Factors to consider before buying solar panels

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 -, 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 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 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 $ 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 $ return each year, pay your $ 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

Buying a solar system can be exciting but it’s also a huge investment. So before you dive in, you need to be as informed as possible about solar panels and other important aspects of your purchase.

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1. Should I buy a solar system?

The cost of installing solar has been reducing over the past five years and will continue to become cheaper. However, the cost of waiting for further reductions is the increasingly expensive electricity bill that you pay your energy retailer. Right now you are effectively ‘renting’ your electricity and in five years you will have nothing to show for the thousands of dollars you have parted with. Should you then decide to purchase a solar system, the system price may be cheaper but when you add your aggregated energy bills you have paid to this price you find it very expensive indeed!

If you invest in solar now you become an energy ‘mortgagee’. You now own a big share of the electricity you consume and your savings start immediately to pay off your investment cost. In five years or so, your power savings have now paid off the cost of your investment. You now own your energy production outright and with a top tier REC solar system you will continue to make savings for a further 25 years at least.

Right now the power companies have secured the right to increase their rates by 25% annually over the next three years. Based on this rate your power bill is tipped to double! With the arrival of solar battery storage now is the time to start investing in solar and take control over your power bills.

Please also take note. There is still a solar government rebate available in all states and territories, and even though it is less than what used to be available in the past, on a 5 KW system for example you are likely to receive over $2,800-$2,900 in rebates.

Some solar installation companies offer financing options, which could reduce your initial outlays. This will allow you to use the electricity cost savings to help finance the repayments.

2. What makes a good quality solar system

A solar power system has a number of considerations to ensure customers are satisfied for many years:

• The solar panels purchased should be from a known and diversified manufacturer, so in future years you can receive follow up service or warranty back up, as the company is still around. Some solar manufacturers are currently in financial distress. Just read up on the internet about these issues.
• Only use high quality inverters with your system. The inverter undertakes the key work in your system. Every time there is a cloud and change in weather the inverter adjusts the output. Therefore an inverter never stops during the day. High quality brands which work well with your REC panels can be recommended by your local REC installer. Please avoid cheap inverters.
• Framing systems. There is no point buying quality panels and inverter and not fastening them with a quality framing system. E-Smart Solar can provide you with solid advice.

3. What is the expected lifespan of a solar power system?

The key components susceptible to failure are the solar panels and inverter. However, high quality products tend to have long life cycles which are reflected in the long warranties available, particularly for solar panels.

A solar panel is a relatively simple device with no moving parts. Solar panels typically have a 25 year output warranty and depending on the quality of the panel can be expected to last beyond this. Also solar panels which are exposed to wind, fluctuating temperatures and weather do deteriorate and each year produce a little less electricity. Cheaper panels, with less UV stabilized backing sheets, cheaper sealants and framing can deteriorate faster and more rapidly.

4. Why should I choose a good brand solar panel?

Arguably, the quality of your solar panels and the inverter are the most crucial factors in selecting a solar system. Over their 25 year output warranty period, solar panels will be subjected to more than 100,000 hours of relentless sunshine, extremes of heat and cold, wind, rain, hail and more. Australia and New Zealand offer an extraordinarily harsh climate for an electrical device.

A good brand like REC, with our extensive testing and quality control helps ensure that you get the most out of your system over its long operating life.

5. When should I NOT buy a solar system?

Most of homes or business premises in Australia can install a system but there are times when you should not buy a solar system. For example:

1. You have no roof space available which is facing North, North East, North West, West or East
2. There is strong shading of trees and building most of the day on your North, East or West facing roofs. Is there is a little shading for part of the day micro inverters or optimisers can offer a solution.
3. You have an asbestos roof
4. When the system has to be installed at a distance from the home (like at farm sheds) and the additional cost of running the electricity wires and poles back to the meter far outweighs the return on investment
5. You are out of your house/premises most of the time, therefore you cannot use the solar power when it is generated. The unused solar power will be exported to the grid. On average your energy retailer will credit you 11c  per kW, making the investment in solar financially less attractive. In this case solar would only be suitable if you also install a battery storage system.

6. As a consumer, what are my rights so I am assured it is a safe investment?

When you purchase a solar system you are protected by a variety of rights, warranties and acts as a consumer.

The overriding protection comes from the Australian Consumer Law which came into effect in January . This Federal Act provides protection for all consumers and is administered at a State level. Businesses are required by law to comply with the requirements of this act and it specifies where responsibilities lay, what rights consumers have and how to take action.

There are several specific issues described in the act which offer protection that is most relevant to solar system owners. They are summarised below:

When you agree to purchase a system, the company you sign a contract with is the first point of responsibility for all issues. They are the prime contractor in law, although they may subcontract some work such as installation and must ensure that all standards, laws and regulations are adhered to by their staff and their sub-contractors.

The company is the world’s best what is a solar cell supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

If you buy imported equipment, the warranty responsibility ultimately lays with the official importer, so it’s important to understand who that is, if it’s not your supplier. That’s why picking brand panels like REC offers you solid protection. However, should you decide to buy a cheaper panel and the manufacturer disappears, or the importer has gone bankrupt, the prime contractor has still a legal obligation to assist you.

Warranty terms and conditions vary by contract and supplier. It is important that you carefully read and compare the terms and conditions from different suppliers so you understand what you may be required to do to comply. For example many panel warranties only include the supply of a new panel. They do NOT include the labour component. REC supplies the panel and pays for the labour within 80 km of population centres.

In the event of a dispute, the Clean Energy Council can assist you with installer related issues. They also have a Solar Retailer Code of Conduct which some, but not all solar retailers have agreed to comply with.

In most cases, if there is a dispute that is unresolvable by discussion, the issue will default to the Australian Consumer Law act and you can approach your State body for advice on what course of action to take. This may include mediation, taking the case to the Small Claims Tribunal or to court.

7. What size panels should I buy?

There are various sizes of solar power panels available. The higher the output, the higher will be the efficiency of the solar panel (as long as the size of the solar panel is the same – of course). A number of years back the 170W to 190W solar power panels were considered a high wattage panel.

Today the technology has improved so that 260W panels are considered a good solid efficiency as more solar power can be produced in a smaller roof space.

Nevertheless some higher efficiency panels can offer a shorter return on investment period.

8. What should I check in a quote before I buy a solar power system?

Make sure the installer comes to your house and has a look, before you get the quote. Internet based solar companies quote you for a system over the can spell trouble for the install quality, as they cannot truly appreciate the individual set up of your house. Are the tiles brittle? Where are any surrounding trees, which can cause overshadowing? What about antennas and chimney locations – which in the future through overshadowing can affect the system output significantly? How old is your switch board and does it need upgrading to handle solar? All these questions only become clear through a proper home inspection.

When internet or call centre based solar companies sell you a system they use the one size fits all approach and give you a standard price. They then negotiate with a sub-contract installer to install your system for an agreed price. So the sub-contracted installer holds all the risk should your system requires extra cabling or is a particular labor intensive installation. It is more likely in this situation for the installer to find the quickest way, not necessarily the best way to install the system. Being a sub-contractor to the company that sold you the system, the installer’s relationship with your system is one of “get in and get out as quick as possible”.

Should you, in the years to come require support for your solar system, the internet based solar company might not be around anymore, or should they still exist are very likely to send a new sub-contractor to check out the issue. It is better, like with your car, to have the same reliable mechanic undertake the ongoing work. Someone who is familiar with your particular circumstances/system.

In general the quotation should provide solar system specifications like quantity of panels, brand and model of panels, system size and likely output per annum in kW/h, capacity and output of the inverter or if micro inverters are to be used the brand and warranty conditions.

A proper, considered quote should include also datasheets of the supplied products. In general make sure your quote includes:

• • Solar PV modules – brand, model and manufacturer’s warranties;
  • Mounting frames – brand, warranties and which part of the roof to be installed;
  • Inverter – brand, capacity and manufacturer’s warranty;
  • Any additional metering cost – if not included in the price, make sure this aspect is clearly outlined in the quote;
  • Travel and transport requirements if not included;
  • Any trench digging if solar to be installed on outbuildings e.g. farms.
  • At hand over, make sure the installer gives you a system user manual.

The quotation should also specify a total price, together with proposed start and completion dates. The quotation should form a basis for your contract with the designer/installer. Deposit requirements for the system by law should not be more than 10 per cent. Usually the majority of the solar system needs to be paid for on the day of installation.

9. What are the ongoing costs of running a solar power system?

It is advisable to organise some maintenance inspections every few years to make sure that all parts of your solar electric power system are operating correctly. With a tilt angle of 10 degrees or greater solar panels are self cleaning. If you install panels onto a flat roof without panel tilts you will need to hose down your panels every couple of months.

10. Is my solar power system insured?

Most home and building insurance policies cover home solar systems on roofs and garages. A solar electric power system is just like the hot water system an extended part of the home.

However most insurance companies would like to be contacted so that you can let them know you added your solar electric power system. Make sure you let them know the value of the system (before rebates) so they will know the actual replacement cost.

For more information, please visit Topcon N-type Solar Panel Supplier.