Retrofitting Existing Grid-tied Systems With Batteries

By mtvSolar | May 16, 2018 | Category mtvSolar

Battery storage was extremely expensive just a few years ago. Now that the prices for lithium packs have fallen by over half in the last year alone, the option to add a battery to your system is much more accessible. This somewhat technical article goes into the options available and why we would recommend one solution over another. Of course, if you have any questions, feel free to reach out to our team of trained professionals with your questions. As with any solar or battery storage system, make sure that your contractor is properly licensed, insured and has NABCEP certified professionals on staff.

Why have a battery?

A standard grid-tied system is designed to directly offset electricity consumption from the utility via net-metering, leading to lower utility bills. Because a grid-tied system does not have a battery, it can not operate when the power fails, and the system will shut down. This is required by code so that utility workers making repairs do not get electrocuted by back-fed power.

Having a battery solution with your solar enables it to disconnect from a non-functional utility grid and continue powering critical loads, a process called “islanding”. The process by which this can happen is explored in our article linked here.

Best Options for Retrofitting

Before reading further, we recommend reading our article entitled “DC/AC Coupled Design Practices for Battery Systems”. This article goes into the technical explanation of AC and DC coupling, and the reasons to choose one or the other.

Regardless, if you want the best solution possible, with the longest battery lifespan, highest reliability and maximum flexibility, the recommended solution would be all DC coupled StorEdge or all DC coupled to a Schneider XW system and zero or very small amount of AC Coupled PV.

Existing SolarEdge system

If you have a SolarEdge system and the StorEdge unit would have the capacity to support your critical loads (5000w continuous), then this is the least expensive retrofit. Nothing on the roof needs to be touched, and the existing inverter can be replaced with a StorEdge and LG battery solution. If your loads exceed the power capacity of the StorEdge unit, an option would be to pull some optimizers and install a Schneider XW system, and AC couple the remaining SolarEdge PV.

Existing SMA system

SMA Sunny Boy systems are also relatively easy to retrofit with a Schneider XW system. Since Schneider charge controllers are 600v, and the Sunny Boys are 600v, it’s a straightforward upgrade to a Schneider DC coupled system. If the system is rather large, we may opt for a DC/AC coupled hybrid design to avoid excess costs of DC coupling the entire array. The other option would be to install SolarEdge optimizers and a StorEdge and LG battery solution.

Existing enPhase system

If your system is built with M190 micro-inverters, then due to the historically high failure rate of that model, we would replace all of them with either StorEdge or a Schneider XW system depending on your budget and loads.

Other options include adding some new DC coupled panels and AC coupling some or all of the remaining enPhase, pulling some enPhase and DC coupling those panels, or pulling all microinverters and converting to a purely DC coupled system. There are many considerations here, including the size of your battery bank and how much PV you have, which would determine the most cost effective retrofit.

Other Considerations

-If there is a well pump or sump pump larger than 1/2hp, we would strongly recommend a Schneider XW system, or replacing the pump with a soft start model that the StorEdge inverter can reliably start.

-You can not AC couple any PV to a StorEdge system. If you want the StorEdge+LG system, we’d have to install optimizers on all existing panels and pull the old inverters.

-If the existing inverter is getting close to 10 years old, it may be better to replace it entirely, as most inverters are standard warrantied for 10-12 years or so.

-If the existing inverter is of a generic brand and/or the company is out of business, it may be best to just replace it. AC coupling is hard on PV inverters and older or lesser quality ones could fail when cycled on/off during an outage, the worst time to fail.

-Total wattage of AC coupled PV can NOT exceed the wattage of a single Schneider XW inverter, even when multiple XW inverters are stacked. Therefore, the largest AC Coupled component is always limited to 6.8kW max, the rating of the Schneider model XW6848 inverter. Another important consideration is installed battery capacity. For example, AC coupling 10kW of PV into an XW6848 with 8 DEKA 210aH batteries would result in equipment damage or meltdown, since you are not only exceeding the inverter capacity, there isn’t a large enough battery buffer to absorb the incoming unregulated charge (refer to the article DC/AC Coupled Design Practices)

-Arrays DC coupled to Schneider charge controllers need to be shade free and on the same roof plane for efficient operation, or we can install Tigo optimizers to mitigate the shading to some extent. StorEdge systems already have very high partial shade mitigation due to the SolarEdge optimizers.

-If a generator is to assist the battery backup, it must be a Schneider XW system, preferably with zero AC-coupled PV, or an interlock to prevent the generator and AC-coupled PV from coming on simultaneously. This is because the AC coupled PV could backfeed the generator if not absorbed by the loads and battery.

Getting Started

Mountain View Solar has more experience with solar and battery systems than any other contractor in the Maryland, Virginia, West Virginia and Pennsylvania service area with 2 Megawatt-hours of installed residential battery capacity going back 10 years. Please fill out our quick and easy consultation form if you’d like to discuss your options with a seasoned professional today!

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