Installer FAQ: Will this energy storage system provide whole-home backup?

whole home backup

The answer is a qualified “yes.” High voltage residential storage equipment manufacturers have made steady progress toward both AC- and DC-coupled residential energy storage solutions that can back up a typical 200A panel.

The ability to sell whole home backup solutions to residential customers is one reason why interest in residential storage has skyrocketed in the last two years. It is easy to forget that 1) having the ability to provide power to all parts of a home’s electrical panel and 2) having the ability to supply sufficient power to meet the demand on a panel are two very different things.

Most of the batteries we’re selling fall in the 10-20 kWh range and are comprised of one or more battery modules. New-to-storage installers should note a few data points on the storage and inverter spec sheets to understand just how effective the system will be at providing whole home backup.

Understanding battery capacity, maximum depth of discharge, battery power (both rated and peak) and inverter in/out capacities are all important factors in setting reasonable expectations for the performance of any whole home storage solution.

For example: a 16 kWh battery paired with a 7.6 kW inverter

The 16 kWh here represents the amount of usable energy within the battery. The depth of discharge percentage describes how much of that energy is available to discharge. The battery’s power (rated / peak) is 7 kW / 11 kW and represents how much power the battery inverter can instruct the batteries to discharge in a given moment. There is typically a seconds-long limit to a battery’s peak discharge while the rated value describes regularly discharged power.

With some quick math, we can get a pretty good idea of how long the battery system could last in a grid outage. For a 16 kWh storage system with 7 kW rated output and 11 kW peak output, we divide the rated output numbers into the system capacity and get our answer: a little over 2 hours. This describes the discharge performance one can expect from the battery, but another key factor in overall system performance depends on the specifications of the inverter.

If our battery’s 7.6 kW inverter has DC battery terminals with a limit of 5 kW, then the math to understand how much power a customer will see actually looks like: 16 kWh divided by 5 kW for a little over 3 hours of backup, without help from the PV array. Under this scenario, a homeowner would also likely need to limit their consumption (sometimes significantly) while in backup mode to stay within the inverter’s specifications.

Thankfully, manufacturers are further refining their storage solutions by introducing soft start solutions and demand management solutions capable of limiting/locking out power to certain loads during a grid outage. These accessories will help ensure that customers’ backup power expectations are met.

Even with coming advancements in residential load management, it remains critical for installers to set proper expectations with their customers about the performance of any solution. In many cases, the system owner should not expect to carry on with their normal energy consumption patterns for long during a power outage. Energy storage is a great way to provide PV customers peace of mind; for PV + storage installers to have that same peace of mind, setting realistic customer expectations is key.

Aaron Bingham is a product manager with BayWa r.e. Solar Systems and is a frequent subject-matter nerd on the company’s Tech Talk podcast. He has worked in the solar industry since 2008 as a senior engineer and program coordinator. 

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  • Anthony Mazzella
    July 28, 2021 09:31:11

    I have an 8.5 Kw thin film PV system and would like to match a battery back up system to it. How can I get some factual info on what system best suits what I have? Our utility company is using Tesl batteries but they never replied to me inquiries.