Can solar panels store energy?

Solar panels generate electricity only when the sun is shining, so how can you store electricity with a photovoltaic system?

Whether you’re a homeowner who is just learning about home solar panels, or a green energy enthusiast who is wondering how the electric grid can use solar energy when it’s dark, a question you might have is whether solar panels can store electricity for later use.

As it turns out, solar panels by themselves can’t store electricity. Any energy that a solar panel generate exits the panel immediately. The panel itself has no capacity for holding electrons. But it turns out that photovoltaic systems can and do store electricity, but they must be designed for this in mind.

Homeowners with solar panels can choose from different types of batteries to keep their lights on when it’s dark outside. Utility companies can also use massive batteries to provide grid-scale storage, but they also have different technologies available to them. Some of these are mature technologies that have been in use for decades, such as pumped-storage hydro.

But there are more exotic energy storage technologies, some of which are just now leaving the laboratories, including gravity storage, compressed air, vanadium flow batteries, and vehicle-to-grid systems.

Read more to learn about the different types of storage that can be paired with solar panel systems so that electricity is available even when the sun isn’t shining.

Batteries for home solar energy storage

Batteries are the technology of choice for homeowners who want to store their solar electricity. Off-grid solar enthusiasts have been doing this for decades with lead-acid batteries, but there’s another type of off-grid home that’s in common use today: motorhomes. In a motorhome, there is a coach battery that provides electricity to the cabin when you’re not at a hookup.

For your home, your choice is the same: lead-acid or lithium ion batteries. Lead-acid has been around longer, but lithium ion is growing in popularity because of dropping prices and higher performance.

There are pros and cons to both types of batteries.

Lead-acid battery basics

All lead acid batteries have the same basic chemistry. The terminals of the battery are connected to plates of lead metal, and the two plates are separated by an electrolyte of sulfuric acid. How that electrolyte is held inside the battery differs. There are three basic types:

Flooded batteries

Flooded batteries hold the sulfuric acid electrolyte in liquid form. This is the least expensive type of lead-acid battery, but this advantage does come with a tradeoff in terms of higher maintenance requirements. One thing to be aware of is that flooded batteries need to be stored in a ventilated area, because charging can result in the production of hydrogen gas, which is flammable.

That hydrogen gas is the result of water molecules splitting into hydrogen and oxygen, so that means you also need to periodically check the electrolyte levels in the battery and top it off with distilled water. Allowing the level to drop too low can damage the battery. Be careful when doing this: you’re dealing with sulphuric acid!

Another thing to be aware of is that flooded batteries must be held upright, or else the electrolyte can spill out, creating a hazardous situation.

Gel cell batteries

Gel cell batteries contain the electrolyte in a thick silica-based gel. The gel is in contact with the lead plates, and electrons can move freely through the gel. Because of this design, the battery can be sealed, except for one-way valves that allow the release of hydrogen gas in the case of overcharging. There are no electrolyte levels to check or water to add, so this type of battery is sometimes called “maintenance free”. These batteries can be stored on their side because there is no liquid to spill. These batteries are about 2-3 times more expensive than flooded batteries.

Absorbed glass mat batteries

Absorbed glass mat (AGM) batteries also avoid a liquid form of electrolyte by containing the acid in a fiberglass mesh. Like gel cells, AGM batteries are called maintenance-free because there are no electrolyte levels to check. They are also sealed and can be stored on their side. They are also more expensive than flooded batteries, but are also more popular and usually less pricey than gel cell batteries.

Drawbacks of lead-acid batteries

One of the main drawbacks of lead-acid batteries is that they aren’t as robust as lithium ion.

Flooded batteries especially must be handled gently, because shocks from rough handling can damage the metal plates. They must also be keep upright to prevent sulphuric acid from spilling out. AGM and gel cell batteries are more durable, and don’t have a liquid electrolyte to spill, but still can be damaged by vibration.

While this isn’t a serious problem with stationary applications like home storage, lead-acid batteries also don’t tolerate serious discharging.

Have you ever had your car battery die, and been unable to charge it again? This is called sulfation, and it happens when a lead-acid battery sits is a low charge state for too long. Sulfate crystals harden on the plates of the battery, lowering its capacity to charge, or disabling the battery completely.

You should use only half of your lead-acid battery capacity

To maximize the life of lead-acid batteries, short discharge cycles are best, and deep discharges should always be avoided. Even a single dischange of more than 50% can permanently reduce the capacity of the battery.

You can maximize the life of your lead-acid battery by maintaining a high charge level, keeping it at least 80% full in typical day-to-day usage.

This is a pretty serious drawback if you are buying lead-acid batteries for daily usage, such as trying to maximize your solar electricity usage under a net billing scheme.

Deep cycle lead acid batteries will have depth-of-discharge (DoD) rating for a given percentage. For example, a battery with a 50% DoD rating of 1,000 cycles means that the battery will tolerate 1,000 discharge cycles where the battery is depleted by half.

Deep cycle lead-acid batteries marketed for solar applications will have 50% DoD ratings around 1,000 to 3,000. While this may be fine for light usage, such as battery backup, it’s less desirable if you’re using your batteries on a daily basis. It means that you really need to buy twice as much battery capacity as you actually need, because you can never discharge your batteries completely (not even once!) or else you’ll end up ruining them.

Lithium ion batteries for the home

A much better technology is lithium ion. This is the same battery technology used in cellphones, laptops, and electric cars.

Unlike lead acid batteries, lithium ion batteries don’t have the same maintenance requirements and can tolerate deep discharges. A lithium ion battery can be discharged by 80% in normal use, and can tolerate occasional 100% discharges without serious issue.

This means that if you need 10 kWh of battery capacity for your home, you can get by with a 10 kWh lithium ion battery, but you would have to buy 20 kWh of lead-acid batteries to safely meet a requirement of 10 kWh of daily power discharge.

In addition, lithium ion batteries last much longer. While the most robust lead-acid deep cycle batteries can last a few thousand 50% DoD cycles, lithium ion exceeds this. For example, the Tesla Powerwall has a 10-year warranty with an unlimited number of discharge cycles.

The main disadvantage of lithium ion batteries is the higher upfront cost compared to lead-acid. However, when you take into account the higher longevity and the ability to use the full capacity of your battery (as opposed to only 50% with lead-acid), the cost differential shrinks or even works in favor of lithium ion.

More lithium ion batteries on the market

With costs dropping, there are more and more manufacturers producing batteries aimed at the home solar market, which is great because just a few years ago this was a non-existent market.

One of the distinguishing things about these products is software. A good lithium ion battery designed for home solar use will have a few important features:

  • Smartphone app. A good battery will let you monitor its status in real time, and tell you how much energy is being sent to the battery from your solar panels, how much energy is being drained, and allow you to control all of its functions right from your phone.
  • Time-of-use control. In many states, including California, the solar energy that you send into the grid is valued depending on the time of day. Under Net Metering 2.0 in California, electricity that you send into the grid during peak hours will be credited to you at a much higher dollar value than electricity you send during off-peak times. With a smart battery, you can program your home to use battery power during peak hours (say, between 3 pm and 7 pm) so that your solar power goes into the grid instead of being used by your home. This helps the utility by reducing peak load on the grid, and helps reduce your carbon footprint even further.
  • Mode selection. Good battery software will let you choose how you want the battery to work on a day-to-day basis. For example, you may want the battery to work in backup mode, saving its power for emergency use only, or you can have it work on a daily basis to make time-of-use rates work in your favor.

Here’s an overview of some of the lithium ion solar batteries on the market.

Tesla Powerwall

Tesla Powerwall2

The Tesla Powerwall is probably the most well-known home energy storage battery on the market. It’s a lithium ion battery with 13.5 kWh of capacity that can deliver up to 5 kW of continuous power. It has a warranty of 10 years and unlimited discharges.

While lithium ion can tolerate 100% discharges much better than lead-acid batteries, those deep discharges still reduce the life of your battery. Given the 10-year warranty that comes with this product, it’s likely that the battery is actually larger than 13.5 kWh to provide some buffer.

Its warranty also guarantees that it will still have 70% of its original capacity after 10 years, which makes for a really robust product, especially given that its intended for daily use.

As far as price goes, this is the product to beat. Tesla’s price advantage likely comes from its massive Gigafactory in Nevada, which allows it produce more batteries than every other carmaker combined.

Price: $6,500 for 13.5 kWh ($0.48 per watt)

LG Chem RESU

LG Chem RESU

LG Chem has a partnership with national solar installer Sunrun to supply the technology behind Sunrun’s Brightbox solar battery.

While Brightbox is a Sunrun product, you can also buy LG Chem batteries directly. There are a number of different products in the RESU lineup, ranging from 3.3 kWh to 10 kWh, not all of which are available in the United States.

The one you’ll find in the US is the 10 kWh model, known as the RESU10H. Like the Powerwall, it also comes with a 10-year warranty, although LG Chem only guarantees that the product will have 60% capacity after 10 years, as compared to the 70% guarantee provided by Tesla.

Price: about $9,600 for 9.8 kWh ($0.98 per watt) although we’ve seen this product discounted to as much as $5,880 ($0.60 per watt)

SimpliPhi

SimpliPhi Power battery

SimpliPhi Power is a California company that has been making energy storage products since 2002. They make a range of lithium ferro phosphate (LFP) batteries that range in size from 1.4 kWh to 3.8 kWh.

While these are smaller than the LG Chem and Tesla products, they’re intended to be combined together to provide the total capacity you need, just like you would with lead-acid batteries.

The 3.8 kWh battery is the easiest to find (you can get it on Amazon) and comes with a 10 years or 10,000 cycles (@ 80% DoD) warranty.

They also have 14 kWh batteries that come integrated with solar inverters from Schneider electric.

Price: about $3,700 for 3.5 kWh ($1.06 per watt) or $18,445 for 14 kWh battery with integrated Schneider inverter ($1.32 per watt)

sonnen

The Sonnen Batterie Eco 4 is a 4 kWh battery that comes with smart software that can be programmed to store and release energy at the optimal time for your electricity rates, just like the Tesla Powerwall.

It comes with a 10-year / 10,000 discharge warranty.

Price: $5,500 for 4 kWh ($1.38 per watt)

Grid-scale storage

Storing enough electricity to power entire cities requires an entirely different scale than the batteries that can power your home, although you might be surprised that batteries can be scaled large enough to do just that. However, there a variety of technologies that are available.

Pumped storage hydroelectric

Normally, hydroelectric dams work by releasing water from a reservoir. But some hydroelectric dams are designed to also work in the opposite way by using electricity to pump water uphill back into the reservoir.

This is done during periods of low demand when electricity is cheap, such as during the night. That cheap electricity is used to storage extra water in the reservoir so that it can be released during peak demand, essentially using water and gravity as a battery. This is the oldest and most widely type of grid-scale “battery” in current use.

One of the largest is the Bath County Pumped Storage Station in Virginia, which has a capacity of 3 gigawatts - even larger than the Robert Moses Power Plant in Niagara Falls.

The main downside of pumped storage hydro is that its deployment is limited to locations where water and typography are suitable. This means that it might not always be available to be colocated near solar and wind resources.

Gravity storage

“Gravity batteries” work similarly to pumped storage hydro: a large mass of something is lifted up and later released to drive a turbine.

That “something” can be any large mass: concrete blocks or even rail cars have been proposed. One company called Energy Vault recently received a $110 million investment to develop their gravity battery that uses 35 ton blocks and cranes to provide grid-scale energy storage. Here’s a rendering of how it would work:

Vanadium flow battery

A vanadium flow battery consists of two tanks of positive and negative vanadium-based electrolytes. As the battery is charged or discharged, electrons move between the two tanks.

One advantage of this type of battery is that the capacity can be increased relatively simply by increasing the volume of the tanks.

This technology is new but starting to be deployed. There is a 8 mWh battery deployed by San Diego Gas & Electric and 200 mWh battery planned for South Australia.

Lithium ion battery

The same type of battery used in your cellphone or Tesla Powerwall, scaled up massively, can be used to power entire cities. This has already been successfully deployed in South Australia, where a 129 mWh battery has been in successful operation for the past two years.

Tesla also recently released their Megapack battery, a 3 mWh battery that can be combined together up to sizes exceeding one gigwatt-hour - enough to power all of San Francisco for 6 hours.

Vehicle-to-grid

In one sense, an electric car is just a big battery on wheels, and most of the time it’s parked. Why not connect it to the grid to provide energy storage service?

Vehicle-to-grid (V2G) technology does just that. When you’re home, you would plug your car into the grid, and your car would intelligently charge its batteries so send extra power into the grid, depending on how the system is configured.

Such a system would likely send power to the grid during peak hours around dinner time, and then charge up the car in the evening and late night hours when demand is low.

So far, there are only small-scale pilot projects for this technology. Nissan in Japan, for example, has a pilot project using Nissan Leaf vehicles. There are no active V2G projects in the United States that I know of yet.

Some of the main problems to solve include two-way charging infrastructure and determining a fair compensation scheme for V2G participants. Using an electric car for grid storage will shorten the life of the battery, so there has to be a reasonably high financial incentive to properly reimburse V2G participants for the added wear-and-tear on their batteries.

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