6 Questions to Ask When Choosing an Energy Storage System

Choosing the right lithium battery storage solution for your solar power system has never been more important.

More frequent and intense storms have led to increased power outages.  This means you may rely on backup power more than ever. In states where you can’t sell excess energy back to the grid, you need to be able to store that energy and use it to power your home. In states like California where utilities charge a time-of-use rate, the right battery storage system can discharge power during peak times so you don’t have to pay higher rates from the utility company.

Reliable Solar Power

Here in El Paso and Southern NM, time of use rates are not yet required and we don’t have many power outages but, they do happen.  As we grow as a community, the chances of the frequency of outages go up.  The higher the demand, the more chance for outages as the utility races to keep up with that demand.  The greatest benefit to storage in the Borderland is “peace of mind.”   Having reliable power in the event of a power outage is the key reason for considering energy storage, especially if you already have a solar system.

Having reliable energy storage will keep your solar system running during a power outage.  Even if the sun is out, when there is an outage, your system will stop working.  This is because your system is tied to the electrical grid.  During an outage, there is no place to send the unused power so your solar system shuts off.  An energy storage system will keep your solar system running.   That solar power now has a place to go.

So now that you know having energy storage is a good idea, how do you choose the right one?

Choose wisely

Choose a reliable lithium battery storage system that suits your specific requirements. You don’t want to overpay for capacity you don’t need.  You also want to avoid shortchanging yourself as well.

Make the right choice and you’ll see a high return on your investment. Make the wrong choice and you could be stuck with a system that doesn’t meet your expectations.  Below are answers to six questions that will help you make an informed decision.

1) How frequently does my battery storage cycle each month?

Storage cycles are the number of times you can charge and discharge a battery.  A lithium battery offers more than 3,000 cycles compared to just 500-1,000 cycles for lead acid batteries.  If you need more than 10 cycles per month, or 120 cycles per year, lithium will be better than lower cost lead acid batteries.  Not only do Lithium batteries handle more cycles, they can save you up to 70 percent in energy and battery maintenance costs. A system programmed for self-supply or time-of-use cycles every day, making a lithium iron phosphate battery, which delivers 6,000 cycles, your most cost-effective option.

Even if your energy storage system cycles less than 10 times per month, lithium batteries come with a 10-year warranty and virtually no maintenance. If your budget allows, lithium is the safer investment than lead acid batteries, which typically offer a warranty of only two to five years. For more information, check out our blog article on how to calculate the energy cost of different battery chemistries.

2) How much energy does my home consume each day?

For battery backup systems or off-grid application, 24-hour energy consumption of the circuits can be calculated on the backup load panel. You may also use a good solar energy and consumption monitoring system like Sense or Solaredge. These can help you learn more about how you are using energy in your home.  The battery system can be designed from this info.  You should then be able to supply power for 24 hours at the very least. The bigger the battery bank, the more autonomy and reserve power you will have.

If your goal is to avoid paying electricity costs by having your solar storage system self-supply your entire home load, the battery bank you choose needs to be larger.   If your utility company uses time-of-use rates, a review of previous electric bills will show your peak usage so you can choose the right-size battery bank.  This is info that can be obtained from your energy monitoring system or in some states, from the utility.

3) Is the battery safe?

There are two main types of battery chemistries used for energy storage applications – lithium iron phosphate (also called lithium ferro phosphate or LFP) and lithium nickel manganese cobalt (also called NMC or lithium ion).

Lithium iron has excellent thermal stability and is more dense. A cobalt-based lithium ion battery, however, can have poor thermal stability. This increases the risk of thermal runaway, which can cause the battery cell to overheat, smoke, and even catch fire.  If you choose to go with a NMC battery, make sure it comes with an internal cooling system such as the Tesla Powerwall.   Good options for Lithium Iron are sonen and Fortress Power’s eVault.  Solar Smart Living has been installing these for years and can testify to their reliability.

4) What cell types are used in energy storage banks and how are cells wired together?

There are three types of battery cells – cylindrical, prismatic, and porch.

Thanks to their small size, cylindrical cells can be easily grouped together to form different battery bank sizes. However, cylindrical cells are heavy and provide low power density and slow heat dissipation.

Porch cells are ideal for consumer products, such as phones, because of their lighter weight and higher power density. Unfortunately, porch cells are difficult to wire together and deliver the poorest thermal stability. The LG Chem RESU series batteries use porch cells.  These also tend to overheat.  Have you ever noticed how your phone has a tendency to shut down when it is hot outside?

In addition to having the lowest material degradation rate, highly durable Prismatic cells have a large capacity, which means you’ll need far fewer cells to form a 48-volt battery bank.

For example, only 16 battery cells are wired together in a Fortress Power eFlex 5.4, which uses cell-to-pack architecture to simplify the wiring process and improve reliability. Companies like Tesla, Panasonic, Simpliphi, and Discover use a few hundred cylindrical cells, which increases the complexity of the battery.  Advances in the way they are stringed together are making them better.

5) What type of Battery Management System (BMS) is used and how well does it communicate with the inverters?

MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) BMS is more widely used in batteries because of its low cost. However, a sudden increase in consumption load or solar production can cause the inverter/charger to overpower the MOSFET, which increases the risk of short circuit and failure.

As companies place more emphasis on quality than low price, we’re seeing a shift to relay-based BMS. At Fortress Power, they build large, solid-state relays into the BMS to support the inverter/charger’s large charge and discharge power.

The best way to ensure seamless operation of your energy storage system is to establish the data communication between the inverter/charger and the battery. The battery BMS reports state of charge (SoC), voltage, current, and temperature to the inverter/charger. Analysis of these metrics makes it possible to adjust settings and schedule proactive maintenance to optimize battery performance and extend the life of your system.

6) In what environment does my energy storage system operate?

If you want to maximize the battery lifespan and system efficiency, your energy storage system should be installed in a temperature-controlled room. If you place the unit in the garage or outdoors, avoid charging the battery below 32 degrees Fahrenheit (0 degrees Celsius) and exposing the battery to direct sunlight.  Never put your battery outdoors and on the south or west side of you home.  It will overheat and ruin your investment.

Fortress Power engineers have created an aluminum cabinet with an IP 65 waterproof rating for the Fortress Power eFlex 5.4. This sleek-looking enclosure:

  • Reduces the impact of extreme temperatures on the battery.
  • Protects the battery from moisture and saltwater corrosion.
  • Delivers five times better thermal performance.

Questions about Fortress Power eVault, Tesla Powerwall, sonnen, or any of our other storage solutions, Solar Smart Living can help.  

Learn more about Energy Storage Systems from Solar Smart Living.

***Original Content for this Article was provided by Fortress Power.

About the author
Alan Morgan

Alan is a solar and sustainable energy advocate. He has served the renewables industry for over 5 years for companies such as Solar City and Tesla and now acts as the Director of Business and Market Development for Solar Smart Living.