Synergy is ahead of the pack when it comes to solar battery systems (or Energy Storage Systems, ESS) and has been installing them for more than a decade. We remain on the forefront of new battery technology. In fact, we feel a battery is so important that Synergy stopped installing solar without batteries in 2019 with few exceptions.

Recall that the utility has various structures whereby they pay you back for the power you export to the grid from your unused solar production. The first of these structures was called Net Energy Metering (NEM1). Subsequent versions are NEM2 and the new Net Billing Tariff (NBT), sometimes called NEM3.

There are two essential system designs using storage batteries, based on what your goals are. One is a backup battery that will provide power to the items you want when there is a grid outage. The other is a savings battery intended to lower your utility bill by storing your excess solar production and using it later instead of exporting it to the grid known as “self-consumption”.  A savings battery will store your excess solar power then discharge it during the peak, more expensive pricing periods known as “peak shaving” and at night when the solar is not producing power. It will result in modest savings when used under NEM1 and NEM2. Under the new NBT, a savings battery is essential and will increase the bill offset and decrease the payback period of a solar system and will quickly pay for itself. A backup battery system under NBT will include a larger battery to be able to self-consume offsetting bill costs and have enough capacity left to meet your needs when the grid is down. Back up capability can be added to a savings battery set up at a later date with no effect to your NEM/NBT status under current rules.

Below I am going to explain the differences of design, battery sizing, installation, and cost between a backup and savings battery. If you don’t know the difference between a kW (kilowatt) and a kWh (kilowatt hour) I have written a separate blog on this here. It may be beneficial to understand these before proceeding since all battery systems have ratings associated with both kW and kWh that will help you understand how a system is properly designed and sized.

A backup battery used as backup power is designed to handle the desired items to be powered when the grid is down. This design determines the size of the battery system in both kW and kWh to result in a system that meets the homeowners needs. A backup battery system requires some equipment that is not needed for a savings battery system. With this added equipment also comes more labor and other material costs. The additional equipment includes a transfer switch and usually a backed-up loads panel. To meet code and keep utility workers safe while working on the lines there needs to be a transfer switch that isolates the backup system power from the rest of the grid when operating in backup mode. There are many items in a house that most folks do not need to be powered in a grid outage such as a pool, hot tub, AC unit, or electric oven. To increase the reliability of the system and provide power to the items most homeowners want backed up (refrigerators, freezers, some lights, internet etc.) and to keep overall system costs lower, a backed-up loads panel (electric sub panel) is usually installed. The circuits that include the items to be backed up are moved to this sub panel. This is what is powered by the battery during a grid outage and the items that are not on this panel will not be powered when the grid is out. Most people that have a back up system also use the battery daily to self-consume their solar power. When using it this way, most battery solutions have a backup reserve which the system will hold for a potential grid outage. If the reserve is set at 70%, the battery will only use up to 30% of its capacity to self-consume daily and 70% will always be available to begin a grid outage.

A savings battery does not provide power during a grid outage. Therefore, it does not need to be sized to handle specific items but is sized to maximize savings. As stated above, It does this by storing excess solar power the house is not using in the middle of the day and discharging it during the more expensive peak pricing times (late afternoons and evenings) and when the solar is not producing power to offset utility costs. Since it does not provide power during grid outages a transfer switch is not required to isolate the system from the grid. Depending on the battery solution the transfer switch may be able to be eliminated which is the case with the Enphase battery storage system and its transfer switch, aka, the Enphase System Controller. Since this design does not provide backup power, there is no need to create a backed-up loads panel and move circuits. This reduces both the equipment and labor costs of the project. There is no need for a backup reserve to hold power for a grid outage so the battery can be discharged down to 5% resulting in a smaller battery to meet the homeowner’s needs.