Issues With PV Cables on Residential Rooftops

Cable management is difficult with rooftop PV installations because the array is coplanar to the roof and it is difficult to see and access the cables once each module is in installed. There are two major issues that occur with these types of installations.

  1. Unprotected PV cables can be damaged by squirrels or other animals that will nest under the array and claw or chew the cables and connectors.
  2. Obstructions under or around the edge of the PV array can hinder drainage and cause leaks into the roof.

To address issue #1, good wire management is one of the best available defenses against squirrels and other nesting creatures. The NEC states that USE-2 (PV wire/cable) must be secured within 24” of a junction box and then every 24” of length. While this is adequate for other installations, roof mounted arrays need better support and protection for the cables. Every 12” to 18” is minimal to prevent drupes in the cable.

The picture below is of a ground mounted array where wire management is easy to achieve. It is exceedingly difficult to achieve this with a coplanar roof mounted PV array. Even with this level of wire management, a squirrel can still chew on the cables; they are still unprotected.

Underneath view

Wire management methods vary depending on the racking manufacturer but none of them fully protect the PV cables from squirrel chewing damage. Here are a few methods.

SnapNrack uses their open rail system for wire management. The plastic clips secure the cables into the rail trough.


Generic cable clips that attach to the module frame.

Generic 1Generic 2


Cable ties (zip ties) These are convenient to use but they must be rated for the lifetime of the array.

Cable tiesCable ties

The problem with zip ties is with their strength and usage rating. The typical black zip tie may not have a sufficient rating for the lifetime of the PV module.

Extreme UV

Another code requirement is the minimum bending radius of USE-2 and listed PV wire. USE-2 has a minimum radius of 5x the diameter of the cable; that is about the curve of a large cup. PV wire usually requires a minimum radius of 8x the diameter of the wire; that is about the curve radius of a good hamburger.

Another method of protecting the PV array cables from damage is to screen the exterior of the array to prevent entry. There are several products on the market but I have heard stories where squirrels still found a way inside the PV array.

Array screen 1Array screen 2

To address issue #2, obstructions under or around the PV array can cause water to enter the roof. The screening could cause a problem if leaf debris reaches the top of the PV array. If leaf debris is not an issue, the screen may be a good solution.

Any time there is a possibility of leaf debris reaching the upper portion of the PV array, it is imperative that there are no obstructing debris from being washed asway. If leaf debris is present, it may be best to keep the array high above the roof and use the best wire management techniques without screens.

nest under PV array
Nest under a PV module
Wires chewed by a squirrel









If there is not a possibility of leaf debris reaching the PV array, screening is the best method to prevent nesting creatures from getting inside the array and damaging the cables and connectors.

Kelly Provence
IREC Certified Master PV Trainer
Solairgen School of Solar Technology

Styrofoam packing

Shouldn’t Packaging of Renewable Components be Renewable Too?

As far as pure energy goes, it doesn’t get better than solar energy. Photovoltaic (PV) energy technologies are improving and growing at an impressive rate. This industry hopes to be a major source of electrical energy on Earth in the not-too-distant future. At the rate it is growing, that will probably happen within 10 to 20 years. That sounds great but there is a dark side to the PV industry’s growth and it is in the way materials are facilitated to the complete installation of a PV system.

Before I talk about this environmental dark side of the solar PV industry it is worth noting its bright clean side. If all the energy required to manufacture and facilitate the installation of the PV system was derived from fossil fuels, it would take only two years of operation to offset that non-renewable fossil-fuel energy. As renewable energies become more dominant that figure will become shorter. Ultimately the process of offsetting the non-renewable, fossil-fuel energy required to manufacture and implement a PV system could be close to zero. Unfortunately, there is still an environmental dark side that has not been offset. This has to do with the negative effects of the front end and back end of product manufacturing, mining and packaging.

On the front end, mining is necessary to get the raw materials to manufacture PV cells, PV modules, inverters, batteries, support structures and electrical conductors. It is possible to reduce the effects of mining by using materials that are more plentiful in the Earth’s crust but there will always be a requirement for mining. It is also possible to recycle materials that are currently reaching the end of their useful life. The obstacle here is usually financial since it is often lower cost to mine for new raw materials than to recycle them from previously manufactured products containing these same raw materials. Interestingly, even if it takes less energy to recycle a material, it may be more difficult and more expensive to completely facilitate the recycling process. When recycling is disadvantaged by these economics, tax incentives or subsidies should be put in place to make recycling more lucrative. The back end of manufacturing is packaging.

Almost every manufactured product is packaged before being sent into the marketplace.

PV modules are packaged on wooded pallets with plastic separators. They are often wrapped in stretch plastic or they are packaged in pairs with cardboard. Plastic is a forever-material that is almost completely non-recyclable. Wood and cardboard-based packaging is much more recyclable. Its recycling should be encouraged though tax incentives or subsidies.

  • Inverters are packaged in cardboard and protected internally with either Styrofoam, polystyrene or corrugated fiberboard. Styrofoam and polystyrene are forever-materials that are non-recyclable and should not be used. Paper based products like cardboard should be used instead.
  • Batteries are typically packaged the same way as inverters but primarily with cardboard.
  • Small balance-of-system components (BOS) are shipped in cardboard, plastic and they use bubble wrap and foam peanuts to secure the items during shipment.

Inverter packaging

Packaging PV products with forever-products that are non-renewable and non-biodegradable is completely unacceptable. The purchaser of these products has a lot of influence over how these products a packaged; the best action is an email or phone call to the sales associate for the manufacturer as well as to the distributor of the products. Solar PV is a renewable product industry, we need to remove non-renewable, non-biodegradable products from the packaging process.

Kelly Provence
Solairgen, Inc.
IREC and NABCEP Certified PV Trainer


At first glance I thought this term was describing a dark German beer that had lost its carbonation. I was wrong but it is actually almost as bad. Dunkelflaute translates to “dark doldrums” and refers to a period when there is no sunlight or little wind for renewable energy generation. This is particularly serious in areas where solar and wind energy provide a substantial portion of electrical energy generation and is even worse at latitudes above 40˚. Germany faces this problem on a regular basis but are very creative in the ways they deal with it. The first solution may appear to be batteries or other means of storing electrical energy which is a partial solution; they also store energy by other means and incorporate smart energy efficiency.

There are solutions that don’t include conventional energy sources such as nuclear and fossil fuels, and thermal energy (steam driven generators) has been the workhorse of the electrical energy industry for the past 100 years. Electrical energy from renewables can be converted to thermal and stored for a short period of time. Electrical energy can be converted to any number of energy stores such as water reservoirs for hydroelectric and compressed air for air turbine generators. It is also possible to convert renewable electrical energy to hydrogen through electrolysis. This process is fairly easy but storing it as a condensed fuel is energy intensive. Smart energy efficiency is another way to reduce dependence on electrical energy when it is in short supply.

Smart energy efficiency sounds good and it is very doable in this age of smart computers but in order for this to work, we have to give up at least some privacy. Every appliance that uses energy will need to be electronically visible and controllable by the smart energy network (SEN). In the U.S. we are seeing this occur with the new listing for interactive inverters. They will be smart in their own accord with electrical energy fluctuation and controllable by the utilities for further control features to make the electrical grid more reliable. California and Hawaii have already implemented these rules and the rest of the U.S. will follow within a few years. Home monitoring of electrical circuits is becoming an affordable norm. It is reasonable to assume that these devices will eventually be required to communicate with a smart energy network. Everything that uses energy will communicate with the network. This may sound futuristic but it’s not and I don’t think it will turn into some dark dystopian landscape of existence where machines control humanity.

A more conservative but less desirable option is to use conventional energy sources such as nuclear or fossil fuels for base level energy demand. Nuclear would get the vote if CO2 emission reduction is the primary concern, but I think what we are going to see is a blend of renewable energy, energy storage, smart energy management and clean conventional energy production.

There is no way to prevent Dunkelflaute but we can progress to a cleaner and stable electrical grid – and the best way to prevent a good beer from losing its carbonation is to drink it faster.

On Grid Solar Farm

Working with Electric Utilities

Balancing the Needs of PV Customers with Utilities

Solar contractors and solar customers often find themselves on the adversarial side of utilities when interconnecting PV systems to the electrical grid. Most of the time this is due to a lack of seeing the other’s side and/or understanding of the other’s position.

Let’s first look at the utility’s position regarding PV systems. Many utilities offer incentives or encourage their residential customers to have PV systems installed on their properties. The reason for this is almost always for the purpose of appeasing the wishes of their customers. This an effort of goodwill because they know that electrical customers like the idea of energy coming from a clean renewable source and most of them also like the idea of owning their own energy. The benefit to the utility is not financial unless the PV system energy reduces load demand during periods when the cost to generate that power from other sources is high.

With residential customers, those periods are mostly in the afternoon and evening and then again in the morning as the sun starts to rise. This being the case, the PV systems with smart energy storage are the only systems that could financially benefit the utility as well as the owner of the system. They store the energy produced during the middle of the day and then feed it into the home and grid during the peak load periods.

With commercial customers the situation is different. The peak load period may be during the normal business hors of the day or 24 hours a day. These customers with PV systems may be shaving loads throughout the day or they may need to shift loads to a time of day other than the solar hours of the day. For the utility to benefit financially, a load demand study must be made for the specific customer. These customers with PV systems will also need energy storage for load shifting, peak load demand reduction and to provide consistent load support during climatic events that vary frequently during the day. The truth is that PV systems without smart energy storage do not save the utility money.

The exception to this would be with smart inverter technology that is controlled by the utility. California and Hawaii are both working through this process and it will become a more universal option in the very near future. However, these smart inverters and smart energy storage systems don’t address another issue for the utility. They understand how to regulate and shift energy supply with the energy sources they now have, the difficult task is how do they do this with customer-based power systems; the customer has far too much independence for their comfort. Customers would need to become liable to the utility for failure problems or let the utility have operational control of the system performance and operation.

Now let’s look at the customer’s position. The customer doesn’t have a choice when shopping for a utility; moving to another location is the only option if you don’t like the utility in your area. This is often a source of discontent for the customer. Customers depend heavily on electrical power for almost every appliance in the home and business. Natural gas, propane and fuel oil for heat are the exceptions.
Electricity seems expensive because we use so much of it; in fact, it is pretty cheap. In order to generate all your own electrical energy on-site, the cost will be excessive. In order to meet the cost per kWh from the utility, the PV system cost needs to be amortized over a 25-year period. Most residential customers do not appreciate how difficult it is to provide constant electrical power and how reasonable the price actually is. Commercial customers are more aware of this yet they would still like to be more independent for the utility if it is possible.

An important factor to both residential and commercial PV system customers is the return on investment. The customer wants the highest price for the kWh generated from their PV energy and they feel justified because they are generating clean energy and contributing to the reduction of conventional fuel sources with known hazards to our environment. This actually makes sense when looking long range. If the utility can absorb some of the cost of integrating PV energy into the grid, the industry will continue to grow and reduce initial coasts and therefore operating cost. Eventually the cost of PV systems with smart energy storage will be cheaper than any other energy source. It is rational to expect a public utility or a customer owned utility to invest in solar for the future by offering premium rates for the solar kWh. The problem with this position is that the rate to the customer will need to decrease over time with greater solar penetration, otherwise the investment from the utility will not have a positive return and the cost of electricity will have to go up. This is a difficult point to make to customers; someone needs to educate the electrical customer on how this will work to their advantage. The salesman for the solar contractor is unlikely do it and the customer will be much less likely to trust that information coming from the utility itself.

Effective open communication between the solar customer, the solar contractor and the electric utility is essential. Avoid adversarial positions and look for positions that benefits each entity.


Kelly Provence
Solairgen School of Solar Technology


Reliable Resources for New Solar Contractors

Online information is abundant these days and misinformation tends to dominate many or most search parameter queries. This seems normal and the information appears to be free but it really isn’t. Usually it’s either written to attract people to buy a product or to attract people to someone’s misinformation-laden ego. Either way, it is difficult to filter the good information from the bad. The purpose of this blog is to provide some basic sources of reliable information for the new solar contractor.

Below I’ve listed the resources needed and information to be a well-informed, competent solar contractor.

  1. Training for the solar contractor. Knowing the design and installation trade and standards is most important and it can be achieved through good accredited training providers and certified trainers. If the training provide is not IREC accredited with IREC certified trainers, don’t waste your time with them, The only exception to this is manufacturers of solar equipment. They will provide good training, but only on their own equipment.
  2. Suppliers of solar equipment. There are many suppliers of solar products and they all differ in the range of products and depth of support they provide to their customers. Small to medium scale PV contractors will need suppliers who offer a wide selection of products from several manufacturers. Large scale contractors may go straight to the manufacturers. Follow the link provided below and select your country. They list wholesalers and distributors of solar products.

  3. Leading solar equipment. One way to determine which solar equipment to purchase is to look at a leading resource such as EnergySage. They have a solar panel database with consumer ratings, They also have an inverter database with consumer ratings, and a database on batteries as well, If you would like a more comprehensive list of solar equipment, go to the California Energy Center’s (CEC) data base, You can also review the products that the leading suppliers carry; this may be best since this will be your primary resource.
  4. Solar energy resource data. There is only one source for historic solar resource data and that is the National Solar Radiation Database (NSRDB). It has been developed from data collected by NREL, NASA and NOAA over the past 50 years. There is no other resource that can provide this data. The best tool to calculate solar irradiation for a tilt and azimuth of a specific site is PVWatts developed by NREL Several companies and organizations use this data in their shading analysis tools. If you use one of these tools, check to verify that their irradiation data is from the National Solar Radiation Database.
  5. Financing for the customer. The list of lenders can be long since the solar industry is growing at a rapid pace and has been providing owners a good return on investment. A good place to start is with the list on the EnergySage website,
  6. Certification and licensing. These terms should not be confused. Certification is an industry merit that is earned by very competent solar workers; the organization that tests individuals and issues certifications is the North American Board of Certified Energy Practitioners,  The certification adds value and credential to the individual and solar contractor. Certification is often required by solar PV system owners and/or utilities who are offering a financial incentive to their customers. Licensing is a requirement by the state and is a separate issue. The purpose of licensing is to make sure contractors have met the state requirement for competence in their field of construction. A licensed electrical contractor is responsible for all the electrical work performed on the solar installation. The license holder must be a permanent part of the solar company such as an employee or partner.

Kelly Provence
Solairgen School of Solar Technology


Graphis of Excess Energy to the Grid

How Solar PV Can Become the Primary Electrical Energy Source

In order to have this discussion, it is necessary to look at electrical energy consumption patterns. Our energy sources have to match the timing of our consumption. The graphs below show the typical daily electrical energy consumption patterns for residential and commercial consumers.

Time of Day End of Hour


Overall peak energy consumption is from 2:00 PM to 6:00 PM. Residential peak consumption is from 4:00 PM to 10:00 PM. Peak solar PV production is typically from 9:00 AM to 3:00 PM. The majority of the solar PV energy is being produced during non-peak consumption hours. If the solar PV system is sized to provide base-loads only, this addresses part of the issue. However, it changes the load demand on the utility grid and makes it difficult to maintain stability of the grid voltage and frequency. The chart below shows the changes to the load demand when solar is added to the grid during the peak solar hours of the day.

2012 represents very little solar generation. Each year shows the actual added solar generation effect and projected solar generation effect on total load demand.

It becomes increasingly difficult and expensive for the utility to meet this type of demand load variation. One solution is to face the PV array toward the afternoon sun. This would help with overall consumption patterns but not for residential consumption patterns. It doesn’t address the issue of cloudy days with little or no solar PV generation. This brings in energy storage as the best solution. The energy storage capacity needs to be large enough to absorb the excess solar PV energy produced during peak solar hours.

Overgeneration risk

Energy storage is the only real solution for solar PV to become a primary electrical energy generation source (EEGS). There are several types of energy storage: Batteries are the fastest growing type of energy storage with Lithium-ion being the leader of that group; the conversion efficiency is pretty high (80% to 90%). Pumped water storage has been used by utilities for this purpose in some sectors of the electrical grid; the energy conversion efficiency is a little lower (70% to 80%). Other forms of energy storage such as fuel cells and capacitors may develop further in the near future but they are not competitors at this time.

Battery storage is the most versatile means of energy storage and it is crossing the barrier of cost effectiveness for shifting loads to a different time of day. To effectively compete with the other EEGS, solar PV with energy storage must have a levelized cost of energy (LCOE) that is lower than the other sources. To calculate the LCOE, divide the total cost of energy by the energy generated during the life-cycle of the EEGS. Twenty-five years is the typical life cycle used for PV systems. The cost of energy includes initial cost and all operating costs during the period of analysis. The kilowatt hour of generation during the period of analysis must be de-rated for non-operational hours and the warranted degradation of the PV output.



8kW PV system with energy storage:       Net Cost: $3.30/W = $26,400

8kW x 5 sun-hours x .85 x 365 days x 25 years x 90% = 279,225 kWh

$26,400 ÷ 279,225 = .094/kWh


500kW PV system with energy storage:  Net Cost: $2.60/W = $1,300,000

500kW x 5 sun-hours x .85 x 365 days x 25 years x 90% = 17,451,562 kWh

$1,300,000 ÷ 17,451,562 = .0745/kWh


20MW PV system with energy storage:  Net Cost: $2.00/W = $40,000,000

20,000kW x 5 sun-hours x .85 x 365 days x 25 years x 90% = 698,062,500 kWh

$40,000,000 ÷ 698,062,500 = .0573/kWh

The above examples show LCOE rates that are competitive in some markets but not in all. The installed cost of solar PV is continuing to drop and the energy storage industry continues to improve and reduce costs. As these trends continue into the very near future, solar PV with energy storage will meet and beat the LOCE of most of its competitors in all markets.

Kelly Provence
Certified Master PV Trainer
CEO, Solairgen School of Solar Technology

Power Conditioner Image

Failure Causes in Solar PV Systems

During the first 10 years in service, the chance of failure within a PV system is approximately 10%. Inverters and other electronic devices account for 85% of all those PV system failures. Only about 1 in 2000 modules will fail during their warranted 25-year life. The system components most likely to fail are the ones with complex electronic circuitry.  The graph below identifies this type of equipment as Power Conditioning; it includes monitoring equipment, inverters, PV optimizers & other DC to DC conversion devices. The causes of failure are from manufacturing defects, improper installation, operating stress and accidents. The first of these is out of the control of the installation and maintenance contractors and the last of these, accidents, is usually outside of their control; the second two, improper installation and operating stress, are not.


Failing Power Conditioning equipment will perform at a lower efficiency than warranted; failed equipment will stop performing its function. Failures due to improper installation and operating stress are both avoidable. Causes of these failures are: Inadequate wire terminations, undersized conductors, environmental conditions that are outside of the equipment rating, inadequate protection from surge voltage and inadequate protection from physical damage. Once the equipment is operational, an infrared camera can detect damage that is occurring from heat at the terminals and inside the equipment. Terminal temperatures should vary no more than 3°C from one to another under the same current load. The internal parts of the equipment should not exceed the max rated temperature listed by the manufacturer. If equipment is failing but still operating, the conversion efficiency can be measured by reading the (Volts x Amps) at the input and output of the equipment. The difference should be no less than the rated efficiency of the equipment. Test six months after installation and then annually thereafter.

Module failures get most of the attention since they don’t usually fail completely but continue to function at a lower output; the causes of failure are numerous. A module is considered failed when one of the following occurs: Breakage, delamination, burned solder joints, browning of a PV cell due to overheating, bad bypass diode or degradation in performance beyond the warrantied percentage. These are covered by the manufacturer’s warranty; depending on the failure mode, proof of failure can be verified with a standard photograph, infrared photograph or by inspection test with a calibrated meter such as an I-V curve tracer. A module is considered failing if it performs lower than other modules of comparable design and type. An IR camera may detect hot spots on PV cells that decrease performance –  an IV curve tracer can determine whether the module is operating below its warranted performance. Test with an IR camera annually, and with an I-V curve tracer for large commercial projects.

The most common long-term failures are: Hot spots due to manufacturing defects in the cells, hot cells caused by high current flow in a de-energized state, potential induced degradation (PID) caused by leakage currents to earth ground, low cell conversion rate due to cracks within the cell, delamination caused by extreme heat and humidity and current loss due to shorts (shunting) between cells where the module substrate is damaged by wind and dust or other natural causes. An I-V curve tracer and an IR camera can identify most of these problems. For residential and some commercial projects module level monitoring will identify problems; it is easy to compare each module performance to the others around it.

The other failure category shown in the graph is balance of system components (BOS). This is generally all the wiring, conduit, switchgear, junction boxes and module support structure. Failure is usually due to improper wire sizing or termination, galvanic corrosion with incompatible metals, materials installed in environments beyond their rating and improperly installed components. Prevention of failure includes testing conductor resistance before startup, capturing IR images at terminals when equipment is operating, measuring resistance to ground on all grounded metal parts and making a visual and torque inspection of all physical and electrical connections. Test six months after installation and then annually thereafter.

The most valuable tool for inspecting PV system performance is a module level monitoring system; most new residential and small commercial projects now have this level of monitoring. The next best tool is an IR camera; since there are now IR devices that connect to smart phones this is a very affordable option. The next best tool is an AC/DC volt-amp meter. You can take readings on the input and output of any equipment and determine the loss factor for that piece of equipment. If you are working on commercial systems, you will want a conductor resistance tester to measure the conductor insulation resistance to leakage currents at the maximum operating voltage. The I-V curve tracer is unnecessary for residential and small commercial projects. They are usually required for large commercial and utility scale projects.

Kelly Provence
Solairgen School of Solar Technology
IREC Certified Master PV Trainer
NABCEP Certified Professional Installer

Calculating Voltage Drop in PV Systems

Voltage drop (VD) is the loss of voltage in a circuit due to the resistance in the electrical circuit. To determine the amount of voltage lost in a circuit, we need to look at three parts: 1. Resistance of the conductor in Ohms (Ω), 2. The length of the circuit conductor, 3. The current flowing through the conductor. A forth component is to compare the VD to the operating voltage in the circuit to see the percent of voltage drop.

  1. The resistance of the conductor per 1000’ (Ω/kFT) can be found in Table 8 and 9 of Chapter 9 in the National Electrical Code (NEC). Table 8 is for DC and Table 9 is for AC. Divide by 1000’ to get the resistance of the conductor per foot (Ω/FT).
  2. The length of the conductor is the full circuit length; for DC and single-phase AC circuits, multiply the one-way distance by 2. For three-phase AC circuits, multiply the one-way distance by the square root of 3 (1.732).
  3. The amount of current flowing in the circuit directly affects the amount of voltage drop in the circuit; e.g. 2 amps of current will double the voltage drop of the circuit with 1 amp of current. Current is represented as intensity of current (I) in the formula.
  4. To determine the VD%, the operating voltage must be used. The operating voltage is dependent on the equipment and how it is connected. The PV source circuit voltage may be the product of modules connected in series or it may be controlled by DC-DC conversion devices and an inverter. The AC operating voltage is simply the nominal utility voltage at the premises.

DC and single-phase AC formula:  VD  =  I  x  Conductor length  x  2  x  Ω/FT

 VD%  =  VD  ÷  Operating voltage

Example: A PV source circuit operating at 9a and 400v using a #12 conductor in a circuit with 50’ of length one-way. Table 8 shows a #12 (7 strand) to have 1.98 Ω/kFT; 1.98   (.00198Ω/FT)

9a  x  50’  x  2  x  .00198  =  1.782 volts dropped or lost in the circuit (VD)

1.782v  ÷  400v  =  .004455  or  .44%VD

If you double the length of the conductor or double the operating current, the voltage drop would also double.

NOTE: This calculation is accurate using current and voltage data either from the PV module Standard Test Condition (STC) or the Normal Operating Condition (NOC). NOC test conditions represent the irradiance and cell temperature during the six critical sun-hours of the day. Both current and voltage are lower than STC. Since they are both lower, the VD% is close to the same as if you used STC current and voltage instead.

The AC side of the calculation is the same as The DC example above, however it may not be as accurate since the inverter rating can be sized from 80% of the PV rating to 135% of the PV rating. This affects the average operating current and the amount of voltage drop. We typically use the inverter’s listed max operating current if the PV array is sized 125% to 135% of the inverter rating. If the inverter is rated the same as the inverter, use 80% of the listed inverter max operating current.

Example: An inverter rated 6000 watts, 240v and 25a is connected to a 7200 watt PV array. The inverter AC output is located 30’ from the AC interconnection point. The conductor used is #10 copper rated 35a with resistance shown in Table 9 of 1.2 Ω/kFT   (.0012Ω/FT)

25a  x  30’  x  2  x  .0012Ω/FT  =  1.8 VD  ÷  240v  =  .0075 or .75%VD

The combined VD% of the DC and AC circuit is .44%  +  .75%  =  1.19%

If the inverter and PV array are the same size, multiply 25a  x  80%  =  20a. This reduced the VD by the same proportion.

We usually consider more than 3% VD in the entire circuit (DC and AC) to be excessive. Increasing conductor size reduces the Ω/FT and reduces the voltage drop in the circuit. The voltage drop percent is a loss factor with energy production. The example above reduces the PV system production by 1.2%.

Three-phase AC formula:  VD  =  I  x  Conductor length  x  1.732  x  Ω/FT


Kelly Provence
Certified Master PV Trainer
Solairgen School of Solar Technology

The Best Methods for Energy Efficient Solar Homes

Solairgen-Installation475x317Your home’s overall energy efficiency is not as exciting to think about as a solar PV system, but it is much more important in most cases. Solar PV systems are getting more affordable, but that doesn’t help much if the overall energy consumption is higher than it needs to be. An energy efficient home will require about half the PV system size as a home that is not energy efficient. There are several low cost improvements that can improve energy efficiency; there are also many improvements that require an investment but provide good return on the investment.

The division of energy consumption in a home looks similar to the following pie chart taken from the Energy Industry Administration report.

Pie Chart of Typical Energy Usage for
Legend for EE Pie Chart

Appliances, electronics and lighting make up 40% of the home’s energy consumption. The lowest cost improvement is changing the energy user’s behavior, e.g., turn off lights and appliances when not in use. Replacing old appliances with higher efficiency ones as they require replacement, such as LED bulbs in long-hour-use fixtures.

Water heating costs can be reduced by several methods; the lowest cost methods are listed first. Insulate the hot water pipes from the water heater to the point of reduced access to the pipe. Buy and install an insulated jacket for the water heater. If your water heater fails and needs replacing, replace it with a high efficiency one. There are models that use a heat pump as the primary source of heat generation with heat coils as backup. Another, but more expensive option is to install a gas tankless water heater.

Heating and cooling together account for about 40% of the home’s annual energy usage. The best savings for the investment is to simply seal all cracks from the exterior to the interior of the home. This would be at the foundation, around doors and windows, and fixtures and access-ways to the attic. The next best investment is to improve the quality of insulation in the attic. The best insulation improvement is to have spray-foam insulation installed in the rafters and vertical walls of the attic. If there is a basement, the walls should be spray-foam insulated as well. Replacing blown-in insulation with spray-foam insulation can reduce heating and cooling costs by up to 50%. That would reduce the home’s total energy consumption by 20%.

It takes some effort and some investment to make the home energy efficient but it will not be nearly as expensive as trying to offset that portion of inefficient energy usage with a solar PV system. A 2000ft² consumes about 1800kWh/month. With an investment in spray-foam insulation, water heating insulation and replacing old appliances with high efficiency ones, it is easy to reduce the home’s energy consumption by 30% to 40% and spend less than $6000. That is about ½ the cost of a PV system to offset the same amount of energy.

Reduce energy consumption first through energy efficiency and then look at offsetting the remainder with a PV system. You may even have some money leftover to include energy storage with the PV system.


Kelly Provence

Solar Shingle

The Future of Solar PV Shingles

Solar shingles seem like the most logical application for the future of solar PV systems, serving two purposes – a roof and a solar PV system – by simply integrating the shingles into photovoltaic roofing. There are problems with the solar shingle system, slowing its mainstream marketability, but may work if the following obstacles can be overcome:

(1) The cost of solar shingles is not competitive with conventional solar electric PV modules. Costs might balance with high penetration into the photovoltaic industry, but that takes time.

(2) The physical constraints of solar electric roofing shingles do not allow for custom fitting to varying roof dimensions. This is an obstacle that is difficult and expensive to overcome.

(3) The solar roofing contractor’s training is limited to the narrow market of solar shingle manufacturers. The manufacturers will not train contractors in areas where sales are not profitable, so the solar shingle market restricts customers’ choices and is controlled exclusively by the manufacturer.

(4) Roof warranties can only be fulfilled by the solar roofing manufacturer and its own factory-trained solar roofers. Repairs may be delayed if there is not a trained contractor in the customer’s area. Roof damage not covered by the solar roofing manufacturer would be difficult or impossible to have repaired by conventional roofers who are not trained to work with solar shingles.

I would like to see this product succeed in the market, but no manufacturer has made a successful long-term run with residential solar roofing to date, and some who are attempting it have yet to make a profit in the photovoltaic industry.

Mainstream success of solar shingle roofing may happen, but it doesn’t appear to be coming in the near future.


Kelly Provence
NABCEP Certified Professional PV Installer
IREC Certified Master PV Trainer
Solairgen, Inc.