Generator & Energy Storage Basics

U.S. battery storage capacity is rapidly increasing, with an expected 89% growth in 2024. 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.

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.

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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.