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Solar plus storage: Solar batteries for home explained
To appreciate why you might choose to install a solar-plus-storage system for your home, you first need to understand how a standard home solar PV system functions.
The typical solar energy system includes solar panels, an inverter, equipment to mount the panels on your roof, and a performance monitoring system that tracks electricity production. The solar panels collect energy from the sun and turn it into electricity, which is passed through the inverter and converted into a form that you can use to power your home.
The vast majority of residential solar energy systems are connected to the electricity grid (or “grid-tied”). When your panels are producing more electricity than your home needs, the excess is fed back into the power grid. Conversely, when your home needs more electricity than your solar panels are producing, you can draw power from the electric grid.
In most cases, you receive a credit on your utility bill for the electricity you send back to the grid. Later, when you are using more electricity than your solar panels have generated, you can use those credits instead of having to pay more to your utility. This process is known as net metering.
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When you install a solar energy system on your home, you can use it to both power your home and charge your electric car for emissions-free transportation. The cost of solar is falling rapidly, and companies from Tesla to Nissan are manufacturing electric cars for your daily use. Now, the ability to install a solar PV system large enough to power both your home and your car is an option within reach. But even with incentives and rebates available for both technologies, most homeowners still can’t afford to install solar and buy an electric car at the same time. Luckily, it’s easy to install a solar energy system today that takes your future electricity consumption into account, if you take a few additional factors into consideration.
How do solar panels charge batteries?
Solar batteries work by converting the AC energy being produced by your solar panels and storing it as DC power for later use. In some cases, solar batteries have their own inverter and offer integrated energy conversion. The higher your battery’s capacity, the larger amount of solar electric energy it can store.
When you install batteries as part of your solar electric system, you are able to store excess energy at your home instead of sending it to the grid. If your solar panels are producing more electricity than you need, the excess energy goes towards charging the battery. At night, when your system isn’t producing electricity, you can draw down the energy you stored earlier in your batteries. You’ll only send electricity back to the grid when your battery is fully charged, and you’ll only draw electricity from the grid when your battery is depleted.
What this means in practical terms is that homes with solar-plus-storage can store excess solar power onsite for use later when the sun isn’t shining. Additionally and in some cases even more importantly, since solar batteries store energy at your home, they offer short-term backup power in the event that there’s a power outage in your area. It is important to note, that without solar batteries, if the grid should go down, you will not have access to the energy being generated by your own solar system due to grid safety issues. With batteries, you will have access to energy in the event of a power outage.
In most cases, you receive a credit on your utility bill for the electricity you send back to the grid. Later, when you are using more electricity than your solar panels have generated, you can use those credits instead of having to pay more to your utility. This process is known as net metering.
How to compare your solar storage options
As you consider your solar-plus-storage options, you’ll come across a lot of complicated product specifications. The most important ones to use during your evaluation are the battery’s capacity & power ratings, depth of discharge (DoD), round-trip efficiency, warranty, and manufacturer.
CAPACITY & POWER
Capacity is the total amount of electricity that a solar battery can store, measured in kilowatt-hours (kWh). Most home solar batteries are designed to be “stackable,” which means that you can include multiple batteries with your solar-plus-storage system to get extra capacity.
While capacity tells you how big your battery is, it doesn’t tell you how much electricity a battery can provide at a given moment. To get the full picture, you also need to consider the battery’s power rating. In the context of solar batteries, a power rating is the amount of electricity that a battery can deliver at one time. It is measured in kilowatts (kW).
A battery with a high capacity and a low power rating would deliver a low amount of electricity (enough to run a few crucial appliances) for a long time. A battery with low capacity and a high power rating could run your entire home, but only for a few hours.
DEPTH OF DISCHARGE (DOD)
Most solar batteries need to retain some charge at all times due to their chemical composition. If you use 100 percent of a battery’s charge, its useful life will be significantly shortened.
The depth of discharge (DoD) of a battery refers to the amount of a battery’s capacity that has been used. Most manufacturers will specify a maximum DoD for optimal performance. For example, if a 10 kWh battery has a DoD of 90 percent, you shouldn’t use more than 9 kWh of the battery before recharging it. Generally speaking, a higher DoD means you will be able to utilize more of your battery’s capacity.
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ROUND-TRIP EFFICIENCY
A battery’s round-trip efficiency represents the amount of energy that can be used as a percentage of the amount of energy that it took to store it. For example, if you feed five kWh of electricity into your battery and can only get four kWh of useful electricity back, the battery has 80 percent round-trip efficiency (4 kWh / 5 kWh = 80%). Generally speaking, a higher round-trip efficiency means you will get more economic value out of your battery.
BATTERY LIFE & WARRANTY
For most uses of home energy storage, your battery will “cycle” (charge and drain) daily. The battery’s ability to hold a charge will gradually decrease the more you use it. In this way, solar batteries are like the battery in your cell phone – you charge your phone each night to use it during the day, and as your phone gets older you’ll start to notice that the battery isn’t holding as much of a charge as it did when it was new.
Your solar battery will have a warranty that guarantees a certain number of cycles and/or years of useful life. Because battery performance naturally degrades over time, most manufacturers will also guarantee that the battery keeps a certain amount of its capacity over the course of the warranty.
Therefore, the simple answer to the question “how long will my solar battery last?” is that it depends on the brand of battery you buy and and how much capacity it will lose over time.
For example, a battery might be warrantied for 5,000 cycles or 10 years at 70 percent of its original capacity. This means that at the end of the warranty, the battery will have lost no more than 30 percent of its original ability to store energy.
MANUFACTURER
Many different types of organizations are developing and manufacturing solar battery products, from automotive companies to tech startups. While a major automotive company entering the energy storage market likely has a longer history of product manufacturing, they may not offer the most revolutionary technology. By contrast, a tech startup might have a brand-new high-performing technology, but less of a track record to prove the battery’s long-term functionality.
Whether you choose a battery manufactured by a cutting-edge startup or a manufacturer with a long history depends on your priorities. Evaluating the warranties associated with each product can give you additional guidance as you make your decision.
