How many solar batteries do I need?

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 how large your solar battery setup should be.

Unfortunately, the answer to the question "How many solar batteries do I need?" is rarely straightforward. For starters, you can get many final answers based on what you want from a solar-plus-storage system. You can optimize a battery system in three general ways: saving the most money, for resiliency, or for self-sufficiency.

Designing a solar battery system to save money

If you want to save as much money on electricity as possible with solar batteries, you'll need to know your electricity rate plan. 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 them 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 the 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 your exact plan - 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 avoid using grid electricity during peak hours mostly 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.

Designing a solar battery system for resiliency

Solar batteries are increasingly used as a resiliency tool 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 depends greatly on how resilient you want to be! A single battery will do the trick if you're only concerned with keeping a few things running during quick outages. You'll need more storage if you're worried about longer outages that could extend into days.

Key takeaway: A single average lithium-ion battery will be enough for most residential homeowners 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 entirely 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 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 remember when deciding how many solar batteries you need to go off the grid are the loads you want to be powered and how long you might need to stay battery-powered.

Key takeaway: Self-sufficiency requires lots of battery storage, especially if you build capacity for extra-long periods with no sunlight (cloudy weather, nights, etc.). Think of 8-12 average lithium-ion batteries like the Tesla Powerwall.

No matter what you want out of a solar-plus-storage system, a few universal factors impact how many batteries you should get. These factors include the electricity loads you need to be powered, your solar panel system's size/production ability, and what you want from a storage system.

Electricity loads

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 to be 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

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 discussed above, knowing what you want from a solar-plus-storage system is essential when 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.

Every solar and battery setup is different, and it's important to consider your unique energy usage profile when 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 to save money

The most important calculation you need to make for a solar battery system 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. 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, 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, 20.7 kWh of electricity is covered by two solar batteries like the LG CHEM Resu 10H.

Example #2: Installing solar batteries for resiliency

Considering solar batteries for resiliency is very similar to the case above: it's all about knowing what you want to power and 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 more expensive and complicated than staying connected to the grid. We go over some example calculations in our blog on the topic. Still, 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.

Whether you're interested in solar-plus-storage or solar on its own for your home, EnergySage can help. Our Energy Advisors can help you walk through your quotes and understand exactly what you're comparing when it comes to the size of your solar or storage system. Sign up for EnergySage to get started today!