Fortress Power on sizing solar batteries for system longevity

In this special edition of Contractors Corner, residential and commercial lithium battery manufacturer Fortress Power lays out the importance of correctly sizing a solar battery. Below is a portion of the company’s Solar Spotlight podcast with Solar Power World, but be sure to listen to the full episode here or on your favorite podcast app to ensure systems are sized for maximum longevity.

Where should contractors start when sizing a battery?

When we talk about battery storage, we have to categorize the different applications, so I would start with a backup or off-grid system. For back-up and off grid, what’s really critical is what kind of appliance you want to back up during a power outage because that determines what you want to put on the backup load panel.

After that, you want to determine the autonomy — e.g., I want to be backed up for a day, two days, three days. And then you’re going to also look into the solar PV array, because that’s going to determine how quickly your battery bank can be charged.

For clients who don’t have net metering or the electricity price is so high it doesn’t make any sense for them to feed solar back to the grid, you really need to look into your PV array size and your daily consumption. Ideally, you want the battery to be charged during the day and then to supply that power to the home during the night. You want to have that full cycle.

Then, you’re going to determine what will be the optimum battery bank size. You don’t want to over-size and you don’t want to under-size, because you want to get the best economics for the customers.

You want to also set up the battery lifespans to be around 80 to 90% for lithium. Lithium batteries do allow you to discharge completely, but every time you completely discharge the battery, you’re going to shorten the battery lifespan. That’s why as a manufacturer, we suggest you use 80 or 90% depth of discharge so you can get a longer duration.

Why is battery sizing important?

Battery sizing is so important for the longevity of the equipment. You want the equipment that you’re purchasing and investing in to work how it’s supposed to work, so first and foremost, battery sizing will ensure that the equipment you’re purchasing is going to do what you want it to do. There are going to be less defects down the road, less hassle, less people coming to do technical support on your system.

Sizing is very important as it’s going to be a good foothold moving forward for the rest of your project. You don’t want to go through all this effort to put in an energy storage system and solar with an inverter and then at one point, you turn on too many appliances at one time as this is going to turn off that battery-based inverter and then ultimately, it’s going to turn off the energy storage system. That is more likely than not a safety feature of the battery, but it’s another point on why we want to size the battery properly and we want to size the critical load panel properly to avoid any unexpected shutdowns and expected turn-offs because of that under- or over-sizing.

Are there tools to help contractors size batteries?

We actually developed a sizing tool for our authorized dealers. It helps you size the essential load panel to basically figure out the battery bank size that can meet the customer’s expectations.

Be creative with using different avenues to size your battery. Then, if you would like to run it through Fortress Power, you’re welcome to email [email protected] that is monitored every single day and can help you size a project whether you’re a homeowner or a solar contractor. It’s certainly good to have an extra set of eyes, especially with sizing a battery, which is still relatively new, kind of how like sizing solar was 10 or 15 years ago.

This podcast is sponsored by Fortress Power