How Do You Size a Solar PV System?
Jeff Hickson LEED AP
I have been thinking about how I size a solar PV system. The answer seems simple. You size it to the customer's electrical load. Is that the right answer though? On the face of it, it would seem so. Over more than a decade of designing my thought has changed and frankly I am better at what I do. It has been driven by my exposure to the processes of others as well as my thoughts on results. I see other designs, the promises made and frankly the lack of engineering support for some of those claims. I also see a need for more advanced questions.
Let's look at the simple way first. We consider the variables that effect solar PV output.
- Geographical position: Where are you in the country and how does your latitude affect solar irradiation (solar input)?
- Local weather patterns: i.e.… is your area more or less cloudy, sunny, rainy or more or less snow. You might be surprised at the regional difference? Even a 10 mile shift in location can have an impact.
- Azimuth of the roof or orientation of the ground array: Is your roof or ground mount facing south? If not, how far off solar south does it face and how does that affect output?
- Pitch of the array: At what angle are the panels and how does this affect output?
- Available roof or ground area: Basically how many panels can we install and will that area support a portion, all or more than the expected electrical load or requirements?
- Shading: What trees, buildings or even terrain elevations would shade the system from the sun and how does it affect the output of the system?
- Panel choice: How does the physical size of the panel affect the design as well as its inherent efficiency? How does the racking affect the design? Is it fixed, does it move?
- The electric utility: Are there utility restrictions on system size or net metering issues to address? Are there variable incentives?
All of these factors are considered to get a final system design. That (believe it or not) is the simple way. We have many engineering systems which aid us in the design and help us get to the right solution. Even data from NASA is used to substantiate the process. For me that is the simple part of the process. I believe in the customers best interest there is much more to consider.
The basic question that needs review is: Does that electrical bill history actually represent the needs of the customer? What do I mean by that? We are missing two deceptively simple things. Both the past and the future have not been considered. Let’s consider the implications of the past:
- Have there been any short term changes in electrical usage that might affect their load profile. A residential question on this might be were there any changes in occupancy during the last year? Did that customer go on an extended vacation? A business would consider changes in equipment, new manufacturing processes and past equipment usage, occupancy and more.
- Did they remove or install equipment that would have increased or decreased load requirements? For example: Did they add or remove a new pool, sauna, freezer, central AC or any other item that would affect the usage of electricity? For a business it is very similar. Was a new welder added, was piece of equipment being used more or less, was a tenant changing how they used the property?
- Was there a construction project? That short term load could have impacted the usage and is that reflected in their electrical history? This is the same for a business.
- Have they done any efficiency improvements during the last year? If so, when during the year were they done? How did that affect the electrical usage? This is the same for business or residence.
We need to also consider the future. What things will change in the future to affect how our customer will use power? Does our design reflect that? Those considerations are similar:
- Are there any changes planned that will affect the future electrical usage. Is that residential customer planning on putting in central AC, heat pumps, hot tub, pool, sauna, electric car or any other item that would change usage? For a business they might consider change of use, adding an electrical furnace for glass blowing, adding manufacturing equipment for more capacity or changing/adding conditioned space.
- Will there be any efficiency improvements planned? Is that residential customer changing to LED lighting, adding Energy Star appliances, having an energy audit with subsequent work or any other work that might change the electrical usage? For a business the same thoughts occur. Are you adding variable frequency drives for motor controls, insulation, and peak shaving usage controls or anything else that might affect the electrical usage?
This article ran longer than I intended. That is actually the point. Sizing a solar system is deceptively simple. Having the correct questions asked is as important as what the system is. Perhaps you have not had the full conversation in the past, or would just like the full conversation now. Maybe you would just like to deal with a company that takes these questions into consideration.
You should reach out. We can help. Our initial consultation is free. Please call Solar Wolf Energy at 888-878-4396 to set up your no cost analysis with a professional solar consultant. You can also request a free quote or send us an email via our contact page located here: Contact Solar Wolf Energy