Inverter Pad Engineering for Utility Scale Solar Farms

The inverter pad is the central location where all AC and DC circuits cross paths in a ground mount solar farm. The incoming DC circuits are protected and are able to be safely isolated at the inverter pad. Similarly, the AC circuits are protected and have an isolation capability at the inverter pad. Plus, there is often a meteorological measurement station, SCADA controls, energy metering, communications, tracker power and station power at the inverter pad. In short, there is a lot going on at the inverter location. In some cases, on larger solar farms, a preengineered inverter pad is replaced with an inverter skid which contains the equipment and most of the prewiring completed. Whether the inverter equipment is placed on a concrete pad, on a skid, or in a container there should be substantial solar design and engineering considerations at this focal point in the solar farm.

Before and After Pouring the Concrete on a Utility-Scale Solar Farm inverter Pad.
Before and After Pouring the Concrete on a Utility-Scale Solar Farm inverter Pad.

DC Conductors

The incoming DC conductors from the solar array will require a large amount of volume underneath the inverters. Not only is the volume large, the bend radii required by the National Electrical Code requires significant space. Proper planning for the incoming DC conductors to the overcurrent protection device is important.

AC Conductors

The outgoing AC Conductors will often travel a short distance from the inverter output to a step-up transformer  or a piece of switchgear. This short distance provides a challenge for the electrician to plan for the conductor’s path and potential obstructions from other solar farm conductors.

Station Power

There has to be a lot of consideration ahead of time to provide power to the various pieces of equipment at the pad. If the solar farm is using single axis trackers, each tracker motor or actuator has a controller which allocates control commands and power to the drive units. The distribution of power under the pad and to the individual pieces and parts can be confusing unless carefully planned.

Meterological Station and SCADA

The measurement and controls associated with a utility scale solar farm or commercial solar farm can be the last 5% of work that takes 95% of the effort. Many of the control wires will run between various pieces of electrical gear and communications equipment. Also, a Supervisoary Control and Data Acquisition (SCADA) system requires at least one phone line and one data line. This is the spaghetti mess of detailed wiring that if, properly planned, can save your precious brain cells.

The Full Picture

Many solar installations are completed with minimal solar engineering and planning services from a professional because this is a budget-driven adolescent industry. However, we have witnessed more than once that the sweet taste of a low price is long forgotten after the bitter results of an unplanned inverter pad.


Putah Creek Solar Farm Flyover Video

This is the 2.6MW Putah Creek Solar Farm located in Yolo County, California. This power plant sells clean electricity to the local utility company through a 20 year power purchase agreement. Blue Oak Energy designed and constructed this commercial utility scale solar farm in 2014.

Rise of the Solar Campus

Establishing a multi-building property as a solar campus is a common sense way to tap into large areas of unused rooftop and parking lot space for solar generation. Once a solar campus is operational the company can rejoice in its brown energy reduction while contributing to a long-term facility investment. The solar campus is one of the more interesting forms of distributed generation for commercial solar applications because all available area is being used for onsite energy production.

Getting Started

When creating a solar campus, there are a few steps that need to take place. First and foremost, the customer needs to decide they are committed to reducing their electricity expenses and determine the financial goals for the project. Is the goal a 5 year Return on Investment (ROI)? Or, is the goal a 10 year Internal Rate of Return (IRR) of 8% or better? Or, is the goal to maximize solar energy generation over the given area and achieve the lowest Levelized Cost of Energy (LCOE)? In most cases, there are multiple goals and we simply need to work through the details together.

Once the basic parameters are established, Blue Oak Energy can then work with the customer to design a solution that will bring/ensure/reap the targeted financial benefits and meet or exceed the project’s overall goals. Our analysts and engineers will then look at the electrical interconnection points and the available space for solar arrays on rooftops, in parking lots (for solar carports) and open land for ground mounted solar arrays.

USVA McClellan Air Force Base Solar Campus in Sacramento, CA
USVA McClellan Air Force Base Solar Campus in Sacramento, CA

Once all the preparatory pieces are in place, the engineering team at Blue Oak can design the implementation plan and ultimately begin installing a solar campus system. The variety of solar arrays located on rooftops, carports and open property will create what we call a solar campus. The commercial solar campus is a beautiful creation because your open rooftops and parking areas will now become energy generating assets!

Before or After?

While it is becoming more common for solar to be incorporated into new construction plans, solar is still, for the most part, an afterthought. This means we will usually be looking at the building or campus after construction is complete and attempting to find a solution to accommodate the solar. When implementing commercial solar projects across existing facilities, we are careful to consider the logistics to enter, stage and construct the solar project. We have worked across many active campuses to install solar facilities and the collaborative effort to install solar is important. We love working with engaged customers and tenants who will help design the implementation and logistics plans to meet their sensitivities.

Financial Impact

While it is unlikely that the output from a solar array can completely extinguish the annual energy consumption of a high energy corporate environment, a commercial solar energy system will greatly reduce peak loads and provide a significant financial return to the host customer. In many retail stores, we have found the annual energy production from a solar array will reduce the energy import from the local electric utility by up to 30%. Typically, the annual energy import may be reduced by 5% to 100%, depending on the site’s energy consumption and the available area for siting the solar array. However, its not always about the reduction to annual energy consumption. Increasingly, utility companies are relying on Time of Use (TOU) energy rate structures and tiered rates based on a baseline of total peak demand. Installing a commercial rooftop, carport or entire solar campus system can have significant impacts to these creative electricity rate structures. Our analyst team is great at running the scenarios to determine impacts to TOU and tiered rate structures.

Google Headquarters Solar Campus in Mountain View, CA

Why hire Blue Oak Energy for your next solar campus project?

When implementing a solar campus, there are many factors and challenges that can impede progress; Blue Oak Energy has the experience and know-how to navigate these complex issues and contingencies. A case in point is Google Headquarters in Mountain View, CA When we engineered the solar rooftop systems and carport solar arrays for Google, we were dealing with a unique scenario. The entire main campus has a single utility company meter which fed a medium voltage distribution loop around the campus. On the Google project, we learned to work with a distributed interconnection architecture and interconnecting the inverter output throughout that medium voltage system at multiple access points.

We have had similar challenges at several Naval Facilities campuses , as well as with other corporate campuses such as Fortinet in Santa Clara, CA. Also, the solar campus project the McClellan Air Force Base in Sacramento, CA is a great example of a solar carport and solar rooftop offsetting brown energy consumption.

To get a better idea of what a solar campus can do, take a look at Blue Oak Energy’s

Fortinet Solar Campus in Santa Clara, CA
Fortinet Solar Campus in Santa Clara, CA

portfolio of completed solar campuses. Solar campuses are a smart way to reduce energy costs, and when combined with solar carports, the possibilities are even greater.

INCREASED NET METERING APPLICATIONS: Virtual Net Energy Metering (VNEM) & Net Energy Metering Aggregation (NEMA)

Increased applications for Net Metering could mean exciting new benefits for utility customers. But before we talk about some of these developments, let’s look at how traditional Net Energy Metering works.

What is Net Energy metering?

Traditional Net Energy Metering (NEM) allows electricity customers with solar capabilities to reduce their electrical load while also receiving a financial credit for onsite solar power production. The NEM customer’s expenses are trued-up annually to include the solar energy contribution for their billing cycle.  Typically in North America, the customer generates surplus electricity in the summer while consuming more electricity in the winter.  The goal is for customers to fully offset their annual energy consumption with solar energy production to result in a net-zero energy import.  What NEM does is allow customers to size their generation to meet their annual load rather than the peak demand

NEM started in the 1990’s and today is widely adopted in over 43 states. Recognized as an important US policy framework, NEM supports direct customer investment in grid-tied distributed renewable energy generation for commercial businesses, residential customers, and public entities. Most importantly, it provides a long term, predictable benefit that approaches the fully bundled retail rate the customer would normally pay to the utility company.  Although in the past NEM has been limited to a single residence or commercial building with a single meter, the policy is continuously expanding its scope.

For example, the California Public Utilities Commission recently initiated a broader NEM application to allow for more complicated ownership structures to increase their commercial solar power production. Many types of commercial buildings and properties are now eligible for these expanded programs. And customers with multiple electricity meters or limited possibilities for incorporating a nearby solar array are now excellent candidates for solar energy production.  Thanks to the California Public Utility Commission and its stakeholders, the rules for new NEM applications are expanding to address previous shortfalls.

What about Virtual Net Energy Metering?

VNEM is a way of allocating on-site energy generation virtually through the utility billing system, rather than by hard-wiring separate systems to each tenant’s electricity meter or electrical load center.  The result is an increased ability to account for net energy metering of multiple customers with a singular grid tie-in that is separate from the electricity load center.

Who uses VNEM? VNEM is ideal when multiple electricity customers share a single contiguous property such as apartments, strip malls, condominiums and corporate complexes, which lack the area or rights to directly install solar power.  With VNEM, a single commercial solar energy system may be installed to cover the electricity load of both common and tenant areas connected at the same service delivery point. The proportion of solar energy owned by the tenants will then be applied to their utility bill. Several pieces of legislature, including The California Community Solar Bill (Senate Bill 43) and the Colorado Community Solar Gardens Act (HOUSE BILL 10-1342) are essentially VNEM policies. Utility companies such as the Sacramento Municipal Utility District (SMUD), Orlando Utilities Commission (OUC) and Seattle Power & Light have all implemented limited VNEM programs to better serve their customers.

NEM Figure 1

Figure 1: Several VNEM customers, image courtesy of

What is Net Energy Metering Aggregation?

The structure for NEMA projects (based in California) differs slightly in that it allows for a single retail customer with meters tied to multiple interconnection points, or “service delivery points”, to offset energy consumption across those meters with a single onsite renewable generator.  However, in order for multiple interconnection points to receive credit for the renewable generator, they must be on contiguous or adjacent properties.  There are some nuances to the definition of “contiguous and adjacent”. For example. the definition does generously allow a public right-of-way such as a road or a power line to dissect the various qualifying properties. The NEMA strategy gets complicated when there are multiple meters on different rate structures or an overproduction of energy compared to consumption.

NEM Figure 2a


NEM Figure 2b


Figure 2: Net Energy Metering Aggregation (NEMA)

How Blue Oak Energy Can Help

Since these applications are a new twist to the traditional solar NEM structure, Blue Oak can help customers navigate decisions about where to allocate energy and why. We know that both VNEM and NEMA require monthly and annual calculations for costs and balance of energy production and consumption, and in California they allow only 1 MW-ac production.  On the positive side, VNEM offers exciting possibilities for multi-tenant buildings and common commercial buildings to install a single solar system to cover common and tenant area loads.  And NEMA opens up larger single customer complexes to solar arrays that can contribute to the customers’ full load without applying the power directly to the same meter.  These niche onsite commercial solar energy generation applications create new potential for developing and delivering distributed generation solar facilities, and we look forward to exploring these new opportunities.

Photovoltaic Energy Generation Water Footprint

By guest blog poster Matt Seitzler, PE, The Davis Energy Group

Drought and Water Usage

As a result of last year’s record low rainfall, at the beginning of this year California Governor Jerry Brown declared a state of emergency asking all of California to conserve water during what would end up being an unprecedented three-year drought. Today California faces this drought where nearly 58% of the state is classified by the U.S. Drought Monitor1 as being in an “exceptional level” of drought;  it’s highest level of drought intensity classification. This heightened state of awareness regarding water has brought the issue of water usage across all sectors of society into focus. One area of particular interest not often discussed, is the issue of water usage in the production of electricity. As the US population grows the stress on freshwater resources is now becoming evident especially in areas like the Southeast of the US, where in some cases water usage via the generation of electricity is greater than that used by residences within certain areas2. In fact in the report Burning our Rivers: The Water Footprint of Electricity, the authors calculate that, for an average US household, the amount of water consumed for electricity generation to meet household loads—using the current portfolio of generation technologies—is five times that of the average water consumption.

Water Footprint for Electricity

All types of electric power generation use water in some form, but not surprisingly this amount is not the same across generation technologies.  As a part of the Burning Our Rivers report, the authors analyzed the water footprint for traditional electricity generation sources (coal, nuclear, and natural gas) as well as emerging resources like renewable energy technologies (solar thermal, geothermal, photovoltaics (PV)). Comparison of water usage is based upon the determination of the water footprint of each

Figure 1. Lifecycle Water Use for Electricity Generation by Fuel Type (Gallons/MWh)
Figure 1. Lifecycle Water Use for Electricity Generation by Fuel Type (Gallons/MWh)

technology in gallons per MegaWatthour (MWh). Water footprint is classified in terms of the sum of the amount of water used, or withdrawn, and the amount of water wasted, or consumed; usually as a result of evaporation of stored water.  One item to note is that while the water footprint is useful in comparing generation technologies, another important aspect not captured in the volumetric water footprint are the thermal effects of certain power generation technologies on the surrounding environments. As water is often used to cool facilities and then it is ejected into the environment, this practice has been shown to adversely affect nearby ecosystems causing algae and loss of animal species.2

Figure 1. Lifecycle Water Use of Electricity (Gallons/MWh)

Water and PV

The Burning our Rivers report analyzes water use during the upstream or manufacturing phase and during the on-site power generation phase of each of the technologies included. In the chart above, findings from the report show that PV is the second overall least users of water on a gallons per MWh basis. Additionally, the production of PV modules and equipment was found to require a minimal amount of water withdrawn and nearly no water consumed during the manufacturing phase. In the on-site power generation stage of the analysis, PV with its low water use requirements during operation and maintenance enabled it to be the second least user of water in that category as well.

Commonly, PV facilities require small amounts of water in their maintenance typically due to the cleaning of module surfaces resulting from soiling from the environment. Previously done by hand, with the introduction of robotic cleaning devices, the use of water can be more precisely controlled while even some manufacturers are reporting positive cleaning results using robotic cleaning devices without the use of water at all.

So, while the use of PV for power generation might require the use of more land and possible animal habitat, it would not be at the cost of additional water loads and the introduction of thermal load into the environment. With the introduction of new energy storage technologies coupled with a beneficial water footprint, PV is poised to be an even more sustainable way to meet our electric power generation needs.


[1] US Drought monitor website:

[2] Burning Our Rivers: The Water Footprint of Electricity, by W. Wilson, T. Leipzig, and B. Griffiths-Sattenspiel,