How to Choose battery storage?

When you picture a battery, the first thing to come to mind is probably the disposable batteries you put in everyday appliances like your TV remote. But did you know you can power your entire house with (much larger) batteries?

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You don't need a home solar panel system to reap the benefits of home battery backup. But you'll get the most out of your system when you pair them together—especially if your utility doesn't pay you much for the excess electricity your solar panels generate and send to the grid. 

We explain how to decide if backup batteries are right for you and, if so, how to get a battery system that fits your needs at the best price.

Home batteries store extra energy so you can use it later. When you only have solar panels, any electricity they generate that you don’t use goes to the grid. But with residential battery storage, you can store that extra power to use when your panels aren’t producing enough electricity to meet your demand.

Most batteries have a limit on how much energy you can store in one system, so you may need multiple batteries if you want to have enough capacity for long-duration backup. Also, most batteries can’t store electricity forever—even the best home batteries will slowly lose charge over time, whether or not you use them.

You don't need solar to install a home battery, but batteries only store energy, they don't produce it. Pairing your battery system with solar panels allows you to truly increase your grid independence and your electric bill savings. Here's how it works: 

But home backup batteries are becoming an increasingly popular choice over home generators. They offer many of the same backup power functions as conventional generators without the need for refueling. While they're more expensive upfront and require an electrician to install, you can "refuel" them for free with the sun's energy if paired with solar panels. They're also much quieter than generators and don't come with emissions-related health concerns.

Most batteries last about 10-15 years, meaning you'll have plenty of time to break even on your investment. While many homeowners can benefit from installing a battery system, they're not right for everyone. Here are a few questions to answer when deciding if you should add a home battery:

Power outages are an occasional nuisance for everyone, but for some people, they're a far too regular occurrence: According to the Energy Information Administration, the average U.S. electricity customer experienced 5.5 hours of electricity interruptions in . However, customers in Florida, West Virginia, Maine, Vermont, and New Hampshire experienced average outages ranging from 10.3 hours in New Hampshire to 19.1 hours in Florida.

Under these policies, you could still have a hefty electric bill even with solar. By pairing your solar panels with a battery, you can program your system to export electricity to the grid only when compensation rates are high and pull from your battery when rates are low, maximizing your savings.

Even if you don't have solar, batteries alone can be worth it if your utility uses a complex electricity rate structure. Time-of-use, or TOU, rates are a form of "time-varying rates" designed to better reflect the actual cost of electricity based on the amount of supply and demand. Utilities have used TOU rates for businesses for many years, but they're becoming an increasingly common way to charge homeowners. Under TOU rates, your electricity cost will vary from hour to hour, day to day, and season to season. With a battery, you can use your stored energy to avoid pulling electricity from the grid when it costs the most. 

Demand charges are also common for businesses and are becoming more common for homeowners. With demand charges, your utility company tracks your maximum energy pull from the grid during any given hour (or even 15-minute period) per month and charges you based on that maximum demand for the whole month. With a battery, you can lower your peak demand from the grid, driving significant bill savings.

If you want to install a home battery but are overwhelmed by the cost, rest assured there are plenty of incentives available that can significantly lower the price. However, the best federal incentive, the 30% tax credit, ends for residential batteries installed after Dec. 31, .

Depending on where you live, you could break even on your home battery storage investment in less than a year. Here are some of the top battery incentives that will either reduce your upfront cost or increase your long-term savings:

In , a 13.5 kWh battery—the capacity of a Tesla Powerwall 3—costs about $9,800 after the federal tax credit based on thousands of quotes through EnergySage. Without the tax credit, that number will jump to $14,000 after this year. This price tag is high, but if you've determined that a battery is right for you based on your answers to the questions we outlined so far, it will pay off over time. 

But if you live somewhere with net metering and a flat, non-time varying electricity rate, the only financial savings from installing energy storage come from avoiding outages or receiving any available state incentives. In those instances, you won't see any more bill savings from adding a battery to your solar panel system.

U.S. battery storage capacity is rapidly increasing, with an expected 89% growth in . Residential battery storage is becoming a popular solution for home backup power, solar energy storage, reducing peak-hour utility charges, and being incentivized to help stabilize the grid. As a result, installing a battery system is becoming more attractive for homeowners, offering cost savings, power independence, and resilience. In this article, we’ll guide you through the key considerations for sizing your battery storage system, including your inverter. Remember, batteries don’t generate power; they store it. So, it’s essential to determine exactly how big of a system you need.

Understanding surge power versus continuous power

Inverters are rated for both continuous and surge (or peak) power. Continuous power is the maximum wattage the inverter can handle over an extended period, while surge/peak power refers to the brief higher wattage it can provide to support the startup of certain devices. When sizing an inverter, it's important to consider both the continuous and surge power demands of each load. Since different devices have varying power needs, understanding the difference between continuous and surge power is crucial for selecting the right inverter.

For proper inverter sizing, assess the power consumption of each load for both continuous and peak usage. Since different devices have varying power demands, understanding these ratings is essential for choosing the right inverter.

Grid-Tied vs. Off-Grid Systems

When purchasing battery storage or a solar system, you have two primary options: grid-tied or off-grid. A grid-tied system is connected to the electrical grid. An off-grid system with solar, however, relies solely on battery storage to power your home when solar isn’t producing power, making proper battery sizing critical to avoid outages.

Understanding load analysis and operation hours for your electrical needs is key to sizing your battery system properly. For example, in a grid-tied solar system, you'll lose power during an outage unless you have battery storage. In an off-grid solar system, power comes from the battery storage, so if it's not sized correctly, you'll face outages when solar power isn’t available.

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Adding battery storage increases energy independence and can lead to long-term savings, especially when electricity prices spike, but the system must be sized accurately. Proper battery sizing depends on several factors: how much electricity is needed to keep devices powered, how long those devices will rely on stored energy, and the actual capacity of each battery pack.

Considerations to make when calculating your battery size needs 

The first step, and most important, is to calculate your energy load profile and estimate the usage required per day in kWh (Kilowatt-hours). Here are some of the main points to consider. 

Calculate your average daily energy consumption rates

The first step is to calculate your daily energy consumption in kWh. You can do this by reviewing your energy bill over 12 months, choosing your highest-demand month, and then dividing by 30 to estimate daily usage. For example, if you use 900 kWh per month, your daily usage is approximately 30 kWh. You can use an online kWh calculator to help determine your daily average energy consumption. 

Identify your critical load

If you’re purchasing the battery for backup on critical loads only, such as your refrigerator, lights, etc., you will want to look at the owner’s manual to help you determine the energy needs of your most important appliances and loads. You can calculate your major appliances with the Department of Energy Appliance & Home Electronics Calculator. 

How long does your backup need to last?

Depending on your location and the average time you experience power outages, you will want to estimate the amount of time you would need battery storage, on average.

Consider solar production

You can install a battery storage system in your home or business with or without a solar system. If you’re adding your battery to an existing solar system, you’ll want to consider the amount of power your system is generating during the day. How much are you consuming of that energy, and how much are you sending back to the grid? 

You can find your system’s power by visiting your monitoring app and looking for daily production. Another helpful tool here is the PVWatts Calculator.

Source: Solar Energy Action

Autonomous days

If you have a solar system, also consider your "days of autonomy," which refers to how many days you can expect to rely only on your battery system, such as during periods with no sunlight (common in off-grid systems). While this varies by location, a common estimate for the U.S. is 3 to 5 days per year.

Design Battery Sizing

Battery capacity is measured in kWh, depending on the battery technology. For example, lead-acid batteries are measured in amp-hours (Ah), while lithium batteries are measured in kWh.

To accurately size your battery pack, follow the manufacturer’s recommendations for depth of discharge (DoD). Most lithium-ion batteries shouldn't be discharged beyond 80%, although using more in emergencies is generally fine. For instance, Briggs & Stratton SimpliPHI batteries can be discharged 100%, but many installers prefer to limit discharge to 80% to preserve battery life. Why is this important? Depending on your battery and its recommended DoD, you’ll need to select a battery that fits that rate. For example, if your battery is 10 kWh, the manufacturer may recommend you only use 8 kWh.

To size your battery, first calculate the power required by your critical loads (the essential devices you need to keep running during an outage) and multiply this by the number of hours you expect to need backup power.

These calculations can be done using online tools, and if you’re combining solar with battery storage, tools like the Sol-Ark Battery & Storage Calculator can help estimate the correct size for both your battery and inverter.

Design Inverter Sizing

Inverters play a critical role in converting Direct Current (DC) power from the battery, usually 12V or 24V, into Alternating Current (AC) power at 230V. This conversion provides stable electricity for various appliances, ensuring essential equipment functions properly during power outages. Inverter performance is closely linked to the size of your energy load. Inverters are classified into three types: resistive load, inductive load, and capacitive load.

Proper inverter sizing is crucial. It must be sized to meet the maximum demand by adding up the power of all the devices that may run simultaneously, from microwaves and lights to computers and entertainment systems. This total determines the inverter size needed. Additionally, temperature derating—when the inverter reduces power to protect its components from overheating—must be considered. The inverter's output can decrease at higher temperatures, so this factor is critical in inverter sizing. For surge/peak power, the inverter must also handle the high inductive surge required when certain devices start up.

For help with sizing your inverter and battery, you can use the Sol-Ark Battery & Storage Calculator.

Generator vs Battery 

Generators and battery storage systems perform many of the same basic functions but differ in upfront and operating costs, maintenance needs, performance, and the ability to be leveraged as a grid-tied tool. A generator produces electricity by burning fossil fuels like diesel or gasoline through an internal combustion engine. In contrast, a battery can be charged from any power source, including the grid, solar power, or even a generator.

Whole-home generators are great for backup power, but they’re not designed to run continuously. Batteries, on the other hand, can serve multiple purposes, such as peak shaving. In areas prone to severe weather where power outages last more than a day, pairing your battery storage system with a generator can ensure you have power throughout the event.

The SimpliPHI 6.6 Home Battery System Difference

The SimpliPHI 6.6 Home Battery System, featuring a scalable, no-wire, stackable design, allows homeowners to easily expand their energy storage. Each unit offers 6.65 kWh of capacity, with the option to stack up to three batteries for a total of 19.95 kWh. For larger needs, the system can scale to six stacks, providing up to 119.7 kWh of capacity and 84 kW of power. This system provides quiet, emissions-free backup power, helping homeowners achieve energy independence and resilience. It's also cost-effective, with a price point 25% lower than competitors before tax incentives.

Learn more about the flexibility of the SimpliPHI 6.6 Home Battery System.