🔋 Battery Storage Products

16 min

Solar battery in garage

While batteries are not required for net-metered solar systems their popularity is increasing exponentially. We can thank all of the advances in Electric Vehicle battery technology in reducing the cost per kilowatt-hour for battery capacity. The next section we will deep-dive on home batteries, their components, popular manufacturers, and more nuances regarding batteries. We will also highlight the benefits you receive when choosing to add a battery to your solar system.

How Home Batteries Work

A home battery acts like a barrier between your home and the power grid. It plugs right into your current electrical system to optimize daily energy use and to provide backup power during a grid failure. It does this by taking in power from the grid or your solar panels and then strategically deploying it on your house.

Home batteries have been around for decades but like the cell phone before the smartphone, home batteries in the past were bulky, expensive, inconvenient, and limited in functionality.

The principle of a home battery system is very simple, but there are some interesting nuances, particularly seasonal behavior which are worthy of further explanation.

Electricity Loads within the home

In order to understand the benefits of a home battery system it is first necessary to understand what you are using. Electricity use within an individual home tends to be very spiky, especially when high-powered appliances such as kettles, vacuums, air conditioning, or dryers are used. We also see higher usage when the house is occupied which for a working family will be predominantly at breakfast and in the evening. This is when lighting and other appliances such as TVs, and consumer electronics are on. The diagram below shows a typical profile of a house through the course of a day:

Electricity Loads within the home

The image above shows a homeowner charging up their battery by noon and exporting power to the grid except for a momentary spike from a clothes dryer during the day. In the evening a typical-use family would charge their electric vehicle using a mixture of solar and battery power and top off their battery by the end of the evening. At night when the sun is down, the battery would self-consume their stored solar power until the cycle repeats itself.

Depending on where a homeowner lives they may not be able to be truly “self-sufficient” off of their solar system. In parts of the “Sun Belt” (CA, AZ) year round self-consumption of solar through their battery might be possible.

Home Battery Components

Solar + storage systems require a number of additional components to make them work. While there are many variations the basic components for any solar + storage system are:

Solar Panels

Solar panels allow you to charge your battery with clean renewable energy. You can have an energy storage system without solar panels but having solar panels is preferable.

Batteries

The battery is the center of your energy storage system. The two most common types of batteries are Lithium-Iron-Phosphate (LFP) and Lithium- Nickel-Magnesium-Cobalt (NMC).

Inverters

Inverters are devices that convert energy from AC to DC. Inverters are needed to convert the DC energy from your solar panels and batteries into AC energy that is used in the home. Some batteries and solar panels now include a built-in inverter in their components.

Critical Load Subpanel

A critical load subpanel is a device that determines what will get powered if the grid goes out. Most batteries can’t power an entire home for very long. To extend the amount of time your battery lasts during an outage you can choose to power only a few critical appliances in your home (such as the refrigerator and lights). The critical load sub panel allows you to choose those critical loads and it’s very important that solar installers set this up correctly for your battery’s capacity. Many times we have to upgrade a homeowners main electrical panel when adding a battery system to their home.

Other Components

There are many smaller components needed to make a solar + storage system work such as AC & DC disconnects, fuses, circuit breakers, switchgear, cables, electrical conduit, mounting hardware, battery enclosures, and safety equipment. Most modern solar storage systems even include software and mobile apps to give even more control to the homeowner.

We are often asked, “How long will I run on battery power?” or ’How long will the battery last?’ The answer is, ’The battery system could run your house indefinitely as long as you don‘t try to run too many things simultaneously!’

Battery Coupling

Solar panels and battery storage equipment work with direct current (DC) and the grid and the outlets in a home use alternating current (AC).

To make a solar panel and battery system work with the grid, the system’s DC power must be converted to AC power. This is done using a device called an inverter. The opposite device which converts AC to DC is called a rectifier. In solar plus storage the term coupling refers to which type of electricity is used to tie the solar and battery together.

In a DC Coupled system, the battery and solar are tied together using DC electricity and share a single inverter/rectifier as per the diagram below:

DC-Coupled Batteries

Advantages of DC-Coupled Batteries

A battery can be installed into a system either on the DC or AC side. The benefits of installing a battery on the DC side is that the battery and solar panels can both use the same inverter, which saves money on equipment and labor costs. DC-coupled systems also tend to be more energy efficient and lower priced.

In an AC coupled system, the battery and solar are tied together using AC electricity and share separate Inverter/Rectifiers as per the diagram below:

AC-Coupled Batteries

Advantages of AC-Coupled Batteries

There are some cases where you may want to install the battery on the AC side. If a solar panel system has already been installed on a home, you may want to use an AC-coupled battery so that you don’t have to rewire the existing solar system. AC- coupled systems are also easier to add to in the future and provide more control since the battery uses its own inverter.

If you are using microinverters on a solar system, then the storage options are limited to AC coupled options. This is because the inverter is attached directly to each panel and there isn’t an opportunity to work with the DC electricity prior to it arriving at the inverter.

Knowing these differences can help you know what energy storage solution you should choose. Different markets have access to different inverter solutions, sometimes due to permitting or utility approval.

Battery Manufacturers

The most popular battery manufacturer in the past few years has been Tesla, with their Powerwall. In recent times the supply of Powerwall has become constrained, and the viability of several other AC coupled batteries has improved. Today there are many other companies producing great quality battery systems with arguably equivalent or better chemistries and at attractive pricing per unit of power stored. Below are the main batteries on the market today divided by AC or DC coupling which affects how they are installed with solar.

AC-Coupled Batteries
SunPower SunVault (Equinox)
Tesla Powerwall 2
Enphase Encharge
Sonnen SonnenCore
DC-Coupled Batteries
LG Chem / Sunrun BrightBox
Generac PWRcell
Tesla power wall batteries

Battery Location

Typically when installing a battery on your house, you have several options. The most popular installation locations are on the side of the house, in the garage, and in the basement. When selecting the installation location, temperature is a significant factor. Most batteries can operate in ambient temperatures from around -15º C to 55º C however maximum performance occurs from 0º C to 30º C. If you live in a very hot area you would not want to place your battery in a location which gets a lot of direct sunlight. If you live in a very cold area you may not want to or be allowed to place your battery outdoors.

Home Electrical System

When building out a solar power system on your property, you have to ensure that we are able to safely connect to the existing home electrical distribution panel. Many times for large PV Solar systems, upgrades to an electrical panel are needed. However some systems could be much more complex and expensive, rendering a project not viable after a site survey review.

Understanding the main panel and your home‘s power system can be critical in determining what options are available to you, especially with battery systems.

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Battery Chemistry Types

Lithium-ion batteries are preferred in the solar industry because they can discharge deeper and have more cycles than traditional lead-acid batteries. They’re the same type of batteries found in your cellphone, laptop, and other electronics. A typical lithium-ion battery will give you around 6,000-10,000 cycles at 90% discharge — so they will have a lifespan of roughly 13-18 years.

The two most common types of lithium-ion batteries are Lithium-Iron-Phosphate (LFP) and Lithium-Nickel-Magnesium-Cobalt (NMC).

LFP batteries are the most commonly used battery type and are preferred for their stability, durability, and performance. They are less prone to thermal runaway, also known as exploding, so they are generally the safer option.These home batteries use the same advanced lithium ion battery cells that power electric cars like Tesla, Chevy Volt, and Nissan Leaf. We work with the best battery technology companies in the world providing compact, automated, cloud-connected home battery systems that require no maintenance and last over 10 years.

LFP Battery Diagram

NMC batteries are the second most commonly used and are preferred for their high energy density, lower cost, and long cycle life. Many companies that already produce batteries for other applications, such as Tesla, prefer NMC batteries because they already have the infrastructure to produce them.

NMC Battery

Battery Warranties

Similar to your laptop or cell phone battery, solar batteries degrade over time. As you continue to use and charge your batteries, they lose the ability to hold a charge. Fortunately solar battery manufacturers provide warranties that guarantee the performance of a battery to a certain level. In this section we will discuss the key elements of a battery system that are typically covered under a warranty.

EnergyPal only offers lithium-ion based battery systems that have minimum 10 year warranty terms. Most homeowners taking advantage of solar-plus-storage systems charge and drain their batteries daily – this is known as a “cycle.“ As an addition to guaranteeing battery life for a set amount of years, some battery manufacturers also provide a warranty for guaranteed performance over a certain number of cycles.

However it’s important to not evaluate battery warranties based on cycles alone because solar batteries gradually lose their ability to hold a full charge as they age. A cycle towards the end of a battery’s life will store and produce less energy than the output of a cycle immediately after installation. The size of the battery is also an important factor to compare alongside cycles. A smaller battery that has a lifespan of 10,000 cycles may actually deliver less energy over its lifespan than a larger battery warrantied for only 7,000 cycles. That’s why it’s also important to consider a battery’s throughput warranty.

A “throughput” warranty is the total energy a manufacturer expects the battery to deliver throughout its lifetime. These warranties are typically stated in terms of megawatt-hours (MWh). Let’s say that a battery manufacturer provides a throughput warranty of 20 MWh. This means that the warranty is valid until the battery stores and delivers 20 MWh–or 20,000 kilowatt-hours (kWh)–of energy.

In addition to providing a warranty for a set number of years, cycles, or throughput, select manufacturers also provide an end of warranty capacity rating. Typically this will be 60 to 70 percent of its original capacity. Higher percentages are better than lower percentage guarantees.

Benefits of a Battery

Up until now, we’ve focused on the “what” and “how” of batteries. This next section will help you understand “Why batteries?”.

There are three motivations on why homeowners get a solar battery system.

  1. Peace of mind by having backup power
  2. Additional savings through energy rate arbitrage
  3. Additional incentives from battery specific funding programs

Peace of Mind

Unexpected power outages can range from a mild nuisance to an absolute disaster for a homeowner. Recently, we saw how unprepared homes in Texas were when there were cold snaps and wide-spread outages. Homes without backup power fell into disarray with pipes freezing and millions of dollars of damages.

For homeowners in California who regularly face power shut-offs due to wind conditions posing a fire risk to the state, there is a frequent frustration with spoiled food and interrupted work-days.

Having a reliable backup option could make a huge difference to a homeowner. As home solar systems are really just about producing cheaper electricity the conversation is much more analytical and economic in nature. The decision to get a battery is more emotional. Who can put a price on the peace of mind of having 24/7 reliable power for a home?

It is important to note that most battery systems do not back up the entirety of a home’s electrical needs. Specific high-power items such as clothes dryers, ovens, or air conditioning are often excluded. A home battery system is intended to cover critical loads (lights, fridges, internet, computer & phone charging).

Time of Use Rate Arbitrage

Time of Use Rate Arbitrage Diagram

Many utility companies, especially on the west coast, offer time-of-use rate plans. These rate plans charge people more during “peak” times and less during “off-peak times”. A homeowner with a home solar battery system would be able to charge up their batteries during low rate times and discharge their batteries power into their homes during peak times. This process can be controlled through software automatically and enables the homeowner to reduce their grid energy usage at peak times.

As a rule of thumb, the bigger the difference in off-peak and on-peak rates the larger the arbitrage opportunity. There are considerations to be made as the process of storing power in the battery and discharging power from the battery results in some losses and therefore increased savings from a battery really only works when there is a decent difference between the on-peak and off-peak rates. This can make sense for homeowners who purchase energy from Southern California Edison and enroll in their TOU-D-PRIME rate plan which has a 24¢/kWh off-peak rate and a 62¢/kWh on-peak rate - a significant range!

Battery Incentives

Homeowners aren’t the only group who benefit from having a battery connected to their solar system as utilities can also benefit. As home solar systems become more popular an issue known as the “Duck Curve” arises. The Duck Curve refers to the shape of demand for utility generated power over the course of a typical day. The need for utility generated electricity in the middle of the day is low as home solar production peaks and sends excess energy to the grid. In the evening as home solar systems are producing less and families return home demand on the grid increases. This uneven demand characterized by a lull followed by a spike creates the shape of the back of a duck (see image below). This low demand followed by high demand is hard for utility owned power plants to respond to as most power plants deliver a high level of consistent power and cannot be powered down or up easily.

Net load chart

Fortunately home battery storage solutions can help create more consistent demand on the grid. Instead of home solar systems sending tons of power to the grid in the middle of the day during peak solar production hours, homes with solar batteries can self-charge their battery and reduce grid overload.

This load shifting can impact the grid in a way to help utility companies improve their profits and therefore not only does the homeowner gain security and potential additional savings from their battery but they are also helping build a more resilient and sustainable grid.

As a result of these benefits some utilities have direct cash incentives to install batteries. Typically this will be based on the usable capacity (kWh) rating of the battery. For example, a 250 $/kWh rebate for a 13 kWh battery would produce a $3,250 rebate to a homeowner which is a significant cost offset!

Another incentive provided to some homeowners to install batteries is performance-based. These programs are called “Demand Response” programs. Here’s how it works. Imagine you were running the power grid for your state. During the middle of the day on a hot summer afternoon, everyone is demanding electricity. Your nuclear plants are constant and can’t increase their power output safely in time. The sun is out but you can’t get any more solar power. Your natural gas plants are firing but they are at their power limit.

You have two options. You could rapidly turn-on an infrequently used coal “peaker” plant which is expensive and idling for 99% of the year or you could call upon thousands of homeowners to discharge their stored solar power into the grid to increase the supply -- responding to the demand.

These demand response programs will pay a homeowner based on the frequency of “events” where they called upon the battery and reward is based on the power (kW) rating of the batteries. Some homeowners are getting thousands of dollars a year off their battery systems just helping the grid avoid turning on infrequently used, higher carbon emitting power sources.

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