Battery backup FAQ

Basics

A home battery stores electrical energy to run electrical household devices at a later time without the need of grid electricity.

There are two primary uses. It serves as a backup electricity source during times when the grid is down. Thus the house can still be powered on during a blackout. It also helps in saving money in situations where time-of-use electricity rates are valid. In such situations the battery can be used to store low priced grid energy or renewable energy like rooftop solar energy and then the stored energy can be used when the grid electricity rates are high.

Traditionally lead-acid batteries were used for this purpose. Those batteries though cheaper lasted only 2-3 years. Currently lithium-ion batteries are used which have a lifetime of 10-20 years and are typically guaranteed for 10 years.

No, having batteries does not mean you must have solar. You can get an emergency battery
backup system that is kept charged by the grid until you need it. Then the grid can recharge the batteries when it becomes available again. Or you can use a backup generator to charge the batteries should your utility be down for an extended period of time. Note that a solar+battery backup system qualifies for the federal income tax incentive. A battery-only system does not.

Currently a single Lithium ion battery holds about 6-8 kWh of energy. This is enough to power low energy appliances like lights, refrigerators, fans, microwaves, computers and home electronics for a day. Heavy duty appliances require multiple such batteries.

Economics

A basic home battery using lithium-ion technology is about $10,000 installed. However, there are generous state and federal rebates which can bring the cost down to as low as $2000. Cost is proportional to battery capacity measured in kWh. Currently such batteries run about $1000/kWh.

This depends on the difference in rates between the highest and lowest times. If say lowest rate is 20c/kWh and the highest is 40c, then a 7 kWh battery can save up to $1.40 per day or $42 per month.

Traditional generators are cheaper upfront, but with ongoing maintenance and fuel costs, a home battery is usually more cost effective over the long run. For example, traditional fossil fuel generators require constant supply of natural gas, diesel, or propane, and yearly maintenance to keep all the moving parts working.

Maintenance

Current Lithium ion batteries are practically maintenance free.

Environment

Yes. Lithium ion batteries are non-toxic and contain over 99% recyclable materials. But you won’t need to recycle your home battery anytime soon because they are warrantied for 10 years and can last over 15 years.

Yes. Home batteries can  reduce our need for dirty fossil fuel power plants. When the neighborhood bands together and installs enough home batteries, they can be coordinated to reduce peak demand and avoid the need for inefficient gas peaking plants. And as the neighborhood installs more and more solar, home batteries will be necessary to save extra solar energy from the day to use at night, allowing us to use more clean solar energy.

Steps to use

Current advanced batteries are well sealed and insulated with adequate conditioning hardware. As such they can be installed both indoors and outdoors. Hanging them on the outside wall near the main electrical panel seems to be a popular choice. Also the indoor garage walls are used sometimes.

Usually the whole process can be finished in one or two days. A city permit is required and it is followed by an electrical inspection before being put in operation.

This is done through an electrical sub-panel which separates out these loads which will be backed up by a battery. This is done at the time battery installation.

Theoretically, a home battery can power your home until the end of time, when paired with solar panels. Solar panels power the home during the day and recharge the battery, and the battery powers the home at night. Without solar panels, the duration of power will vary depending on the size of your battery and how much energy is used.

Yes, net-metering is unaffected while using a home battery but in case of net-metering unless time-of-use rates are used or unless power backup is a necessity a home battery may not make sense.

No. These are high voltage installations and require a licensed electrician.

This can be done easily with a smartphone app that is supplied by the manufacturer. The battery is Wi-Fi connected and it also helps in energy monitoring.

Yes, home batteries are totally safe with lot of safety mechanism already built in.

Rebates

The California Public Utility Commission’s Self-Generation Incentive Program (SGIP) provides incentives to support existing, new, and emerging distributed energy resources. SGIP provides rebates for qualifying distributed energy systems installed on the customer’s side of the utility meter. SGIP funding in the residential sector currently focuses on energy storage. Rebate availability depends on the level of allocated funds and the demand for those funds. More information on SGIP can be found at https://www.cpuc.ca.gov/General.aspx?id=11430

It depends. If the battery is only charged by a renewable energy system located on your property, then the federal ITC can be taken. It may also be required that the battery must be installed within one year of the renewable energy system’s installation and that it be installed at your primary residence. Beginning on 1/1/2020, the ITC will be at 26%, dropping to 22% on 1/1/21, and to 0% on 1/1/22. (Consult a tax expert for a definitive assessment of your ability to make use of the ITC.)