Blog Post

Grid-Tied vs. Behind-the-Meter Energy Storage: Putting Power Back in the Hands of End-Users

All over the country, winter storms are known to cause power outages that last for days or weeks at a time. As a result, upstate New York homeowner Bob Schluter designed a fully-automated, net-zero smart home with the ability to fully function off the grid for three weeks or more. This solar+storage installation combines a 72kW dual-axis tracker solar PV farm with a 163kWh battery bank. The system can switch seamlessly between solar, the batteries, the grid and a backup generator (if needed), to ensure zero interruption of power to the home in the event of a centralized grid failure. This seamless integration is made possible because Schluter owns these distributed, behind-the-meter assets, not his utility. Additionally, the system’s software can detect when the grid is down and “island itself.” It also knows when the grid is “back up” and can go “back online” automatically.

NY State: Award winning fully automated home with integrated PV, power storage, grid and generator.

After Hurricane Sandy, it took New York utilities 13 days to restore power to 95 percent of their customers. This is the potential vulnerability of grid-tied assets that distributed, behind-the-meter assets, such as energy storage solutions, are designed to eliminate.

Power on the Your Terms

Since the energy generation of a PV system does not match a typical end-user’s energy usage, the industry relies on rate structures to deal with energy produced, but not consumed. These structures are typically net-metering (NEM) agreements or feed-in tariffs (FIT). With a NEM and FIT, end-users receive a credit from the utility for excess energy production, essentially helping to lower their electricity bills. Depending on the rate structure, the inverter in these grid-dependent systems will decide whether to feed energy to the grid or store it for later use.

In the event of grid failure, independent power-producing inverters attached to the grid are required to turn off. This prevents the DC-to-AC inverters from continuing to feed power into small sections of the grid, known as ‘islands’. Powered islands pose a risk in the event of grid failure, therefore, distributed assets and inverters must detect islanding and immediately stop feeding the utility lines with power. This is known as anti-islanding. A grid-tied solar power system is required by law to have a grid-tied inverter with an anti-islanding function that senses when a power outage occurs and shut itself off.

Energy storage assets that can’t be relied on without utility grid access and centralized software platforms are not necessarily assets for the end-user. Rather than creating energy security and resiliency in times of catastrophic or local failure, these systems promise the same chokehold over end-users that a grid-tied home or building represents without PV or storage assets.

Conversely, behind-the-meter assets that are owned and controlled by the end-user allow critical systems to operate and ‘fail back’ to local, autonomous control to meet the needs of the end-user. During a centralized grid outage, this is the most basic kind of resiliency as it utilizes equipment that has the capability to remain operational in the absence of the grid or centralized control systems. It is also the essence of what we, at SimpliPhi Power, mean by “Power. On Your Terms.”

In the case of distributed generation with battery back-up, also known as ‘intentional islanding’, the inverters immediately disconnect from the grid and direct the distributed generation to power local electrical loads in the building. When batteries are combined with an energy generation source and inverters, end-users do not lose power at night when the grid and sun are down. If their storage system runs on low voltage, they also do not require high-voltage utility personnel or electricians to get their systems up and running or ‘back online’.

In this respect, distributed assets can meet the needs of, as well as be owned by, both the utility and the end-user during normal operation. At the same time, these assets can continue to meet the needs of the end-user during grid failure. This serves the utility in terms of the value and security those assets represent to its ratepayers.


Accessing Remote Assets without Wi-Fi

In today’s marketplace, very often distributed PV plus storage systems and technologies require centralized grid and/or internet connection to maintain access, warranties and functionality (with the exception of companies like SimpliPhi). Because distributed solar plus storage assets can provide security and resiliency during grid failure, it is important to consider how the central control and ownership of those assets can be deployed and utilized by the end-user during grid failure, while maintaining the warranty. When designing storage plus PV assets, it is essential to understand how these critical systems can continue to operate and “fall back” to local, autonomous control when the grid or communications network is down or compromised, even if they are owned and operated by a ‘virtual’ third party.

To build true resiliency, the microgrid, even if grid-tied, must be able to disconnect, or island, from the grid in the event of centralized grid failure, such that the local assets continue to provide power for local electrical loads. This capability is often overlooked in the debate on the importance of and rush to implement smart cities, virtual power plants and the ‘Internet of Things’. When centralized software platforms control a microgrid’s functionality and access, by definition, it is vulnerable to cyber attack and catastrophic events, thereby jeopardizing the safety and security of people on a local basis.

The overriding reason to invest in site-specific, locally controlled power assets is that they can be relied on when the grid fails. The more distributed and the less dependent assets are on a centralized software and hardware infrastructure to function or be serviced, the more resilience they offer end-users. In the event of a grid failure, distributed assets that are safe, simple, low voltage and easy to service, without trained electricians or IT experts, ensure the homeowner, business interest, school, hospital or community are more likely to maintain access to power, as well as to get them back ‘online’ without being dependent on the utility. When these distributed end-user owned and/or controlled considerations are taken into account, solar plus storage systems are able to meet the needs of the end-user and function on their terms delivering value throughout their lifetime, particularly during times of centralized and catastrophic failure when they are needed most.