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Tesla Powerwall Home Battery
Learn about solar & home battery

How home battery works

A home battery added to your solar system can help you achieve true energy independence.

What is a home battery?

A home battery enables homeowners to store and use their own electricity independently of the Utility. It’s frequently paired with a solar system so you can store excess electricity generated by your solar panels and use it at night (when solar panels cannot generate electricity), during power-outages, or to sell back to the Utility.

Home batteries are typically made from lithium ion (the same as you would find in your phone battery). The standard home solar battery is the size of a laundry basket or mid-sized LED television. Some sit on the ground, some are wide and flat and mounted on a wall. They are generally installed in a garage or outside where they can be protected from the elements or extreme temperatures. They are safe, quiet with just a small internal cooling fan that runs as needed, and aesthetically unobtrusive (sometimes without visible wires coming in or going out). They weigh between 100 and 300lbs, depending on battery capacity and components. 

The term ‘solar battery’ is nearly synonymous with ‘energy storage’ and ‘home battery’. They are all referring to the same thing; batteries large enough to power a home.

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Why would I need a home battery?

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Solar power can only be generated when the sun shines. The excess power generated is sold back to the utility for credits you can use at night. But here’s the catch: under TOU (Time-of-Use) billing structures, the rate they pay you for daytime generated electricity is less than the rate charged for electricity at night. And you use most of your electricity at night. So daytime generation might not cover your nightly use, meaning you still need to pay the utility company for some power.

Home batteries solve this problem. They store electricity generated by your solar panels in the daytime for use in the evening, meaning you can run your house on solar day and night. Whatever you don’t consume then gets sent to the utility for further savings. And you remain connected to the grid in case you need a little extra juice: if your batteries run out of electricity, your system will automatically fall back onto utility grid power. This system of solar+battery use is called self-consumption.  It differs from off-grid in that you stay connected to the grid and might occasionally buy electricity from the grid.

As an additional benefit, you can control the time of day you export excess power back to the grid, meaning you can export it during peak hours for higher rate credits

For back-up power

Grid-outages and rolling blackouts have affected millions of Californians recently. Most solar systems turn off when the grid goes down. Home batteries give you back-up power from your solar system, helping keep your family safe and comfortable.

To go Off-Grid

Home batteries can enable you to go off-grid and achieve true energy independence. Paired with solar, you can generate and store electricity completely shielded from the outside world. This is a great option if you are in a rural location, or building a home where an off-grid system might be cheaper than connecting to the Utility, or just want your home to run off 100% clean energy from a renewable source.

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How long do home batteries last?

Most home batteries are warrantied to last between 10 and 20 years but can continue to operate and save you money long after. Many variables contribute to the lifespan of an energy storage battery. A battery used infrequently, such as for backup power, or part of an overall backup power generator system, will outlast a battery cycled every day. The frequency in which a battery is used and the depth in which it is discharged are the leading factors in battery life. 

How do I compare the cost of different home batteries?

The cost of energy storage is measured in two ways and it is important to understand the difference.

Battery cost per kilowatt hour (kWh): Cost per kWh gets a lot of media coverage. Tesla and other battery manufacturers are in a race to see who can build batteries the cheapest and take the greatest market share as a result. Costs range from $150 to $1,000 per kWh hour currently; $100 per kWh is the arbitrary goal in which, theoretically, batteries will be cheap enough for mass adoption. Battery cost per kWh does not take certain important financial factors into consideration, however, such as installation cost, maintenance, or the number of cycles until the battery is out of warranty or no longer operational. As with most technology, energy storage technology will rapidly decrease in cost over a short period of time.

Installed cost per kWh: This measure is much more valuable to homeowners. This is the final price to the owner after the battery, peripheral components (such as an inverter), installation, and all other costs are factored in. The average residential energy storage system costs less than solar.

How home batteries can take advantage of TOU billing

Under Time-Of-Use (TOU) billing, the customer gets charged different rates for electricity depending on the time of day. For example, 1 kWh might cost 52 cents at 5 pm but only 37 cents at 6 am. 

If you sell your excess solar back to the utility during the day, you may not have enough credits to power your home in the evening. But sell your excess solar back to the grid in the evening and you will. A solar and battery system will do this automatically.

You can even take advantage of arbitrage (also known as TOU arbitrage). You can charge the battery from the grid when utility electricity is at its cheapest, but instead of using the energy later, sell it back to the Utility at a higher rate than purchased. Using the pricing example above, a battery could be filled with 37 per kWh energy at 6 am, then excess energy sold back to the utility at 5 pm at a 15 cent per kWh profit. In theory, a battery could be used for nothing more than a daily cycle of buying cheap and selling high, thus offsetting the owner’s electricity bill.

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