Solar Storage in Practice
Solar energy storage works by adding a battery (or batteries) to the solar system installed on the home. It can provide peace of mind and, in many cases, real ROI.
There are two main solar storage use cases for homeowners to consider:
☀️To have backup power when the grid goes down
🚀To take advantage of Time-Of-Use rates to lower their electricity bill
Here, we'll take a look at both, as well as the information homeowners and installers need to make a decision on what makes the most sense for their solar system.
Using solar energy storage for emergency backup
Many homeowners purchase energy storage simply as a backup in case the power from the grid goes out. With wildfires, thunderstorms, hurricanes, and other natural disasters becoming more frequent in recent years, many homeowners are looking to storage as a peace-of-mind purchase.
What many solar homeowners don’t realize is that if the grid goes out during the day their solar panels will no longer work. This is because their solar panels are tied to the grid and they turn off as a safety precaution if they no longer detect power coming from the grid. A battery, however, can continue working in these cases, which is why it’s a good way to prepare for emergencies.
Using solar energy storage for electricity bill savings
A growing reason homeowners get solar energy storage is to take advantage of time-of-use (TOU) rates. This is when a utility company charges higher rates at certain times of the day and lower rates at other times. The utility companies do this when there is higher demand on the grid, in the evenings when everyone is at home using power, for instance.
A battery can get around TOU rates by charging during the day, either from the cheaper grid energy or from the solar panels, and then discharge that power during the evening. This allows the homeowner to capture the excess energy their solar panels generate and use it later once the sun has gone down, reducing their utility bill.
We'll look at more details below.
Storage for self-consumption primer
With net-metering policies like California's Net Billing Tariff (i.e. NEM 3), storing solar energy in batteries when the sun is shining, and using it when it's not can have serious ROI.
When a homeowner has their batteries set to self-consumption mode, the system is constantly monitoring how charged the battery system is — whether it’s fully charged, at its manufacturer’s minimum charge, or somewhere in between.
Then, if the solar system is producing more energy than the home is consuming at any given time, it makes a decision based on the battery’s state of charge.
If the battery is fully charged, meaning that it can't hold any more energy, then it'll sell that excess back to the grid. We can think of this like collecting rainwater in a big bucket to use in your garden. If the bucket is full of water, you can't add any more, so it needs to go somewhere. In solar's case, that excess power goes back to the grid because there's nowhere else for it to go.
But, where it gets interesting is if the battery state of charge is less than 100%, meaning that it's not fully charged. In this case, it's going to use that excess energy from the solar system to charge the battery rather than selling it back to the grid. During the day, NEM 3 export rates go as low as 0 cents per kilowatt hour (kWh) — daytime usage is generally lower, and the sun is largely shining, so there’s a lot of solar available. This makes selling it back to the grid not very cost effective, to say the least. So storing this energy for later use, say in the evening when energy can cost over $.80 per kWh (yes, you read that right) really adds up.
To determine whether storage for self-consumption makes sense for a homeowner, modeling usage and generation out hour-by-hour is key. Once you have that information, the homeowner can make an informed decision.
Solar Battery Terms
There are a few key terms specific to solar battery storage. Knowing these terms is helpful when comparing types of solar batteries and understanding their advantages.
Cycles
A single “cycle” of a battery is a single discharge and recharge. Life-cycles refers to the number of times a battery can be charged and discharged before it needs to be replaced. Although all batteries can be discharged to 100%, many battery types should only be discharged to 70-90% of their total capacity. This is called the “depth of discharge.” Cycles are also important because some manufacturers use number of cycles for warranty tracking.
Depth of discharge
The depth of discharge refers to how much of a battery’s energy is discharged before it’s charged back up again. Many types of batteries shouldn’t be discharged completely because it can lower the lifespan of the battery. Many lithium batteries, for instance, should only be discharged 90% before they should be charged back up again.
Lifespan
The estimated lifespan of a battery is how long it will last before it should be replaced. Lifespan can either be measured in number of years or number of cycles. A lithium battery, for example, has an estimated lifespan of 13-18 years or around 6,000-10,000 cycles.
Types of solar batteries
There are three main types of solar batteries: Lithium-ion, lead-acid, and flow. The most commonly used type of solar battery is Lithium-ion, of which there are two main types.
Lithium-ion
Lithium-ion batteries are the same type of batteries found in your cellphone, laptop, and other electronics. They have become preferred in the solar industry because they can discharge deeper and have more lifecycles than traditional lead-acid batteries. A typical lithium-ion battery will give you around 6,000-10,000 cycles at 90% discharge.
The two most common types of lithium-ion batteries are Lithium-Iron-Phosphate (LFP) and Lithium-Nickel-Manganese-Cobalt (NMC). NMC are the most widely used, but LFP batteries are gaining in popularity and will soon be the dominant battery in the market.
LFP batteries are preferred for their stability, durability, and performance.
NMC batteries are preferred for their high energy density, lower cost, and long cycle life. Many companies that already produce batteries for other applications, such as Tesla, prefer NMC batteries because they already have the infrastructure to produce them.
Lead-acid
Lead-acid batteries are the same type of battery used in your car. They are an older technology that is mostly used for off-the-grid and DIY applications. Lead-acid batteries are a very well tested technology, but they lack the energy density of lithium-ion. This means that you’ll need a lot more lead-acid batteries for the same amount of energy storage as a lithium-ion setup.
The other drawback to lead-acid batteries is that they have a lower discharge capacity and number of lifecycles. Most lead-acid batteries have a recommended discharge of 60%, meaning you shouldn’t discharge them lower than 60% or you’ll reduce their lifespan. They also have a lower cycle life of about 1,000-3,000 cycles before they need to be replaced.
Flow batteries
The newest battery technology is what’s known as a flow battery. A flow battery uses a water-based liquid (Zinc Bromide) that flows between two tanks.
The two main advantages of a flow battery are its discharge capacity and its safety. Flow batteries can be discharged to 100% of their capacity and won’t lose any of their lifespan as a result.