Sizing a Grid-Direct PV System
When you set off on the task of sizing a grid-direct PV system you need to account for many variables. As you find out in this article, the overall process results in a PV system that’s based on the client’s budget, the available area, the annual energy production and consumption at the site, and your choice of materials for the job (notably, the array and the inverter).
In comparison to stand-alone PV systems, grid-direct sizing is much less complicated. You also don’t have to make nearly as many assumptions and leaps of faith. Because the utility grid is always (well, almost always) present, and because a grid-direct inverter operates in parallel with the utility, you don’t have to size the array to meet electrical consumption at any given time.
If the loads within the building require more power than the array can provide, the utility can supply the difference. And even though the PV system goes down when the utiliity goes down, the system automatically restarts when the grid comes back to life.
A grid-direct system, therefore, should be viewed as a way to supplement (rather than replace) the utility grid.
Note: Sizing a grid-direct PV system requires you to know a few temperatures and be able to make calculations with those temperatures. The calculations get pretty messy when you try throwing Fahrenheit into the mix, which is why all the temperature related calculations presented in this article use Celsius.
Evaluating Budget and Available Array Area
Whenever you’re looking to install a grid-direct PV system, the overall system size will generally be limited by one or more factors, but the first two you need to consider are your client’s budget and the amount of space available for the PV array. People with unlimited budgets and unlimited space for a PV system are out there, but those clients are few and far between.
Before you establish anything else, find out what your client’s budget is. Of course, everyone would love to offset 100 percent of their electrical energy consumption with solar power, but most people simply can’t do that. So before you spend too much time sizing the best-possible system for your client’s desires, find out what his budgetary realities are first and work from there.
By establishing your client’s budget in the beginning of the process, you can prevent yourself from wasting a lot of time and energy designing a system that never has a chance of being installed.
The next limitation to consider is the area available for mounting the array. For the majority of grid-direct PV systems, this area is the roof of the house or business. (Other options include the ground outside of a building or the top of a pole.) To determine the amount of space available for the system, you need to perform a site survey.
After you conduct a site survey, you can calculate the available square footage by measuring the length and width of the array area and multiplying those two values together. You can then take that area and multiply it by the module’s power density, or the number of watts per square foot for the module you’re thinking about using based on the client’s budget and what’s available from your suppliers.
The resulting number gives you an idea of the total power in watts (W) that can be placed in the available area.
I recommend you don’t attempt to use every last square inch of a roof surface when installing an array. Squeezing a few more modules up there whenever possible is tempting, but don’t do it! Cramming the roof will not only make the installation more difficult but it’ll also make it tricky (and possibly down-right dangerous) to try and maneuver on the roof when the system needs maintenance.
Also, wind loading at the edge of the roof is much greater (requiring more mechanical support), and some jurisdictions have requirements based on access for firefighters and other safety personnel. (The same goes for array sites other than roofs — when it comes to placing PV modules in a given spot, less is definitely more.)
Be sure to consider “dead” spaces (the areas around the array that can’t be used due to shading or the need to maintain sufficient paths for access) as well as the small spaces between the modules when determining the overall area available for the PV array. Suddenly that decent-sized roof may be very limited to account for all of these issues.
To account for dead spaces, simply subtract the area needed for the access paths from the overall area and use any shading-analysis tool to determine the area that’s unavailable due to shading.
By taking these areas out of the picture, you can establish a realistic idea of the total power rating of the potential array. The size of the PV array may change over the course of your design process.
For example, you may estimate that you can fit 24 modules on a roof, but as you progress, you may realize that the string configuration requirements won’t allow that many modules. Or perhaps you have to consider a different module type entirely.
Consider your initial estimate of the available space a starting point and use it only for reference.
Estimating the Site’s Annual Energy Production
When you know roughly how much space you have for the array, you can begin to estimate the annual energy production for that site. This estimate is helpful to have when designing grid-direct PV systems because you need to evaluate the type of agreement the utility will be willing to enter into.
You can estimate the annual energy production in a variety of ways. For larger commercial systems or in situations where advanced techniques are required, you can use modeling programs to evaluate multiple scenarios and change parameters to estimate the energy production (PV*Sol and PVsyst are two popular programs).
For residential and small commercial systems, one of the best ways to estimate energy production is to use the PV Watts tool. PV Watts is a free Web-based tool provided by the National Renewable Energy Laboratory. Many people in the solar industry use PV Watts on a regular basis. To use it correctly, you need to know
- The amount of power the PV array you’re thinking of using can potentially produce (in kilowatts [kW])
- The tilt, orientation, and shading effects of the array
- Some basic information about the equipment you’re thinking about using (the PV Watts program makes some good assumptions about the performance of the equipment)
The PV Watts tool takes the information you enter and creates a total solar resource factor (TSRF), the percentage of the solar resource available for that specific site in relation to a perfectly oriented and shade-free array.
It then provides a simple report that estimates monthly and annual energy production values measured in kilowatt-hours (kWh). It also calculates the dollar value for that energy, which is nice because this is a value everyone can relate too (unlike peak sun hours or system efficiencies).
I suggest you use the first version of the PV Watts calculator until you become familiar with the program. Then you can move on to Version 2 if you like. (I rarely use Version 2 because Version 1 gets me all the information I generally need.)
Either way, PV Watts is relatively self-explanatory; it even has helpful links for commonly asked questions and ways to manipulate the program based on your needs.
The annual energy production you calculate at this point is only an estimate. Don’t treat it (or let your client treat it) as the promised production value.The energy produced from the array will be affected by installation methods to an extent, but ultimately, the available solar resource dictates how much energy is produced. So if your client experiences an abnormally cloudy or sunny season, the energy-production number you gave him will be off.
Sizing the Array to Meet Your Client’s Energy Consumption
I promise that if you’re in the PV industry for more than two weeks, you’ll be asked this question before you finish telling someone what you do for a living: “How big of a PV array do I need to power my average-size house?” My favorite response: “Gosh, I’m not sure. What color is your house?”
Neither question makes much sense, does it? Sure, I can figure out what the average American household’s energy consumption was two years ago, but that number rarely means much to anyone individually.
The energy consumption of an individual, family, or business has little to do with the size of a house or commercial building and more to do with people’s lifestyles. Does your client have all-electric heating? How does he heat his water? Does he have an electric car hidden in the garage, just charging away? Is his business home to a welding shop or a number of refrigerators?
After you establish the maximum array size and estimate the site’s annual energy production, you need to look at some utility bills (preferably all 12 bills from the previous year) to get a sense of how much energy the house or business consumes and what the impact of a rooftop or yard-based PV system will be.
You also want to guarantee that the energy the system produces provides the maximum financial benefit for your client based on the agreement with the utility. I explain what you need to know in this section.
Determining annual energy consumption: Grid-direct PV systems have the advantage of built-in energy record keeping from the utility provider. To determine the annual energy consumption for the house-hold or business in question, simply collect the last 12 months’ worth of bills from the utility.
This snapshot will give you a great idea of the amount of energy consumed annually (so long as you look at the total kilowatt-hours consumed). Of course, if you have access to more than one year’s worth of utility bills, go ahead and take a look at them all. The extra information can only add to your knowledge of your customer’s energy habits.
One thing to ask is whether there are any recent changes to the electrical consumption. For example, did your client install a solar hot-water system a few months ago? Or did he just put an electric water heater in the master bed-room suite? Such changes affect future electrical consumption, which is why you should always base your estimates on the most recent information.
Note: If you can’t obtain the energy records for the client’s current home or past home because the current electrical consumption is dramatically different, you may have to estimate the annual energy consumption by using the same process I
describe in next article for sizing battery-based systems.
Looking at contract options with the utility: Your client has a few options for entering into a contract with the utility, but the most common approach is net metering. In a net-metering agreement, the utility agrees to “pay” your client the exact amount it charges him for energy — given that your client doesn’t produce more energy than he uses in a given time period (typically a year). The exact restrictions are included in the interconnectionment provided by the utility.
Usually, if a PV user produces more energy than he consumes in a year under a net-metering agreement, the utility can say thanks very much for the extra energy and move on.
The PV user (in this case, your client) doesn’t get any extra accolades or cash for producing more energy than he consumes.
Many utilities allow customers to essentially bank up their kilowatt-hours when they produce more than they consume — kind of like when your extra calling-plan minutes roll over. At some point, though, the utility will start over; this is known as the true-up period.
Some utilities make this true-up period occur monthly, which means your client doesn’t get the full benefit of the energy produced in any month that he doesn’t use that same amount of energy. As the installer and designer, be sure to confirm with the utility how its true-up period works before getting too far into the design process.
Another component of net metering to consider is the time-of-use metering option (TOU). With TOU, the utility charges your client different rates based on the time of day he uses energy. Typically, TOU rates are highest during the middle of the day when overall consumption peaks.
This means that for a PV system installed under a net-metering agreement that uses TOU, the times of the day when the PV array is producing the greatest amount of energy correspond to the times when the utility rates are at their highest.
If your client can maximize PV production and minimize consumption to correspond to peak energy rates, say he’s at work all day with as many appliances powered down as possible, the overall effect can be beneficial financially.
The other contracting system that may be available is the feed-in tariff system (FIT). With a FIT contract, your client doesn’t really care about annual energy consumption because there’s no direct relationship between his PV array size and his energy consumption due to the fact that he receives more money per kilowatt-hour generated from the PV array than he’s charged from the utility.
So if your client signs a FIT contract with the utility, you want to design the PV system so it maximizes the amount of energy the array produces regardless of your client’s energy appetite.
Note: Regardless of the utility contract, I encourage you to advise your client to conserve energy as much as possible even though he doesn’t really have to worry about comparing his energy consumption to the size of the array. Energy conservation should be the mantra of anyone installing a PV system. Besides, conserving energy helps your client financially — the less energy he consumes, the less he has to buy.
Using consumption and contract options to select an array’s needed power value: At this point, you’ve collected data about how much energy your client consumes and established a starting point for the total amount of energy the proposed PV array will produce annually. Now you can begin comparing the two numbers and working with your client to establish the best option based on the utility agreement he has to enter into.
If your client consumes less energy annually than the PV array will likely produce, he’s in great shape. He can either reduce the overall wattage of the array or be prepared to overproduce annually and not receive full financial credit for every kilowatt-hour produced by his system.
A more likely scenario is that the energy produced by the PV array is less than the energy consumption of the people in the building. In this case, you can work with your client to help him reduce his overall energy consumption (common techniques for this include changing to compact fluorescent light bulbs and installing better insulation).
You can then compare the estimated production to the assumed reduced energy consumption and establish the percentage of electrical energy that will be provided by solar power and the associated dollar savings.
For example, if your client’s PV system can produce 2,000 kWh but the client consumes 5,000 kWh annually, the PV array will offset 40 percent of his total energy consumption.
Keep the true-up period that the utility uses in mind (see the preceding section for the scoop on this). If the utility looks at the values monthly, you need to evaluate the production and consumption ratio on a monthly, rather than annual, basis.
Make sure your client is aware of how the interconnection agreement works so he isn’t surprised if he doesn’t get full credit from the utility because he produced twice as much energy as he consumed.
Applying the right consumption numbers and contract options may seem like a lot of upfront work, but after you go through the process a few times, you can establish some good working numbers based on the equipment you like to use, local utility requirements, and the site-specific information you gather. Having these numbers in mind helps you finalize the proposed array’s design, including its size.
Getting Ready to Match an Inverter to an Array
After you nail down the right PV array wattage for a client’s grid-direct system, you’re ready to establish the relationship between the inverter and the PV array. To do so, plan to look at all the electrical characteristics — power, voltage, and current.
I suggest looking at power, voltage, and current in that order so you can narrow down your inverter choices with each step. Another benefit to following this sequence? The final check, current levels, typically becomes a simple verification.
Every inverter manufacturer has online PV sizing calculators to help you with matching an inverter to a PV array. Even though I use them on a regular basis, I do so as a check of my calculations, not the source of my calculations.
As a PV system designer and installer, it’s vital that you perform your own calculations instead of relying on the online calculators. Here’s why:
- The potential for error: The data plugged into the online calculators is entered by humans who’re fully capable of making mistakes. Trust me; I’ve seen some of these mistakes firsthand.
- The inevitable disclaimer: Every inverter manufacturer requires you to accept a disclaimer before you can access its tool. In essence, the disclaimers all say the same thing: The information here may or may not be accurate, and if you use the tool, we (the manufacturer) aren’t responsible for any problems.
- The National Electrical Code (NEC): One section of the NEC says that “you shall do the calculations.” At some point, the local electrical inspector may ask you to show him these calculations. If you can’t, you run the risk of having your system fail inspection. (Shall in NEC terms is pretty strong language. It’s equivalent to the sideways look you used to get from mom and grandma — something not worth messing with.)
Occasionally, you may want (or need) to use multiple inverters in a system. In grid-direct systems, the most common reason for having multiple inverters is that the desired PV array wattage is greater than a single inverter can handle.
Many of today’s inverters are rated at less than 10 kW, but fewer options exist between 10 kW and 30 kW, so if the PV array wattage falls into this range, you may need to design for multiple inverters.
Another popular reason for having more than one inverter in a grid-direct system is that sometimes the PV modules are installed in different locations or sometimes they’re all on a roof but pointing in different directions.
Matching Power Values for Array and Inverter
All inverters are rated by their maximum continuous power output, which is measured in watts or kilowatts. (More often than not, this number is incorporated into the inverter’s model number, giving you a quick idea of the inverter’s rating.) This value is the AC power output. Inverters actually limit their power output, so you can use this value to figure out the maximum power input coming into the inverter from the PV array.
When you look at PV module ratings, you find that the power output ratings are based on standard test conditions (STC) where the cell temperature is 25 degrees Celsius and the intensity of the sun is equal to 1,000 W/m². PV modules rarely operate in these conditions due to the motion of the sun across the sky and increased PV operating temperatures.
So, the power output from the array (in other words, the power input coming into the inverter) is typically considerably less than the rated values.
On top of that, both the inverter and the array (along with all the other equipment in a grid-direct system) have efficiency losses that must be taken into account. Fortunately, you can quickly estimate all of these system inefficiencies by adopting the industry standard that any PV system will operate at approximately 80 percent overall system efficiency, which means that about 80 percent of the array’s rated value will be “processed” by the inverter and pushed on to the load center.
Even though a PV system as a whole can be more efficient or less efficient at times, approximately 80 percent is a good industry standard to use when relating the PV array output power to the inverter’s output power.
So how do you get started? Well, because you’ve already established the approximate wattage of the PV array you want to install, you can now take that array wattage and relate it to the inverter’s rated output power. Because inverters are limited in the amount of power they can process, it doesn’t make a lot of sense to try and make an inverter work any harder than it can.
You therefore need to put enough, but not too much, PV power into the inverter in order to have the inverter efficiently produce AC power on the output side. The industry standard for turning PV-rated DC power into AC output power is 80 percent. You can use this value to help determine the size of the inverter based on the array you’re working with.
For example, if you go through the process I describe in the beginning of this article and decide that your client is a good candidate for a 5 kW array, you can use that array size to narrow down your inverter choices. Because the overall system efficiency will be around 80 percent, you can multiply the 5 kW array size by the 80-percent efficiency rating to calculate the minimum inverter rating.
5 kW array x 80% efficiency = 4 kW minimum inverter rating
This calculation tells you that for a 5 kW array, you should have at least a 4 kW inverter. This inverter-sizing methodology is within the specifications of most ers. Of course, you should confirm with the inverter’s manufacturer that it supports this sizing method.
To find the maximum rated input power from the PV array, refer to the inverter’s specification (spec) sheet. You can put less power on an inverter than what the calculated maximum array size suggests, but you should put on more only in very rare circumstances (and only after careful consideration of the effects). If you were to put on more power, the inverter would limit the output to its rated value, so your client wouldn’t get any benefit from the extra modules anyway.
Believe it or not, I encourage you to put fewer modules on inverters than what they can handle. If you think about the last example, if you went ahead and put 5,000 W on that 4,000 W inverter, you’d be limiting the overall system efficiency to 80 percent.
On days when the efficiencies are higher than normal (say, a cool day with high irradiance conditions), the inverter would have to limit its output, resulting in a lost opportunity.
Also, I don’t like to see inverters pushed so hard for long-term reliability. An inverter’s internal temperature rises as the number of watts out of the inverter rises. As with all electronics, the hotter an inverter runs, the shorter its life will be.
When I design a PV system, one of my first goals is to multiply the PV array’s power value by 87 percent rather than the 80-percent maximum value. Doing so gives me a larger inverter wattage that’s closer to the PV array’s wattage. By keeping the PV array size closer to the inverter’s rating, you keep the inverter cooler, increase its life, and allow the array to operate at maximum efficiency values at all times.
So in the earlier example, the minimum-size inverter for a 5,000 W array would be:
5,000 W × 87% = 5,000 W × 0.87 = 4,350 W
However, a 4,350 W inverter isn’t an option (because inverters come in nice round numbers like 4 kW or 4.5 kW), which means you need to look for an inverter that’s close to this value. You may be able to find a 4,500 W inverter, or you may need to jump all the way up to a 5,000 W inverter. (Note that this efficiency level allows the system to operate at approximately 87 percent, a value that won’t limit the array’s power output. The real world may of course interfere, but the 87-percent efficiency level is still a solid starting point.)