Why would a customer want batteries, and what purpose is served by them?  Several years back, lead acid batteries were still installed on grid tied, battery backup systems, but most modern systems are various lithium chemistry batteries, and they come in different sizes.

There are 2 purposes to batteries on modern solar, called “paired storage”, meaning the batteries are paired to the grid and able to export (sell) energy to the grid.  Many people prior to NBT on PG&E have installed batteries, and that was generally for backup power.  There are 2 main reasons that paired storage is installed:

1. Production shifting during the day (most important to the utility)
2. Backup Power during outages (most important to customers)

Is it possible to have one option and not the other? Yes.  On NBT a customer needs to install paired storage (usually 5-20kwhr) to let the system shift the high production during the day to the evening grid shortages, helping the grid out.  The reason that the customer wants the system to do this is that the tariff policies have been designed to incentivize by means of evaluation of power, measured by the kilowatt-hour (KWHR), to not send power into the grid in the middle of the day (like NEM 1 & 2 customers) if possible, but to store this power for (a) keeping the customer from buying power during evening peak time, and (b) to export power into the grid in the peak time, to be paid for the power.  During certain times of the year, the buy-back rate for export is much higher than the customer ever pays for power from the grid.  NBT systems can be done with paired storage, but only part of the bill will be offset by $.  In other words, the system might produce as much energy as the customer uses in a year, but the customer will not offset all they spend, since too much energy exports into the grid during the peak times of the day, and only a little bit exports into the grid in the peak hours, since they are no batteries to assist with that, as the sun is going down.

In our experience, in the NBT environment, customers are split on wanting to have the batteries provide backup power in addition to the store/sell function described above.  This is because there is some extra cost (usually around $4500) for extra labor and materials, like a transfer switch.  Many customers want a large standby generator, since battery systems supply limited power output, or many customers already have a portable generator.  The discussion of having batteries but not having power during outages is something that we are detailed about in clarification with new customers.