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I am a retired electrical engineer who worked in the solar panel industry for a number of years. There are many thing about solar that are not explained well, or in some cases distorted. I would like to help clarify some of those.
Is solar right for me?.
There is not a simple answer to that question. It depends on many factors, including where you live, the cost of electricity, the availability of electricity, the orientation of your home, and many others.
Resources
Much of the information presented here is based on the "Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors" published by the National Renewable Energy Laboratory (NREL) in Golden Colorado. The information in that document is based on data collected and computer models. The data collected was between 1961-1990, and conditions may have changed slightly since that time.
Solar Facts
Sunlight varies during the day even if the sky is clear. This is due partly to the amount of atmosphere the light travels through before striking your solar panel. Around mid-day the light is nearly overhead but this does depend on the latitude where you live and the season. At that time, the light absorbed by the atmosphere is minimum. In the early morning and late afternoon the light travels through the atmosphere at an angle and more light is absorbed. When directly overhead, the sunlight intensity is about 1000 watts per square meter. That means if you have a solar panel that measures about 40 inches by 40 inches, it will be exposed to approximately 1000 watts of sunlight. Double the size of the panel and you will double the power available.
Of course not all that sunlight is useful for producing electricity, and some is lost as heat. Very high grade solar materials used by NASA may have an efficiency or 40%, but normal consumer grade panels usually only are able to convert between 10-20% of the sunlight to useful power. If you have room for more panels on your roof, twice as many 10% efficient panels will produce the same power as a lower number of more efficient panels.
The angle the light strikes your solar panel affects the amount of power it can produce. The maximum power will be produced when the sun is exactly perpendicular to your panel since the light is most intense at that time. Some commercial systems use a tracker to tilt the panels so they always face the sun perpendicular in both vertical and horizontal directions. This does increase the amount of power produced, but the sun angle relative to the atmosphere cannot be overcome. Trackers include one and two axis trackers, one axis only tilts the panels in an East-West direction, while two axis include North-South tilt. Trackers are expensive to purchase, install and maintain, so they are seldom used for residential systems. Most residential solar panels are mounted at a fixed N-S tilt equal to the latitude and facing as near to South as possible. This results in the optimum that can be achieved with a fixed system. Note that most homes do not have steep roofs that are equal to the latitude, and often the roof area facing directly South is only a fraction of the total roof area. As a result installation of solar shingles on a roof will result in less than optimum performance.
Sunlight varies with the season, with a minimum around December 21 and a maximum around June 21 in the northern hemisphere. The further you live from the equator, the more pronounced this effect is. Even though the sun may shine for 12 hours during the day, the effective time is less than that. The NREL document referenced above lists the average amount of light collected in one day in terms of the number of hours of full sunlight equivalent, taking into account the atmosphere and angle factors mentioned above.
The electric company does not sell you Watts of electricity as such. Instead they calculate how many watts you use multiplied by the number of hours. This is normally reported and kWH, i.e. the number of thousands of watts times the number of hours. This is actually a measurement of total energy consumed. The NREL document lists the average available sunlight in the same units, kWH per day.
Here are some examples of the average full sun effective hours per day for various locations in the USA according to the Solar Radiation Manual. The data appear to include cloudy days since the maximum for some areas occurs in months other than June. There is a +/- 9% uncertainty in the values presented.
Daytona Beach, FL Fixed array, tilted to latitude 29 degrees
Average hours per day varies from 4.1 - 6.3 hours per day from month to month, with a yearly average of 5.2 hours per day.
Phoenix, AZ Fixed array, tilted to latitude 33 degrees
Average hours per day varies from 4.9 - 7.5 hours per day from month to month, with a yearly average of 6.5 hours per day.
Topeka, KS Fixed array, tilted to latitude 39 degrees
Average hours per day varies from 3.4 - 6 hours per day from month to month, with a yearly average
of 4.9 hours per day.
Anchorage, AK Fixed array, tilted to latitude 61 degrees
Average hours per day varies from 0.6 – 4.6 hours per day from month to month, with a yearly average
of 3.0 hours per day.
Shading
The effect of shade from nearby trees or buildings is another consideration. If a solar panel is even partially shaded, it will likely reduce the power it can produce by far more than you might expect. This can be overcome to some degree by installing local inverters on each solar panel rather than one big inverter for the entire array. In some cases shading may cause permanent damage to a panel unless it is designed to protect against this.
Rated power of panels
Solar panels are tested and rated for power output at room temperature. As with all semiconductor devices the voltage of each individual solar cell will decrease as the temperature increases (typically 2mV per degree Celsius). A typical solar cell will produce about 400-600 mV, with an average of about 500 mV. When placed in sunlight, a solar panel will rise in temperature by about 20 degrees Celsius above the ambient air temperature. If the outside air temperature is around room temperature, this will cause the voltage to drop by about 40 mV, or around 8% of the room temperature value, reducing the rated power by about 8%. So a solar panel rated for 100 Watts would typically produce about 92 Watts due to the temperature increase. In colder climates the panel will produce much closer to it’s rated power, but this usually occurs in winter when the available sunlight is less. The power will likely be slightly less in summer but then the sunlight is more intense too.
Knowing the average daily full sunlight equivalent and the expected power from a solar panel, we can calculate the expected kWH per day. The yearly average for most locations in the USA is about 5 hours, multiplied by 92 watts (100 watt panel) means you could expect about 460 WH or 0.46 kWH of energy per day for each 100 watt panel. The inverter is somewhat less than 100% efficient, and probably 90-95% efficiency is reasonable, resulting in about 0.425 kWH per day from each 100 watt panel. A typical system might be rated at 3 kW (30 each 100 watt panels), producing about 12.75 kWH of electricity per day averaged over a year.
Check your electric bill to see what you are paying for a kWH of electricity now. This varies widely around the country. I have heard of rates over $0.30 per kWH in California, while I am paying less than $0.10 per kWH through my REA Co-op. You may also be charged extra for peak times of the day. What makes economic sense to one customer may not apply to another.
If you are in an HOA, be sure to check the rules. Some HOA’s take a dim view of solar panels and prohibit them is some cases.
Grid Tied System
Unless you live on a mountain top with no power poles for miles, it is not common to have a battery storage system. Instead most residential users opt for a grid tied system. The idea is to produce more electricity than you need during the daylight hours and sell it back to the power company. At night you will buy the power back, hopefully breaking even at the end of the day. There are some pitfalls to be aware of with this type system.
1. There is no guarantee that the power company will continue buying your power at retail rates in the future. Many solar customers in states like California and Nevada have been blindsided when the power company decides to pay wholesale rates for power you generate and sell it back to you at retail rates. This frequently totally destroys the concept of paying for the system in 10-20 years as advertised when you bought (or worse yet leased) the system.
2. The grid tied system will stop producing power when the power company system goes down due to a downed power line or blown transformer, etc. This is for safety reasons, since it would not be safe for a lineman to be working on a power line that is supposedly dead. A safety switch device is required for all grid tied systems to prevent this hazard.
3. Your power bill will likely not drop to zero even if you produce all the power you need every day. Most power companies charge a monthly service fee just for being connected whether or not you use power. My power company charges $27.50 per month for example.
Myths debunked
Several websites claim to debunk some solar myths, and in some cases they are accurate. In some cases they are a bit misleading; for example claiming solar panels still produce power when it is cloudy. While this is true, they do not tell you that the amount of power produced may be so low as to be almost useless. The fact is half intensity of sunlight produces half the power of full sunlight or perhaps slightly less.
Be cautious of the claim that if you save $1500 per year on electricity that you will pay for a $30,000 system in 20 years. While the numbers seem to add up, consider what you could earn if you invested the same $30,000 in something that pays interest or dividends such as a CD or mutual fund. The S&P 500 claims to average about 10% per year return over many years. If you invested in an index mutual fund, you could easily average a $3000 return each year, pay your $1500 electric bill and have money left over to grow in value. Even a CD paying 5% per year would pay your electric bill, and you would still have the initial investment at the end of 20 years.
Consider the condition and life of your roof. If you need to replace the roof before the solar system wears out, you could incur a significant additional expense to remove the solar panels, replace the roof, and re-install the solar panels.
A solar system will increase the value of your house. I suggest talking to a few realtors to see if this is reality or not. This is especially true if you are considering leasing a system.
Bob Butcher
Is solar right for me?.
There is not a simple answer to that question. It depends on many factors, including where you live, the cost of electricity, the availability of electricity, the orientation of your home, and many others.
Resources
Much of the information presented here is based on the "Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors" published by the National Renewable Energy Laboratory (NREL) in Golden Colorado. The information in that document is based on data collected and computer models. The data collected was between 1961-1990, and conditions may have changed slightly since that time.
Solar Facts
Sunlight varies during the day even if the sky is clear. This is due partly to the amount of atmosphere the light travels through before striking your solar panel. Around mid-day the light is nearly overhead but this does depend on the latitude where you live and the season. At that time, the light absorbed by the atmosphere is minimum. In the early morning and late afternoon the light travels through the atmosphere at an angle and more light is absorbed. When directly overhead, the sunlight intensity is about 1000 watts per square meter. That means if you have a solar panel that measures about 40 inches by 40 inches, it will be exposed to approximately 1000 watts of sunlight. Double the size of the panel and you will double the power available.
Of course not all that sunlight is useful for producing electricity, and some is lost as heat. Very high grade solar materials used by NASA may have an efficiency or 40%, but normal consumer grade panels usually only are able to convert between 10-20% of the sunlight to useful power. If you have room for more panels on your roof, twice as many 10% efficient panels will produce the same power as a lower number of more efficient panels.
The angle the light strikes your solar panel affects the amount of power it can produce. The maximum power will be produced when the sun is exactly perpendicular to your panel since the light is most intense at that time. Some commercial systems use a tracker to tilt the panels so they always face the sun perpendicular in both vertical and horizontal directions. This does increase the amount of power produced, but the sun angle relative to the atmosphere cannot be overcome. Trackers include one and two axis trackers, one axis only tilts the panels in an East-West direction, while two axis include North-South tilt. Trackers are expensive to purchase, install and maintain, so they are seldom used for residential systems. Most residential solar panels are mounted at a fixed N-S tilt equal to the latitude and facing as near to South as possible. This results in the optimum that can be achieved with a fixed system. Note that most homes do not have steep roofs that are equal to the latitude, and often the roof area facing directly South is only a fraction of the total roof area. As a result installation of solar shingles on a roof will result in less than optimum performance.
Sunlight varies with the season, with a minimum around December 21 and a maximum around June 21 in the northern hemisphere. The further you live from the equator, the more pronounced this effect is. Even though the sun may shine for 12 hours during the day, the effective time is less than that. The NREL document referenced above lists the average amount of light collected in one day in terms of the number of hours of full sunlight equivalent, taking into account the atmosphere and angle factors mentioned above.
The electric company does not sell you Watts of electricity as such. Instead they calculate how many watts you use multiplied by the number of hours. This is normally reported and kWH, i.e. the number of thousands of watts times the number of hours. This is actually a measurement of total energy consumed. The NREL document lists the average available sunlight in the same units, kWH per day.
Here are some examples of the average full sun effective hours per day for various locations in the USA according to the Solar Radiation Manual. The data appear to include cloudy days since the maximum for some areas occurs in months other than June. There is a +/- 9% uncertainty in the values presented.
Daytona Beach, FL Fixed array, tilted to latitude 29 degrees
Average hours per day varies from 4.1 - 6.3 hours per day from month to month, with a yearly average of 5.2 hours per day.
Phoenix, AZ Fixed array, tilted to latitude 33 degrees
Average hours per day varies from 4.9 - 7.5 hours per day from month to month, with a yearly average of 6.5 hours per day.
Topeka, KS Fixed array, tilted to latitude 39 degrees
Average hours per day varies from 3.4 - 6 hours per day from month to month, with a yearly average
of 4.9 hours per day.
Anchorage, AK Fixed array, tilted to latitude 61 degrees
Average hours per day varies from 0.6 – 4.6 hours per day from month to month, with a yearly average
of 3.0 hours per day.
Shading
The effect of shade from nearby trees or buildings is another consideration. If a solar panel is even partially shaded, it will likely reduce the power it can produce by far more than you might expect. This can be overcome to some degree by installing local inverters on each solar panel rather than one big inverter for the entire array. In some cases shading may cause permanent damage to a panel unless it is designed to protect against this.
Rated power of panels
Solar panels are tested and rated for power output at room temperature. As with all semiconductor devices the voltage of each individual solar cell will decrease as the temperature increases (typically 2mV per degree Celsius). A typical solar cell will produce about 400-600 mV, with an average of about 500 mV. When placed in sunlight, a solar panel will rise in temperature by about 20 degrees Celsius above the ambient air temperature. If the outside air temperature is around room temperature, this will cause the voltage to drop by about 40 mV, or around 8% of the room temperature value, reducing the rated power by about 8%. So a solar panel rated for 100 Watts would typically produce about 92 Watts due to the temperature increase. In colder climates the panel will produce much closer to it’s rated power, but this usually occurs in winter when the available sunlight is less. The power will likely be slightly less in summer but then the sunlight is more intense too.
Knowing the average daily full sunlight equivalent and the expected power from a solar panel, we can calculate the expected kWH per day. The yearly average for most locations in the USA is about 5 hours, multiplied by 92 watts (100 watt panel) means you could expect about 460 WH or 0.46 kWH of energy per day for each 100 watt panel. The inverter is somewhat less than 100% efficient, and probably 90-95% efficiency is reasonable, resulting in about 0.425 kWH per day from each 100 watt panel. A typical system might be rated at 3 kW (30 each 100 watt panels), producing about 12.75 kWH of electricity per day averaged over a year.
Check your electric bill to see what you are paying for a kWH of electricity now. This varies widely around the country. I have heard of rates over $0.30 per kWH in California, while I am paying less than $0.10 per kWH through my REA Co-op. You may also be charged extra for peak times of the day. What makes economic sense to one customer may not apply to another.
If you are in an HOA, be sure to check the rules. Some HOA’s take a dim view of solar panels and prohibit them is some cases.
Grid Tied System
Unless you live on a mountain top with no power poles for miles, it is not common to have a battery storage system. Instead most residential users opt for a grid tied system. The idea is to produce more electricity than you need during the daylight hours and sell it back to the power company. At night you will buy the power back, hopefully breaking even at the end of the day. There are some pitfalls to be aware of with this type system.
1. There is no guarantee that the power company will continue buying your power at retail rates in the future. Many solar customers in states like California and Nevada have been blindsided when the power company decides to pay wholesale rates for power you generate and sell it back to you at retail rates. This frequently totally destroys the concept of paying for the system in 10-20 years as advertised when you bought (or worse yet leased) the system.
2. The grid tied system will stop producing power when the power company system goes down due to a downed power line or blown transformer, etc. This is for safety reasons, since it would not be safe for a lineman to be working on a power line that is supposedly dead. A safety switch device is required for all grid tied systems to prevent this hazard.
3. Your power bill will likely not drop to zero even if you produce all the power you need every day. Most power companies charge a monthly service fee just for being connected whether or not you use power. My power company charges $27.50 per month for example.
Myths debunked
Several websites claim to debunk some solar myths, and in some cases they are accurate. In some cases they are a bit misleading; for example claiming solar panels still produce power when it is cloudy. While this is true, they do not tell you that the amount of power produced may be so low as to be almost useless. The fact is half intensity of sunlight produces half the power of full sunlight or perhaps slightly less.
Be cautious of the claim that if you save $1500 per year on electricity that you will pay for a $30,000 system in 20 years. While the numbers seem to add up, consider what you could earn if you invested the same $30,000 in something that pays interest or dividends such as a CD or mutual fund. The S&P 500 claims to average about 10% per year return over many years. If you invested in an index mutual fund, you could easily average a $3000 return each year, pay your $1500 electric bill and have money left over to grow in value. Even a CD paying 5% per year would pay your electric bill, and you would still have the initial investment at the end of 20 years.
Consider the condition and life of your roof. If you need to replace the roof before the solar system wears out, you could incur a significant additional expense to remove the solar panels, replace the roof, and re-install the solar panels.
A solar system will increase the value of your house. I suggest talking to a few realtors to see if this is reality or not. This is especially true if you are considering leasing a system.
Bob Butcher
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