Battery storage has revolutionized the way we use solar energy at home. For years, one of the major appeals to installing rooftop solar was the ability to feed that power to the grid and zero out home electricity bills — even achieve a negative monthly balance.
But times have changed.
Utilities are rolling back the compensation they once awarded homeowners for their solar production while simultaneously increasing electricity rates. This is happening at the same time extreme weather is causing longer and more frequent power outages. The trifecta of events has made a compelling case, financially and psychologically, for installing battery storage.
Adding your own source of backup power offers peace of mind during a storm or heatwave. It also gives you control over when you use your solar power, and how much (if any) you wish to send to the grid.
In this solar battery buyer’s guide, we answer questions many homeowners ask when considering this game-changing technology.
What is a Solar Battery?
A solar battery is a device that stores the extra energy that your solar system generates beyond the amount needed to power your home.
In order to operate, solar panels need sunlight. While they still work on cloudy and rainy days, photovoltaic (PV) panels produce the most energy in direct sunlight. Once captured, that energy is converted into electricity and routed through an inverter in a form you can use in your home.
Production tapers off as the sun goes down, and your system won’t produce power after sunset. Therefore, your solar panels don’t have any way to store power if you don’t immediately use it. Unless you have an off-grid system, your PV panels are connected to the grid, which lets you draw electricity from your local utility when your solar isn’t producing enough power to meet your needs.
Some days, your PV system will create more energy than your household needs. Without a battery, that energy can only be sent back to the grid for credits on your utility bill — a process called net energy metering (NEM) or net energy billing that is becoming less and less financially rewarding, especially in California. If you’d rather save that excess energy to use later on, such as during an outage or at night when the sun isn’t shining, you’ll need to add a battery to your PV system.
Battery storage lets you self-consume as much of your solar power as possible, which reduces your need for electricity from the grid. Not only does this maximize the value of your PV system, but it also helps you reduce your utility bill while protecting your home if the grid goes down.
Limited Power Availability: Keep in mind that the amount of power your solar battery provides during an outage is limited and will depend on several factors, such as whether or not it’s connected to solar panels, how many electrical loads it powers, and its overall configuration.*
Different Types of Solar Batteries
In recent years, lithium-ion batteries have emerged as the preferred option for residential storage. Their high energy density means you can pack a great deal of power into a small space, which is ideal for garages where many home batteries are installed.
There are two popular types of lithium-ion batteries that use different materials for the cathode, the positive side of the battery. Both use the same carbon-based anode (usually graphite) and have a similar construction, but their working principle differs. There are benefits and downsides to each.
Lithium iron phosphate (LFP)
These batteries combine iron phosphate with lithium carbonate. They’re a newer technology, used by FranklinWH and Enphase.
Pros
- Materials are abundant and less toxic than NMC batteries
- Have a longer lifespan due to a slower rate of degradation
- Can better withstand high temperatures, making them safer and more stable
- More eco-friendly and easier to recycle at the end of their life
Cons
- Pricing varies; upfront costs can be higher when production is limited
- Newer technology can mean supply constraints
- Lower energy density, making them bulkier and heavier for the same storage capacity
Nickel manganese cobalt (NMC)
These batteries are a combination of nickel, manganese and cobalt. This technology has been around for longer and is trusted by leading manufacturers like Tesla and Generac.
Pros
- A trusted battery chemistry used in smartphones, laptops and electric vehicles
- Higher energy density (used a lot in EVs) so they take up less space than LFP batteries
- Able to store more energy and require less maintenance
Cons
- Have a shorter lifespan than LFPs due to more rapid degradation
- Poses an environmental risk (cobalt), making them harder to recycle
- Small risk of thermal runaway if the battery overheats (which can ignite in extreme cases)
Looking for the latest technology from top equipment manufacturers?
How To Choose a Solar Battery
Depending on where you live, your storage options may be limited. Remember that technology continues to evolve, so if an installer or dealer in your area doesn’t have your preferred type of solar battery now, this may change in a few months.
Installing Solar and a Battery Together or Separate?
Perhaps the biggest factor to consider when choosing a battery is the order in which your equipment will be installed. Thanks to advances in technology, you have three options:
- Install solar panels first, then add a battery later (or retrofit an existing PV system)
- Install a standalone battery first, then add solar later
- Install solar panels with battery storage together
Residential solar batteries are classified based on how they integrate with your solar panels, called coupling. Batteries can only store power in the form of direct current (DC). But if your solar system feeds your battery, this DC energy is already converted into AC power that you use in your home.
Therefore:
If you already have solar: AC-coupled batteries are preferred for retrofits because your solar system was already installed with DC-to-AC power conversion.
If you install a standalone battery first: This equipment isn’t coupled to existing solar, but it operates in the same manner as an AC-coupled system, as it’s powered by grid electricity.
If you’re installing solar and a battery together: DC-coupled batteries are more cost-effective since you can store DC power directly from your solar panels and eliminate unnecessary conversions.
Read on about AC-coupled vs. DC-coupled systems
Power Rating and Capacity
A solar battery’s power rating tells you how much electricity the battery can provide (measured in kilowatts or KW), which indicates how many electrical loads or appliances it can power at once. For instance, many batteries on the market today have a continuous supply of about 5,000 watts (5 KW).
The storage capacity tells you how much power your battery can hold (measured in kilowatt-hours or kWh). A battery’s usable capacity can vary considerably by manufacturer, ranging from 3 kWh to 18 kWh. The smaller-capacity batteries can be wired together, giving you the flexibility to tailor your system to meet your household needs. Batteries with a larger capacity are designed to last longer on a single charge — ideal for standalone applications with no solar panels to recharge from.
When combined, these ratings tell you how much power to expect. For instance, if your battery can provide 5 KW of continuous power with 13.5 kWh of storage:
- It can send 5,000 watts of electricity to your home at once.
- It can maintain this maximum output for about 2.5 to 3 hours (13.5 kWh / 5 KW = 2.7 hours).
- If you choose to power the most essential (lights, fridge, Wi-Fi and a few outlets) and drop that load to 1,500 watts (1.5 kW), it can last for around nine hours (1.5 kW x 9 = 13.5 kWh) on a single charge.
Remember, if you have solar panels connected to your battery, as long as the sun is out, your system can recharge your battery with solar energy and continue supplying power to your home.
Sunnova now offers the Powerwall 3 with an industry-leading 11.5 KW of continuous power.
How Many Solar Batteries Do You Need?
As illustrated above, a battery’s power rating and capacity will determine the number of batteries best suited for your home. Tesla and Franklin offer high-capacity storage systems where homeowners may need a single battery to meet their needs, while other manufacturers like Enphase and Generac provide smaller, stackable options.
To determine how many batteries you’ll need, ask yourself how you intend to use that battery based on your energy goals. Each household and battery brand is different, so be sure to discuss your unique situation with a Sunnova solar expert. Here’s some guidance to get the conversation started.
Goal: Energy Resiliency
If you live in an area prone to rolling blackouts or weather-related outages, you might want a reliable source of backup power during a grid interruption. Because solar panels alone can’t store power, in order to save your own solar energy to use later on, you need a battery. You can also install a standalone battery that charges from the grid and stores power in case of an outage.
Oftentimes, Sunnova customers find that two batteries provide enough backup power to keep their essential appliances running for the duration of an average outage.
Goal: Cost Savings
If you live in an area subject to time-based rates, where utilities charge more for electricity when demand is highest (such as 4-9 pm in California), you might want protection from these peak rates. A battery lets you save energy and use it strategically in place of grid electricity.
Even if you’re on a flat-rate structure, a solar battery lets you use more of the free energy you produce from your rooftop and less electricity from the grid — providing potential savings in both the short and long term.
Sunnova customers with the primary goal of saving money on electricity often find that one battery is enough to make a difference on their monthly utility bills.
Goal: Total Energy Independence
If you want to completely cut ties with your utility company, or perhaps you’ve built a house in an area with no utility service, this is considered going ‘off the grid.’ To achieve complete self-sufficiency, you’ll need to plan carefully to ensure you have power during inclement weather.
It will take a number of batteries to power an off-grid home, which can be costly.
Therefore, if you plan to go off-grid, it’s best to use a combination of sources, such as a few batteries and a standby generator if you have access to propane or natural gas. You can also add load control to strategically power your essential loads when sunlight isn’t abundant, and extend the duration of your solar batteries.
Read all about load control
How Long Do Solar Batteries Last?
The lifespan of your solar battery will depend on a handful of factors, such as:
- Extreme temperatures: Most batteries perform best between 68°F and 77°F. In extreme heat, solar batteries may degrade faster when exposed to temperatures above 85°F for prolonged periods. High temperatures can also cause chemical reactions inside your battery that hinder its capacity and reduce its lifespan. Placing your battery in a shaded area can prevent premature degradation.
- Frequency of use: If you reserve your battery for emergency power in case the grid fails, your battery may outlive your solar array (25, even 30 years). If you plan on using your battery daily — to supplement the power when your PV isn’t producing — it may last 10 or 15 years. And if you use your battery as your primary power source, it could last less than a decade.
- Depth of Discharge (DoD): The percentage of your battery’s capacity, that can be discharged without damaging its health. The standard DoD of a lithium-ion battery is 90%, so if your battery’s capacity is 13 kWh, you can use 11.3 kWh without degrading the battery. Experts say maintaining a partial charge and discharge will reduce stress, so avoid full cycles if you want to prolong your battery’s life.*
While these factors are important for longevity, investing in a high-quality solar battery ensures fewer replacements and consistent performance.
When you purchase a solar plus battery system through Sunnova, you’ll be covered by our Sunnova Protect® Platinum plan. This includes our end-to-end services, from system design to monitoring to ongoing maintenance for 25 years. If your battery fails during that timeframe, we’ll replace it.*
Don’t wait until you’re sitting in the dark. Find out how you and your family can benefit from battery storage.
