Solar projects are largely known by the panels they use. It is good to confirm that a panel is rated as Tier 1 category by Bloomberg. All Tier 1 rated panels now come with 25 years of production warranty. While panels are by far the most important and expensive component, there is more than panels that determine the overall quality of a solar system. If panels are the heart for a solar plant, invertor(s) is the brain. Invertor is required to convert the direct current (dc) generated by the panels into alternating current (ac) that runs the household appliances. Any downtime of an invertor directly means revenue loss, and therefore, its selection becomes equally important. Next comes the racking system which holds the panels together on a roof. Most racking systems are pre-engineered to carry the panel weight (which is not much, usually around 2 to 3 lbs/sq. ft.), but maintaining electrical continuity among the various rails and clamps becomes important. Finally, wires and cables. Though very small part of the cost, high quality and extra-jacketed wires ensure long life of a system and prevention of ground faults. This is also a major area where workmanship and skillfulness of a solar company can play a large role in ensuring 25 year system life. Since many components have to come together to generate ac power, good design and workmanship play as big a role as good quality equipment in a solar system.
Home owners often get confused among the different choices they have these days for selecting an invertor. A “string” in a solar design means a number of solar modules electrically connected in series to achieve a desired voltage. A series connection essentially means all modules have to be at the same orientation and without any shade. Any shade or dust, debris etc on any module in a string affects the performance of the entire string. Just as how a chain is as strong as its weakest link, similarly, a solar string is as strong as the module with the most shade or dust. Therefore, any single module can severely impact the performance of an entire string if it’s shaded or unusually dusted. Most importantly, a small amount of shade can cause a severe production loss depending on the module orientation. For example, a 10% shading on the short edge of a panel can cause a nearly 90% drop in output from the panel.
To overcome this problem in a conventional solar string design, module level electronics such as micro invertors or optimizers are required particularly for residential installations surrounded by trees etc. These devices are capable of tracking and optimizing module level performance and also additionally have the ability to comply with the latest NEC requirement of “Rapid Shutdown” by rooftop solar systems.
Micro invertors – A micro invertor is a very compact device that is usually placed underneath a module, converting dc power from each module into ac and avoiding the above discussed drawbacks of string design. Since the dc to ac conversion takes place right next to the modules, micro invertors eliminate longer, higher voltage dc wire runs on rooftops, and are considered to be inherently safer designs particularly for residential installations. Leading micro invertor makers such as Emphase offer 25 year warranty.
Optimizers – Similar to a micro invertor, an optimizer is also a module level device that allows a string to bypass a particular module that may be shaded or unusually dusted, and thereby essentially avoiding any weaker link in the chain. Optimizers however are dc to dc convertors and still require a string invertor to convert the dc power into ac. Since optimizers normally work at a higher dc voltage (compared to the 240 Vac residential voltages), one advantage they offer is less losses due to voltage drop. Solar Edge optimizers also have a 25 year warranty, while its invertor has a standard 12 year warranty, which can be extended for a fee.
In terms of output comparison, empirical evidence of the projects installed and studied by us indicates no meaningful difference between the two technologies. Some installers argue SolarEdge optimizers perform marginally better, which could be because of the higher dc voltage. However, there does not seem to be a meaningful improvement in output of one over the other. Thus, a choice between the two competing technologies boils down to price, where the optimizer solution is a clear winner, so far.
One last thought we would like to leave home owners with is that as storage technologies continue to improve and battery cost keep coming down, batteries will see widespread implementation and their integration with a string invertor/optimizer would certainly be easier.
Massachusetts is well known for its renewable energy and SREC incentives for promoting solar and other forms of renewable energy. However, Rhode Island’s RE Growth (REG) Program is equally strong in terms of offering incentives to home owners to go solar. Updated on 1 April, 2017, the REG Program offers home owners an attractive solar tariff of 34.75 cents/kWh for 15 years or 30.85 cents/kwh for 20 years for up to 10kW (DC) systems. Under this Program, home owners and businesses can sell solar generation by dispatching the energy directly into the grid through a separate energy meter. Larger systems can earn the following tariff in Rhode Island:
11kW – 25kW Systems are eligible to get 27.75 cents/kWh
26kW – 250kW Systems are eligible to get 22.75 cents/kWh
251kW and above will have to participate in competitive bidding
REG coupled with 30% federal ITC can reduce return on investment from solar to around 5 years.
Rhode Island has another attractive grant via the RE Fund (REF) Program, which offers cash grant of up to $1/W capped at $10,000 for home owners. Under the REF Program, home owners can use solar generation for self consumption through Net Metering and thereby offsetting the utility’s electricity bill. In addition, system owners are also eligible for 30% federal ITC as well, making the overall payback period similar to the REG Program.
System owners can choose either the REG or the REF but not both.
Different states have different requirements for qualifying or licensing a solar installer. Some
New England states such as Massachusetts and Rhode Island only require that a solar
system be installed by a licensed electrician while other states such as Connecticut, Florida
and California etc have a separate license for solar contractors. These requirements are
necessary but not a sufficient condition for a well performing and long lasting solar system.
Neither the electrician licenses nor the state level solar contractor licenses fully prepare an
installer to deal with the intricacies of modules string design, detrimental cold temperature
voltages, AC/DC ratios and many other design complexities of a solar system.
The North American Board of Certified Energy Practitioners (NABCEP) PV Installation
Professional Certification is one of the few credentials, if not the only one, that prepares an
installer with rigorous training in design and implementation of complex solar systems and
focuses on minute design details such as solar production optimization, wire and fuse
sizing, use of extra-jacketed PV Wire, design of un-grounded inverter systems, UV-rated
wire ties, use of appropriate raceway to account for thermal expansion etc. While it’s crucial
to select good quality modules and inverter, it is equally important to understand these
design subtleties and implement them well. NABCEP PV Installation Professional
Certification is increasingly getting adopted by many states and towns as a required
qualification and prospective system owners can benefit hugely by making sure the installer
has this credential.