Solar energy is a great resource, but it’s not always available when we need power the most. Our customers use a lot of energy during periods when the sun is either headed for the horizon or has already set, including late summer afternoons and early evenings.

Coupling a photovoltaic (PV) system with energy storage – known as solar-plus-storage – can help solve that dilemma.

That’s true at the utility level, where our largest battery storage system at the Wilmot Energy Center banks energy generated when the 314,000 solar panels on the site are most productive. It’s also true in customers’ own homes.

“Customers are adding systems to store energy when the sun shines during the day and their usage is lower,” explained Neil Saunders, Energy Program Engineer for Tucson Electric Power, noting the concept is called “solar shifting.”

“When the sun goes down, they discharge the energy from the battery to power the home. That’s a good thing for solar homeowners and for the grid.”

Energy storage systems allow homeowners to take advantage of the midday sun to generate and store electricity. During times of peak demand, when rates are higher on our time of use pricing plans, the stored energy can be released to power some or all of the home.

Adding a battery system to your home can also add energy security. While grid-connected solar systems without storage will shut down automatically during power outages for safety reasons, solar-plus-storage systems can continue to generate electricity and use stored energy.

“With solar-plus-storage systems, it is possible for a customer to remain in power when the TEP grid power goes down,” explained Don McAdams, Principal Energy Services Engineer. “This is done by installing a ‘smart switch’ at or near the main service panel that is wired to sense the loss of electricity from TEP and automatically opens a contact that isolates the solar-plus- storage system from the grid. When the system senses power is restored, it will reconnect to the grid.”

While energy storage systems provide convenience, resiliency and peace of mind, there are a few things to keep in mind before you invest in one.

Storage size

The first consideration is determining the size of your storage system. The right capacity depends on your PV system, your total household energy use and the length of time you would like to power critical appliances and electronics.

“Installers tend to stack batteries if homeowners want more stored energy capacity. Batteries can range from 3.5 to 13.5 kilowatt hours each,” noted Saunders, “and you may need several to provide the stored power you want.”

Battery storage systems are flexible in that they can provide a lot of power in a short time or less power for a longer time, unless your system is producing energy while you are using it. If you want to run your whole household on power from your storage system, you may only have electricity for a few hours. By reducing your load, you will have electricity for a longer period.

“Some customers install enough batteries to provide whole home backup for several hours,” McAdams noted. “Most homeowners, however, determine what their critical loads are such as the refrigerator, microwave oven and various home electronics, and move those loads off the main service panel to their critical loads panel that can be served by their energy storage system.”


Storage systems are expensive, and the more batteries you need, the greater the cost. They range in price from $12,000 to $22,000, depending on the brand, number and size of batteries and other factors. Available federal tax credits can help offset the upfront cost, but there is likely to be a long payback period.

Limited lifetime

Another factor to consider is the useful life of the batteries, which is 7-10 years. Be prepared to replace them at least once during the life of your solar energy system.

Battery systems also are not 100 percent efficient, as some energy is always lost through energy conversation and retrieval. “Tesla, for example, has a 10-year warranty on its lithium-ion battery, but there is some degradation of performance over time, so you’ll need to eventually replace them,” Saunders said.

Because extreme heat or cold can further reduce battery performance and life, Saunders recommends placing your storage system in a garage or room with conditioned air.

Battery disposal

Since batteries contain heavy metals that are reactive, they should never be discarded in the trash to avoid the risk of fire or contamination of the soil or groundwater.

Fortunately, an increasing number of qualified e-recyclers can safely recycle the batteries. Several are located in the Phoenix area.

Adding energy storage to a new or existing solar system can help homeowners meet their green energy goals, provide greater resiliency and reduce energy usage during on-peak times when rates are higher on time-of-use pricing plans. As technology advances, battery performance, cost and recycling options will improve.

“Energy storage systems have been around for a while, and they’re getting better and better all the time,” Saunders said. “Solar customers thinking about adding battery storage should rely on their installer to recommend a system that’s right for their system size and meet their expectations and budget so that it can provide the power they need, when they need it.”

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