By John Matheny and Emily Collet —
Uses for Batteries with Solar PV Systems
A grid-tied battery system can provide two primary functions: Emergency Backup Power and Energy Arbitrage (self consumption). Some batteries can be programmed to do a combination of functions. Reading this article will help provide some background information: https://solarworksca.com/net-energy-metering-nem-solar-homes-solar-works-sonoma-county/
Emergency Backup Power Mode
In the event of a grid failure, batteries can be used for back-up power purposes. The most commonly installed batteries on the market can provide 10 kWh of energy, and a maximum of 5,000 watts, enough power to run a freezer or refrigerator and provide some lighting when the grid is down. The solar PV system recharges the battery during the day (if the sun is out).
Energy Arbitrage (Self-Consumption) Mode
In this mode, the primary function of the batteries is to reduce net usage during times of day when imported energy is costly. Assuming the batteries are charged from the previous day, they help reduce the amount of energy imported from the grid during the portions of the day when energy is most expensive (and in the highest demand). Once the sun comes up, a grid-tied solar system charges the batteries first, then power is allocated to home loads and any excess energy is exported to the grid. This helps make the grid more stable by decoupling demand from production. Here’s why: Batteries store energy when the grid-tied solar system is producing and discharge energy when there is higher electricity use, usually in the evening. Whether this is cost effective or not is dependent on a lot of factors, the main one being when energy is used. This is one of the reasons that supplying Solar Works with hourly interval usage data is so important in ensuring we can design the best system for you.
The illustration on the left shows the usage and production of a household with a traditional PV system with no battery. The red represents usage coming through the meter, the green curve is solar production; and the blue shows “Self-Consumed”, which means the energy produced by the solar and directly used by the household. The green shows energy that is exported to the grid. Click on image to see larger view
The illustration on the right shows a PV system with a battery set to Energy Arbitrage mode. The production and consumption are identical to the graph above. The green now represents energy that is produced by solar, but stored in the battery for use later in the day. By doing this, the consumer will avoid paying a high cost for energy that is consumed in the evening, after the solar production is low, but before the cost of energy reduces (this time is different for each rate schedule). The battery is programmed with the rate schedule so it knows when to charge, and when to discharge to maximize its effectiveness. Click on image to see larger view
Incentives and Subsidies
Customers who install battery systems charged by solar or other renewable power source might be eligible for the 26% Federal Tax Credit (FTC) on the cost of the battery.
Additionally, in California, the Self Generation Incentive Program (SGIP) offers rebates for battery installations on a per kilowatt hour (kWh) basis.
For more information about financial incentives and advantages, please visit these sites: