As more and more different types of DC to AC inverters come onto the market for solar and storage management there is confusion about what each type does and where they should be used. Particularly, the term “hybrid” seems to be applied to many different types of inverters without any real definition. Here, I will talk about the different types of inverters.
Modern inverters can be roughly categorized into four different types; PV Grid Interactive; PV Grid Interactive w/ Storage; Storage Grid Interactive; and Battery to AC. There may be some overlap between these types and certainly there is sometimes interaction between the different types.
PV Grid Interactive, also known as Grid tie inverter, Grid following inverter, or when micro-inverters are bundled with a PV module, an AC module. This is the most familiar type of inverter for solar systems and one that simply takes DC current from the solar array and inverts it into AC current that can be fed into the local utility grid. They do require a grid connection to operate and generally cannot form an AC output on their own (there are a few exceptions now). These inverters can be separated into sub-groups; Micro-inverter; String inverter; Central inverter. These inverters are listed to UL-1741. Examples of PV Grid Interactive inverters includes; Enphase, SolarEdge; SMA Sunny Boy; Fronius; and many others.
PV Grid Interactive w/ Storage, often considered an ESS (Energy Storage System), is a newer version of a PV Grid Interactive inverter that will also accommodate the addition of energy storage. Sometimes called “hybrid inverters” this terminology is used for other types of inverters and is not very useful for describing this type of inverter. The primary purpose of this type of inverter is the same as a grid interactive inverter, but can also use energy storage to absorb or release power when it is more useful, and not just when the sun is shining. Energy storage can be used to shift the solar production from the middle of the day to other parts of the day and night to enable self-supply and load shifting, particularly when the local utility does not allow net-metering or has a very low feed in tariff. Usually these are supplied with CT’s to measure the current at the service entrance so that they can supply stored energy to the whole house, and/or curtail the feed to the grid. These are often (but not always) made to be able to separate from the grid and form their own grid output to run backup loads during a grid outage. Most are able to function as only a PV grid interactive inverter without the storage installed, so that energy storage can be added later. This type of inverter can be sub-divided into two types, those with batteries having their own internal voltage conversion, that operate at a similar voltage as the PV ie: 280V to 450V, and inverters with voltage conversion so that they can operate with 48V batteries. The higher voltage batteries are generally a lithium type with direct communication between the inverter and the battery bank. The 48V type can use most 48V nominal batteries, including lead or lithium types with or without direct communications. Inverters with approved storage can be listed to UL-9540 as an ESS (using section 706 of the NEC). Examples of PV Grid Interactive w/storage inverters includes; SolarEdge EnergyHub; Generac; Sol-Ark.
Storage Grid Interactive is an inverter mated to energy storage as an ESS and as a single package, and without their own PV input. They are made specifically to store energy at certain times of the day and feed that energy back into the house at other times of the day. They can be used alone to simply shift energy from one time to another without solar, or can be mated via AC coupling to work with a separate PV grid interactive inverter to make a complete grid-tie PV system with storage. These inverters with approved storage are usually listed to UL-9540 as an ESS energy storage system. Examples of Storage Grid Interactive inverters includes; Enphase Storage; Tesla Powerwall; OutBack Mojave ESS.
Battery to AC inverters have been around longer than the other types represented her. These are known as: battery inverter; off grid; stand alone; hybrid; static; islanding; bi-directional; grid forming; inverter/charger, and other names. This is a more general range of inverters, from simply converting DC to AC for use with loads; to some that have a transfer from the AC input; to charging from the AC input; to grid-tie capability. What distinguishes these is that they always need a battery to operate. They can be made to use many different voltage battery banks with 12 volt, 24 volt, and 48 volt being the most common. These can be further divided into two types, those that will feed PV power into the grid and those that will not. Those inverters that can feed the grid do so by sending PV power, in excess of what is needed to keep the battery charged, into the loads or grid. There is the need for charge controllers to process the power from the array to the battery. Some inverters have the charge controller built in. The grid tie versions also work fine as strictly off grid inverters. Battery inverters that cannot feed into the grid may still be able to use grid or generator AC power to charge the battery, or transfer to pass through to the loads. Grid feed type inverters with approved storage may also be listed to UL-9540 as an ESS. Examples of grid tie battery inverters include; OutBack Radian; Schneider XW. Examples of non-grid tied battery inverters include; OutBack; Schneider SW: Magnum; Samlex; Morningstar; and many, many others.
Written by: Brad Bassett, AEE Sr. Application Engineer