2018 Solar Energy Storage and Battery Options

An In-depth Guide

Solar Energy Storage is generally achieved through batteries (sometimes called residential energy storage units, or RESUs). These are rechargeable units that allow you to store excess solar energy for later use. Unless your home is connected to the grid through a net metering arrangement, the sunlight shining down on your solar panels all day while you’re out at work will just be wasted. But if you have a solar battery installed, you’ll be able to use the energy stored within it at night, during periods of cloud cover, or at other times when your solar panels are not producing adequate energy.

How much does solar storage cost?

The cost of your battery depends on whether you add it to an existing solar array or whether you install your solar panels and battery at the same time. Adding a battery to an existing array is about $3,000 to $5,000 more expensive than getting a battery installed at the same time you get your solar panels installed.

Batteries typically cost $10,000 to $15,000, including installation and additional hardware. Over the lifetime of your solar system, batteries will comprise the largest component cost.

While this might sound like a lot, the federal government offers a generous 30 percent Investment Tax Credit for anyone purchasing a solar power system (including solar batteries). However, the ITC will only cover the cost of your battery if 75 percent of the energy used to charge it comes from solar energy (as opposed to the electrical grid).

Some states offer helpful incentives, too. California’s Self-Generation Incentive Program (SGIP) offers some of the best opportunities to get a solar battery. SGIP offers consumers the financial support for investments in a variety of advanced energy storage systems – including solar batteries. In 2017, the SGIP set aside over $16 millionto promote small residential energy storage. If you’re a California resident interested in solar batteries, contact your local electrical utility to find out how to apply for the SGIP.

For more information on incentives that might help you get a solar battery of your own, check out PowerScout’s overview of purchasing incentives.

When are solar batteries worth it?

If you experience power outages regularly, solar batteries can help you get through them without an interruption in electrical service. And if your solar system is totally off the grid, solar batteries are crucial; without them, you won’t be able to use electricity at night or during overcast weather.

Even when you remain tied to a reliable grid, using battery power is preferable to using grid energy, since it could further decrease your electric bill. And reduced electric bills, after all, are one of the best reasons to go solar in the first place.

What the heck are amps and volts?

Amps-the shortened form of “ampere”-are a unit for measuring the number or volume of electrons moving through an electrical current. A related measure, amp-hours, is used to indicate the number of amps moving through a current over the course of an hour. For example, a three-amp-hour battery will produce three amps for one hour. However, if you draw more energy (more amps) from your battery, you will not be able to run it for the full three hours. Drawing six amps from that same three-amp-hour battery, for instance, will only provide power for 30 minutes.

Volts are a measure of energy potential. The more volts a battery has, the more force can be used to propel electrons through the battery.

Amps and volts are two of the most common units used to measure battery power. The actual power you get from your battery (its wattage) increases as you increase both amps and volts.

Battery Cells Are Added Together to Increase Amps and Volts

How long do batteries last, and why?

Batteries don’t last forever. As you use your battery each day, its ability to hold a charge diminishes. Some batteries, for instance, can be fully charged and discharged 500 times; superior batteries remain effective after over 5,000 cycles. The best batteries will be able to sustain a high number of charge cycles. These variations in cycle life are due to the differences between different types of batteries and different manufacturing techniques.

Battery lifetimes are also determined by environmental conditions. If your battery is stored in an excessively hot environment, its ability to operate effectively will be compromised and its operational lifetime will also decline.

Lithium-ion batteries have quickly become the industry’s dominant battery

How are sealed and non-sealed batteries different?

There are two types of lead-acid batteries: flooded (“non-sealed” or “wet-cell”) and sealed. Flooded batteries contain a mixture of sulfuric acid and water. This sulfuric acid compound needs to be topped up over time at a frequency that depends on the battery’s rate of use and efficiency.

Sealed batteries, on the other hand, don’t contain liquid and don’t need to be refilled. There are two kinds of sealed batteries: absorbed glass mat (AGM or “starved electrolyte”) and gel. AGM batteries contain acid-saturated fiberglass mats between the positive and negative battery plates (technically known as the cathode and anode, respectively). Gel batteries, as their name implies, contain an electrolyte suspended in gel.

Flooded batteries have longer lifetimes than sealed batteries, but they’re also more expensive. And since you have to regularly add liquid to flooded batteries, they’re also more work. Sealed batteries, conversely, tend to charge faster than flooded batteries, and their efficiency rates remain more stable than those of their flooded counterparts when exposed to temperature fluctuations.

How much do batteries weigh?

The precise weight of any battery is usually proportional to its voltage and capacity(measured in amp-hours). Batteries with higher voltages and amp-hours typically weigh more, regardless of whether they’re lithium ion or lead-acid. Most standard batteries (the type you’d find in an automobile) weigh 40 to 60 pounds. But a larger unit like Tesla’s PowerWall can weigh well over 200 pounds.

Lead-acid batteries are far less energy-dense than lithium ion batteries. As a consequence, it takes a lead-acid battery weighing at least six kilograms to produce the same amount of energy that a one-kilogram lithium ion battery can produce.

Nickel-metal-hydride batteries are even more energy dense. A 1-kilogram nickel-metal-hydride battery can store 150 watt-hours of energy, while a 1-kilogram lithium ion battery can store 100 watt-hours at most. However, nickel-based batteries are being phased outdue to environmental concerns and the costs associated with their production. Most nations in the EU, for instance, banned nickel-cadmium batteries in 2004.

How can I safely store and handle the different types of batteries?

All batteries are safe when handled correctly. To store and operate your battery correctly, read the user guide that comes with it to ensure you’re complying with relevant safety standards.

From a safety standpoint, flooded lead-acid batteries are the most problematic of all batteries. The sulfuric acid in lead-acid batteries is more corrosive than the acids in most other battery systems, and improper storage or handling can cause dangerous leaks. Never store a flooded lead-acid battery on its side. Always keep lead-acid batteries upright and store them in a cool, well-ventilated area. Place a spill tray beneath each to prevent potential leaks or spills from damaging either your floor or the battery beneath (if your batteries are stacked in a battery bank).

When adding acid electrolyte solution to flooded batteries, always pour the mixture slowly and carefully according to manufacturer instructions. Keep neutralizing solutions on hand in the event of a spill. Store the solution in a container made of glass, polyethylene, or another appropriate material.

Among the two types of sealed batteries, gel batteries are typically considered safer than AGM batteries. In most cases, they don’t require storage in ventilated spaces, since they don’t vent gas unless they’re overcharged.

Keep all batteries away from sources of heat, flame, or sparks like radiators and hot water heaters. Don’t wear jewelry, and don’t place tools, cables, or other conductive materials near your battery. If any of these objects comes in contact with the battery terminals, they could cause a short circuit.

The anatomy of a battery cell (commonly used in home batteries)

Are there any special considerations when choosing a battery for an off-grid location?

Not really, no. Choosing a battery really comes down to balancing your desire for functionality and convenience with your desire to save money. Lead-acid batteries are more affordable than lithium ion batteries, but they are less efficient and can’t be discharged as deeply. Compared to sealed batteries, flooded batteries are typically cheaper and have longer lifespans, but they require more maintenance and care. However, any of these battery types can be used in an off-grid or grid-tied location.

How do I choose the right battery size for my house?

The first step is to determine the amount of energy your solar panels produce per day. To calculate this amount, you could either use a daily average or the amount of energy necessary to meet your needs during the winter peak, when you get the least sunlight. If you’re off-grid, your best bet is probably to use the energy you need to power your home during the winter peak. If you choose to use your daily average, look over your past energy bills and add up your annual kilowatt-hour total. Divide this number by 365 to determine the number of kilowatt-hours you use per day.

If you’re in an off-grid location and don’t receive a home electric bill, you’ll need to calculate the kilowatt-hour usage of your home the long way. This means tabulating the total wattage of each of your devices and appliances, then multiplying that number by the number of hours you use it per day. For instance, if you have 10 LED light bulbs, each of which is rated at eight watts, and you usually use each one six hours daily, you’ll need to develop a battery bank capable of sustaining at least 480 watt-hours (10 times eight times six) for lighting per day.

When using this wattage calculation method, create a digital spreadsheet to keep track of all your appliances, their wattage, and the number of hours you use each one.

After figuring out the watt-hour expenditures for everything in your house, add all the totals together to arrive at your home’s daily energy total. For instance, if you have a TV that uses 100 watt-hours, a fan that requires 300 watt-hours, and a computer that uses 400 watt-hours, plus the 480 watt-hours of lighting mentioned above, your home’s total daily energy use would be 1,280 watt-hours (480 + 400 + 100 + 300) per day.

You can use the daily watt-hour total to determine the amount of power your battery bank needs to sustain your home for one day. Just divide your daily watt-hours by the voltage of your solar system. For instance, if you’re developing a battery bank for the 1,280 watt-hour system described above and your solar system has a voltage of 24, you’d divide 1,280 by 24, yielding a quotient of 53.3 amp-hours. In other words, your battery bank needs at least 53.3 amp-hours of storage to provide power for one day.

You’ll also need to determine how deeply you expect to discharge your batteries. Totally discharging your batteries can shorten their lifespans. The recommended depth of discharge for lead-acid batteries is usually 25 to 50 percent. In other words, manufacturers recommend that you only use 75 to 50 percent of the battery’s total energy. Lithium ion batteries, on the other hand, can be discharged down to around 20 percent of their total storage capacity.

Since discharge rates require you to always leave at least some portion of the battery energy in reserve, you’ll need to upsize your battery bank by an appropriate amount to ensure you have enough usable energy. For instance, suppose you need 100 amp -hours and your batteries can be discharged to a depth of 20 percent. In this case, you’ll divide 100 amp-hours by 0.8 (80 percent, or 100 percent minus your maximum discharge depth of 20 percent), leaving you with a final tally of 125 amp-hours.

Once you know how much battery energy you need to sustain your appliances and electronics each day, determine the amount of backup power you want. For instance, you could install a battery bank that would provide you with enough energy for one day (equal to your amp-hour total), half a day (by dividing your basic amp-hour total in half), two days (by doubling your basic amp-hour total), and so on. Just remember: the larger your battery bank, the more expensive it will be, and the more space it will require.

Tesla Powerwall

The undisputed king of the solar battery market is the Tesla Powerwall. Let’s look at the specs behind its flashy name.

The Powerwall is a 269 lb. lithium-ion battery that fastens onto the wall of a house. Tesla produces the battery module and pack, which are similar to those used in their electric vehicles.

Capacity: The Tesla Powerwall 2 has the capacity to hold up to 13.5 kWh on a full charge. The creation of this second generation of Tesla batteries has more than doubled the 6.5 kWh capacity of the Powerwall 1. This is more than enough to get by for a few cloudy hours and can even power the home’s vital appliances for up to 24 hours in the event of a black out.

Pricing: The Powerwall comes at a price of $5,500, or $407.40 per kWh. This includes the unit itself, plus its inverter, but does not include the cost of installation. Tesla estimates that installation could cost anywhere between $800 and $2,000 depending on your market location and labor rates.

Warranty: Tesla has an unlimited 10-year warranty. However, like all batteries, the solar wall’s ability to hold a charge will begin to lessen over time and Tesla offers no minimum performance guarantees.

LG Chem RESU

The LG Chem RESU Battery is a lithium-ion home energy storage device. It comes in three different sizes, the heaviest of which weighs in at 214 lbs.

This product is still relatively new to the market, and comes from the South Korean company, LG, a trusted name in the solar industry. LG is a solar panel and module producer that is gaining ground in the US market. The RESU is priced similarly to the Tesla Powerwall, but it does not match its heavy hitting competitor’s storage capacity.

Capacity: The LG Chem RESU battery’s three sizes have a capacity of 3.3 kWh, 6.5 kWh, and 9.8 kWh, respectively. While coming in below the advertised capacity of the Tesla Powerwall, the RESU stands by a guaranteed minimum capacity of 60%.

Pricing: The LG Chem RESU prices range between $4,400 and $8,800. A 3.3 kWh RESU costs about $1,333 per kWh. The 9.8 battery at the higher price point would be $897.95 per kWh. This however does not include the cost of installation or the inverter. From a purely pricing standpoint, the LG Chem RESU battery does not offer the cost effectiveness of the Powerwall, wherein you get more capacity for less money per kWh.

Warranty: The LG Chem RESU battery has a 10-year warranty, just like Tesla, with a 60% performance guarantee in that time period.

sonnenBatterie eco

Another huge competitor for the Tesla Powerwall comes from the German company sonnenBatterie. Their product, the eco, is another lithium-ion home battery that integrates fully with a solar power system. It comes in a variety of different sizes, ranging from 377 lbs. to 800 lbs. While the eco has the potential for higher levels of energy capacity, it also comes at a higher price per kWh.

Capacity: The eco comes in seven different sizes, the lowest of which has a capacity of 4 kWh, while the largest comes in at 16 kWh. Other sizes include 6, 8, 10, 12, and 14 kWh versions. Two of the eco’s variations have a capacity higher than that of the Tesla Powerwall.

Pricing: The smallest eco, with a capacity of 4 kWh, comes at a price point of $5,950, or $1,487.50 per kWh. This includes a built-in inverter but does not include the cost of installation. Other variants of this battery can cost upwards of $10,000 prior to installation, which would be $714 per kWh for the largest 16 kWh model.

Warranty: The eco comes with a 10-year warranty, just like the Powerwall. It surpasses Tesla in this regard by offering a performance warranty, which guarantees that the product will never fall below 70% effectiveness within the period of that decade.