Solar batteries are becoming more and more common for individual homeowners, and just like solar panels, the exact battery installation you should get depends on your unique situation and what you want to get out of energy storage. The question “how many solar batteries do I need?” doesn’t have a straightforward answer – in this article, we’ll outline the major factors that contribute to the size of a battery system, and around how large your solar battery setup should be.
Considering that the average solar battery is roughly 10 kilowatt-hours (kWh) in size:
- If you want to save the most money possible, you’ll need enough battery storage to cover your energy usage when your solar panels aren’t producing – somewhere around 2-3 batteries
- If you want to keep the power on when the grid is down, you’ll usually just need one solar battery
- If you want to go off-grid completely, you’ll need far more storage capacity, more along the lines of 8-12 batteries
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The number of batteries you need depends on what you want
Unfortunately, the answer to the question “how many solar batteries do I need?” is rarely straightforward. For starters, you can get to many different final answers based on what you actually want out of a solar-plus-storage system. There are three general ways you can optimize a battery system: for saving the most money, for resiliency, or for self-sufficiency.
Designing a solar battery system for saving money
If you want to save as much money on electricity as possible with solar batteries, you’ll need to know what your electricity rate plan looks like. Generally, on a flat-rate structure, you’ll want enough storage capacity to rely on the grid as little as possible. The more you can store from your solar panels and use later, the better your long-term savings will be. The other main option for an electricity rate is a variable-rate plan – for these, it’s important to make sure at a bare minimum that you have enough storage capacity to ride out the high-cost times of day.
Key takeaway: To save the most money with solar batteries, you need enough energy storage to keep your home self-sufficient during peak electricity pricing hours. Peak pricing hours differ based on where you live and what exact plan you’re on – read our article on determining what plan you’re on to learn even more. In the end, this will roughly end up being about 2-3 average lithium-ion batteries like the Tesla Powerwall to mostly avoid using grid electricity during peak hours and when your solar panels aren’t producing power.
Importantly, this doesn’t mean you are self-sufficient and don’t need the grid at all – it just means that you’re maximizing your solar panel installation and using as much of your own solar electricity as possible.
Batteries and net metering
If you are mainly concerned with saving money from a solar battery installation, and you’re also in an area with one-to-one net metering, solar batteries probably won’t help you to save much extra. When you have one-to-one net metering, the electrical grid essentially acts as a giant battery for you, but instead of storing electricity and pulling it back out later, you’re collecting credits for electricity you provide to the grid that you can redeem later when you pull electricity from the grid.
Designing a solar battery system for resiliency
More and more often, solar batteries are being used as a resiliency tool for when the grid goes down. With a battery system installed, you can keep your house and essential appliances energized through extreme weather conditions and grid failures. As you can imagine, the number of batteries you should get for resiliency reasons depends a lot on how resilient you want to be! If you’re only concerned with keeping a few things running during quick outages, a single battery will do the trick. If you’re more worried about longer outages that could extend into days, you’ll need more storage.
Key takeaway: For most residential homeowners, a single average lithium-ion battery will be enough to keep the lights on during most power outages.
Designing a solar battery system for self-sufficiency (or off-grid)
If you want to design a solar-plus-storage system so you can go fully off the grid, you’ll need a significantly larger battery bank than for any other use case. We have a whole article that walks through example calculations you might have to do to get set up off the grid – check it out to see just how big you’ll need your solar array and battery bank to be.
The key factors to keep in mind when deciding how many solar batteries you need to go off grid are the loads you want powered and how long you might need to stay battery-powered.
Key takeaway: Self-sufficiency requires lots of battery storage, especially if you’re going to build in capacity for extra-long periods with no sunlight (cloudy weather, nights, etc.). Think on the order of 8-12 average lithium-ion batteries like the Tesla Powerwall.
What factors impact solar battery system sizing?
No matter what you want to get out of a solar-plus-storage system, there are a few universal factors that impact how many batteries you should get. These factors include the electricity loads you need powered, the size/production ability of your solar panel system, and what you want out of a storage system.
A “load” is anything that consumes electricity. In the case of solar and home batteries, electricity loads are appliances and systems like refrigerators and air conditioners that use electricity around your home. It’s important to know how much electricity you need to provide to your appliances when designing a storage system – depending on what you need powered, you’ll need more or less storage capacity.
Learn more about how to calculate electrical load in our article on the topic.
Size and production of your solar panel system
In order to charge up a battery with electricity, you either need to pull from the grid directly or charge it with solar electricity. Whether you’re looking to save money or build up resiliency in your home, charging a battery with solar energy is usually the way to go. As such, it’s vital to know how much solar energy your panels produce, and how much they’ll be able to charge up your battery system during the day. It may be the case that you need more storage capacity than your panels can fill, so you might need to install more solar panels to get the full benefit from your solar energy storage setup.
Your individual needs
Last, and as we talked about above, knowing what you’re looking to get out of a solar-plus-storage system is essential when you’re deciding how many solar batteries you need. Generally, off-grid systems need the most batteries, and systems designed for resiliency and savings can be significantly smaller.
Bonus factor: flat vs. variable electricity rate
If you are on a variable electricity rate plan like time-of-use or demand charges, this can also influence the ideal size of your solar battery system. In some areas with variable rate plans, you can “arbitrage” during peak pricing times, which means you can sell stored electricity to the grid for a higher price than you would have gotten at other times. As such, it might be beneficial to install slightly more storage capacity to take advantage of this higher return for your electricity.
Importantly, you can’t just install tons of batteries, store tons of electricity, and sell it all back at a premium to the grid later. Most utilities have limits on how much you can arbitrage, and you likely won’t make a profit over the course of the year with this strategy. However, it’s still a good thing to keep in mind as a backup plan in case you do end up having to pull electricity from the grid at high-cost times.
For the vast majority of installations, sizing a solar battery system to cover 100 percent of your expected electricity usage when the sun isn’t shining will save you the same amount of money whether you’re on a flat-rate or variable plan. The goal is the same: pull as little electricity from the grid as possible!
How to calculate the number of batteries you need
Every solar and battery setup is different, and it’s important to take into account your unique energy usage profile when you’re shopping around for solar and storage options. Any potential solar installer you might work with is a great resource for this – they’ll be able to walk through the ins and outs of your unique property and how you can achieve your goals. In broad strokes, here’s what the calculations for sizing an energy storage system might look like for three different use cases:
Example #1: installing solar batteries for saving money
The most important calculation you need to make for a solar battery system designed to save you money is the amount of electricity you use when the sun isn’t shining, i.e. the amount of electricity you can’t rely directly on your solar panels for. You’ll need to know a few things about your electricity usage to come up with this number, namely:
- The amount of time (hours) you can’t produce solar electricity
- The electricity demand from all of the appliances/systems you want to run during those hours
From there, you can calculate the number of batteries you need by multiplying the electricity demand of the appliances you want powered by the number of hours they’ll need to be powered. This isn’t perfect – some appliances like a dishwasher just need to be run once, and aren’t an hourly electricity load. You can estimate though: for example, if you have a 1,500 watt (W) dishwasher, a 3,000 W air conditioner, an 800 W refrigerator, plus lights, WiFi, and miscellaneous appliances that consume 1,000 W of electricity, that’s about 6.3 kilowatts (kW) of electricity you need from a storage system for the hour you run your dishwasher, and 4.8 kW of electricity for the rest of the time. Assuming a four hour peak pricing interval, that’s 20.7 kWh of electricity – covered by two solar batteries like the LG CHEM Resu 10H.
Example #2: installing solar batteries for resiliency
Thinking about solar batteries for resiliency is very similar to the case above: it’s all about knowing what you want to power, and for how long. Taking the electricity load above for the hour you use your dishwasher (6.3 kW), you can cover that load with one battery for an hour plus, which is more than enough to last through a typical power outage.
Example #3: installing solar batteries for self-sufficiency/going off-grid
Going fully self-sufficient with solar batteries is an expensive and more complicated process than staying connected to the grid. We go over some example calculations in our blog on the topic, but generally, you’ll need to plan for extended periods of low solar production because unlike staying connected to the grid, you can’t just fall back on utility-supplied electricity when it’s cloudy.
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