The Importance of Optimum PV Array Size for Solar PV Systems

Residential Solar PV Ground Mount SystemThe question of optimum PV array size is the first question to be answered when designing the PV system. There are several factors that have to be considered to arrive at the best size. The customer budget, the available area for the array and the purpose of the PV array are always primary considerations. The budget is usually the most flexible since the system can be financed. The second may be difficult to get around if the location options are limited and restrictive. The third part is where most mistakes are made. There are generally two types of systems, (1) stand-alone and (2) utility grid interactive.

The stand-alone system must be designed to serve the electrical loads throughout the year. The PV array is usually sized to serve the loads in the month with the highest ratio of electrical load demand verses available solar energy. As long as the electrical load demand is known and the solar resource is known, the calculation is made for each month of the year (Avg. daily loads kWh ÷ Avg daily insolation ÷ Power conversion efficiency % = ratio) The ratio is the exact size of the PV array necessary to offset 100% of the energy consumed in that critical design month. The hard part is usually determining the kWh of electrical consumption.

The stand-alone system can be off-grid with generator backup or grid supported. If it is grid supported a portion of the house electrical loads are served by the stand-alone system. The PV array is sized for the stand-alone portion of the house.

The graph below show a typical PV system output capacity compared to the electrical consumption for each month of the year. The PV array is exaggerated in size to cover the worst ratio months of the year.

Graph 1

Grid interactive systems must consider how much power can be fed into the electrical system and remain compliant with the utility interconnection agreement and the best power offset value. The most common interconnection agreement is Net Metering for the billing cycle. It is typically set to offset up to 100% of consumption with solar PV generation during the monthly billing cycle. Energy generation in excess of 100% is usually compensated at avoided cost, about 1/3rd the rate of retail. The obvious objective here is to not generate more than is consumed for each month of the year. The challenge is to project generation and compare it to consumption throughout the year.

The months that usually control the PV array size are in the spring and fall. We consume less energy during these periods because of the reduced need for heating and cooling. Coincidentally, these are usually the best two periods for solar PV generation because of clear skies and lower temperatures, April and May are typically the best generation months.

The graph below shows the typical electrical usage for residential customers. The PV system that is designed to offset 60% of the annual electrical consumption is generating 100% of that consumption during the month of April.

Graph 2

A third type of system is a self-consumption type system that requires energy storage and is designed to connect to the grid but not sell into the grid. The PV array size usually follows the same rules as the interactive guideline above, but it can be more complicated and requires a smart control system with consumption and generation metering to keep everything in check.

It is OK to oversize a stand-alone system but with interactive systems keep an eye on the low-consumption vs. high generation months.

Kelly Provence
Solairgen
www.solairgen.com

706-867-0678
info@solairgen.com

Image of a Poor Solar PV Installation

How to Prevent a Substandard Solar PV Installation

No one wants a poor PV system installation, but it happens from time to time. The good news is it’s 100% preventable but you should first understand the causes of a bad installation, and then learn how to prevent it. We tell you how.

Problem #1: The installer is unskilled and unknowledgeable about the correct installation process, but how do you determine that?

Solution: Screen the installer. Ask for references and evidence of their experience such as pictures, invoices, permits and inspection reports.  Ask how much training they received, and where they received it. You can even ask to see their graduation certificates. Inquire about industry certifications (NABCEP) and whether they have a contractor’s license.

If they can’t or won’t provide any of the above information, don’t contract with them no matter what they promise you. A reputable installer will be eager to provide their credentials.

Problem #2: A skilled and experienced installer wants to install a brand-new product that is not fully understood or tested in the industry. It is not uncommon in this industry for changes in PV products to outpace the contractor’s full understanding of them. Don’t be the guinea pig.

Solution: Insist on tried-and-true system components unless it’s just an improvement over a product that has been around a while. It’s best to see how those brand new products hold up during beta testing. Find out by going to the internet and doing some independent research. Don’t be afraid to tell your installer what you learned and that you prefer another product.

Problem #3: The contract price is too low for the contractor to make a profit. Incorrect bidding is common when a contractor is inexperienced, but underbidding occurs occasionally even for experienced installers.

Solution: It is best to find out the going price for the installation prior to accepting a bid. If you don’t know the going price, get more than one bid and then compare. Most companies will give you a generic bid without a problem.

Some Advice: If the installer you select has underbid the installation, it may save you money in the long run to offer a fair, renegotiated price. Some contractors are very honorable and will do the same good job even if they lose money, but some will not and that may cost you more down the road. And remember, a deal too good to be true is usually exactly that, untrue.

In summary, the real responsibility is on you, the customer. Take a day or two to learn about the PV products, the installation process and who the good contractors are. There are many organizations out there to help you. Here are a few links to get you started: American Solar Energy Society, Solar Energy Industries Association, NABCEP, IREC, and The Solar Foundation.

Finding skilled, experienced solar PV installers isn’t difficult, and eliminates the frustrating and very expensive future problems of a bad, or failed, solar PV installation.

Kelly Provence
IREC Certified Master Trainer
Solairgen School of Solar Technology

Graphis of Excess Energy to the Grid

Alternative to PV System Net Energy Metering: Improving Grid Stability

What alternative to net energy metering (NEM) with PV systems will improve the grid stability and reduce consumer dependence on it? Self-consumption energy storage PV systems are the answer.

Solar PV installations continue to grow in the residential and commercial sectors. Integrated energy storage continues to grow along with the PV installations, and both of these are growing at a strong pace in the industry.

NEM can be calculated on a daily, monthly or annual basis; most NEM agreements are calculated monthly. Whether the consumer generated energy is used by the consumer or sent back into the grid, the energy consumption is reduced at the retail rate. The problem with this setup is that most of the energy goes back into the grid during the middle of the day (solar hours) when consumer energy demand is low; this does not improve grid stability, and can make it worse. The graph below shows the typical energy consumption vs. solar energy generation for a residential customer.

Graphic of Energy Out to Utility

PV systems with energy storage can change this with the batteries storing energy that would otherwise be sent into the grid.

Graphis of Excess Energy to the Grid

The stored and redistributed solar energy will improve grid stability in residential by reducing demand during peak usage times. If time of use metering (TOU) is in place, this setup will reduce energy consumption during afternoon and evening high rate periods. This increases financial savings to the PV system owner.

The following graph shows the typical U.S. grid load demand curves for all electrical energy users.

Total Demand

Residential electrical energy customers with interactive PV systems are supplying power during the time of day when residential load demand is not at its peak. PV systems with energy storage can operate as self-consumption systems and improve their return on investment. Grid stability is improved to the point where grid operators can see reduced operating cost as this type of PV system penetrates more of the NEM market.

This type of PV system can also provide the customer with critical load circuits that will operate during a grid outage. Everyone wins with these systems and NEM policies are no longer a factor with the customer’s financial rate of return.

Kelly Provence