trees that may block the solar rays.

If you would like to get the sunlight hours for different cities in the United States, you can see the solar radiation sunlight hour data here.

Everyone asks this question at some point.

How much solar is needed? Most of the time, this question is asked from a homeowner to a solar professional.  The answers of course vary.  What is usually assumed is the homeowner is asking about how much solar is needed in order to take care of all the electricity needs of the house.  Already there are all kinds of assumptions going on in the head of the homeowner.  But lets look at this question, “How much solar do I need?”

suntech solar panels

The answers are unique to every house, and there are several variables involved(location on the planet, shade from trees which would limit solar production, heat factors, snow factors, roof angles, or ground angles towards the sun, and then there is a very complicated variable of the local state incentives for solar power.

Let us start by doing an example of a house in San Francisco, California.  According to data from the department of energy, San Francisco receives 5.2 hours of sunlight on average per day per year.(What the heck does this mean?)  Well this means that if you took all the days in the year(the long summer days, and shorter winter days) and added up all the hours of sunlight, and then divided by 365, you would be left with a number.  The number is going to be more than 5.2, but all sunlight hours are not created equal.  This means that when it is 8:00am the solar panels do not produce as much electricity as they do compared to lets say the middle of the day.  Either way we need to come up with a number that equals the average number of sunlight hours per day per year that can create maximum output on the solar panels, and that number is 5.2.  Please realize, you want this number to be as high as possible.  For example, in some parts of Arizona this number can be over 7 hours per day, which is dramatically different than lets say 5.2 sunlight hours per day.

unirac solar panel mounting kit

Shade from trees is not good for making solar energy if the shade hits the solar panels.  That is a basic concept, so lets not freak out about that too much.  Some people go ahead and cut down some parts of trees in order to maximize the solar panels production.

Heat is a factor, but for much of the US, it is relatively small compared to factors that can reduce energy production.  The bottomline is worry about shading from trees before you worry about the solar panels being too hot or cold.

The angle of the solar panels towards the sun is a very important variable.  For the most part, the best angle for generating the most electricity throughout the year is this equation: Your location’s lattitude minus 15 degrees.  For the San Francisco Bay Area, that comes out to about 22 degrees give or take.  So solar panels around here should be mounted towards the sun at about 22 degrees.  That will make the most energy.  Yes of course, if you have panels that rotate during the day, they surely will make more electricity, but that type of setup is very costly and most homeowners don’t go that route, so lets not get into that too much.

solar panels mounted on roof

In general, you want to point your solar panels to the south at the angle we talked about in the paragraph above.  Since we are north of the equator, that is why we have the panels face south, it would be different if we lived down in Argentina.

So lets do a little math.

Look at your utility bill.  You should see the number of kilo watt hours of electricity you used for that particular month.  That is the number we need to know. Say for EXAMPLE you used 900 kwh for that particular month.  That would come out to 30 kwh/day since there are about 30 days in the month.  That would mean, your solar system would need to produce 30 kwh in AC to power your home, and keep up with your needs ON AVERGE.  So, the next logical question is, what does that mean in terms of a solar system size?

Here is the answer:

30 kwh per day means that in the exmple of San Francisco, where we have 5.2 sun hours per day on average, we need to make about 5.8 kwh for each of those sun hours.

Since Solar systems are not 100% efficient, some energy is lost in the process which can’t be avoided.  So, we need to find out how many solar panels are needed to do that for us. Lets assume that 20% of whatever is produced on the panels is lost in transmission.  So, the 5.8 kwh really needs to be 7.25 kwh produced at the panels which after losing 20% will really become 5.8 kwh.

That means that we need a 7.2 kw solar system.  From a post from a previous solar installer on this blog, he said systems are running about $6/ watt.  A 7.2 kw system is equal to 7200 watts.  So $6 x 7200= $43,200

WOW that’s a big number.  Now we need to look at the incentives, since the average price of electricity in the country is about $0.12/kwh.  That means that this system producing 900 kwh/month is saving: $108.00

That is not a great looking financial equation right now.  So we need some help, and there are incentives for that.  Take for example, the 30% federal tax credit.  That is a big number.  Then there are all types of local incentives, many times from the local utility company, and other times there are solar incentives on the city level.  Also, look for state level incentives.  In California for example, this type of system would be eligible fo rabout $10,000 in state level rebates.  Not bad, and definitely changes the financial equation.

Please post your comments, I will try to answer them all.