Thinking About Retrofitting An Existing PV Plant with Storage?

Commercial and Technical Considerations for Adding Storage to Existing PV Projects

The benefits of coupling Solar + Storage are becoming more widely understood by the power sector. On a conceptual level, the idea just makes so much sense. Solar, while wonderful from the perspective of being a totally clean and free source of energy (the sun), is intermittent and not always available when you need it. Complimenting PV with battery storage can be a great solution to those limitations.

As a consequence, a number of solar developers and asset owners are asking themselves: Why wait until my next new project to incorporate storage with solar?  Can I just add storage to my existing operating assets or already approved projects?

If you’ve found yourself asking the same questions, then read on to learn more about some important issues to keep in mind when considering repowering solar with storage.

AC vs. DC Coupling for Repowering

When considering any Solar + Storage project, the first question you need to answer is whether to build an AC-coupled or DC-coupled system.  In an AC coupled solar plus storage system, the batteries (i.e. storage) are charged after the solar energy has passed through the PV inverter, i.e. such systems have two inverters, one for the storage and one for the PV, the connect to the transmission grid. In a DC-coupled system, the battery is charged on the DC-side of a single gird-tied inverter. While there can be a debate as to the relative merits of each approach, DC-coupling clearly makes more sense for most projects where you will be repowering (i.e. retrofitting) solar with storage for the following reasons:

1. Much Easier Interconnect Process: While there can be many reasons to consider DC or AC coupling for various Solar + Storage use cases, retrofitting storage into an existing PV plant (or even one that isn’t built, but has an interconnect agreement already) mitigates strongly in favor of DC-coupling.  The reason for this is that, when coupling storage on the DC-side of the inverter, you are not altering the AC capacity of the system. When AC coupling, the utility could look at the AC capacity of the system as the sum total of the AC capacity of the battery and the PV, which could force the need to alter the interconnect agreement. In many cases renegotiating the interconnect agreement may not be a possibility. Certainly, in almost all cases, revisiting an interconnect agreement would at the very least be costly and time consuming.

As an example, for a 5 MW PV system with a 3 MW battery, a utility might be inclined to look at such a battery energy storage system (BESS) as an 8 MW system if it were AC coupled, while it would look at it as a 5MW system if it were DC-coupled.

2. Potential Tax Benefits*: Another point in favor of DC-coupling for repowering efforts is the potential tax advantage.  When charging storage on the DC side, the cost of the storage system is eligible for the ITC. As the cost of storage is significant, the credit provides a dollar for dollar reduction in taxes owed.  In addition, if you claim the commercial ITC, you can typically also take advantage of accelerated depreciation to reduce the overall cost of the installation and/or the storage system itself.  While the ITC is a credit, depreciation is a deduction meaning it reduces a business’s taxes by calculating the sum of the amount of the depreciation multiplied by the business’s tax rate.  Click here to read a great article about solar and storage and the ITC from NREL. 

3. Opportunity for Tax Equity Investors*:  Whether in a fixed or time-based flip structure, or even a pay-go structure, in new or existing PV plants, the potential tax advantages mentioned above regarding DC-coupling can be particularly attractive for tax equity investors. If you are currently a tax equity investor, have the appetite for tax equity or are looking for a way to lower your taxable income, a DC-coupled retrofit of storage into even a new operating asset, either the ITC, or accelerated depreciation, or the ITC in conjunction with the accelerated depreciation, can substantially improve the ROI of a solar plus storage deployment and/or provide significant tax advantages beyond all of the other benefits that complimenting the PV installation with storage provides. 

Killing Three Birds with One Stone

We’ve all heard the cliché about “killing two birds with one stone” in reference to solving two problems with one solution. Well, taking the cliché a step further, augmenting an existing PV project with storage on the DC-side with Alencon’s unique SPOT PV Harvesting system for PV-centric coupling of Solar + Storage can literally kill three birds with one stone by providing system owners the following benefits:

1. Adding Storage – Alencon’s SPOT PV harvesting system provides a very unique solution in its own right for DC-coupling of Solar + Storage. The SPOT allows storage to be added directly to the same DC-bus as the inverter. Adding storage to an existing PV project makes solar a truly dispatchable energy resource and thus opens additional revenue opportunities for that plant.

2. Improving PV Yield – While facilitating the addition of storage, the SPOT will also provide the additional benefit of injecting granular MPPT into the PV system which could serve to boost DC yield if some degree of PV panel mismatch exists in the array, as can often be the case for aged solar arrays.  While additional DC yield may not be that attractive to PV only plants with a significant DC-overbuild, it can be particularly attractive when the opportunity to capture all of a plant’s DC generation can be achieved with a DC-coupled battery, a technique known as clipping recapture.

3. Possibly Replacing an Inverter – At the moment, many PV plant owners are getting to the point of considering inverter replacement. This could be a particularly daunting challenge for owners of plants with 600-volt strings, as finding 600-volt inverters to replace their original inverters is virtually impossible in today’s power electronics marketplace. As such, only 1000 or 1500-volt inverters may be available for older plants with 600-volt rated PV strings. In such instances, the SPOT, with its unique Galvanic Isolation-based approach to DC-DC optimization, can help bridge the voltage gap between the older panels, the new inverter and the battery’s charge/discharge voltage range on the DC-bus.

Technical Considerations

Like any new technique in large scale alternative energy, it would be naïve to assume retrofitting storage in an existing PV plant is an “off the shelf solution.” Such a decision will require thoughtful technical planning. Here at Alencon, based on our own experience with such projects, we offer the following advice when considering a retrofit of storage into an existing PV plant:[if !supportLists]

1. Galvanic Isolation – When retrofitting storage into an existing PV project, the importance of galvanic isolation between the PV and DC-bus to which the inverter and battery are connected cannot be emphasized strongly enough.  Having galvanic isolation between the DC-bus offers the ability to flexibly ground all of the system’s components in a very intuitive manner while protecting the inverter and battery from any potential harmful faults that occur in the PV array. This can be particularly helpful in aged PV systems which may be more prone to arc and ground faults than newly constructed systems. Galvanic Isolation also solves the problem of potentially injecting reverse bias from the DC-bus into the PV array. Galvanic isolation essentially brings the same level of safety to DC-coupled systems that AC-coupling inherently offers, i.e. complete electrical separate of source (PV) and load (battery).

2. System Controller – One design consideration that is critical when considering a DC-coupled Solar + Storage system is the plant controller or SCADA. This piece of technology is responsible for governing the behavior of the (BESS). This technology is sometimes available from the inverter provider, though is most typically best sourced by a third party, specialist provider of such systems.

If you are considering a retrofit of storage into an existing PV plant and have any questions on doing so, please do not hesitate to contact us at and we will be glad to offer you any further guidance we can.

* Please note that this article is not meant to provide tax advice - please consult your tax advisor whether the tax advantages discussed here are suitable for or are applicable to you.