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Simplifying home solar + storage system sizing with Sol-Ark’s online calculator

Written by: Leonard Parker | solar news | March 30, 2021

Sol-Ark provides one of the most powerful solar hybrid inverters on the market, and to better help installers design systems with batteries, they’ve launched a proprietary online battery calculator tool. Sizing up a solar + storage system is complicated (and missteps often result in poor performance and customer complaints) and, with this calculator, Sol-Ark has made your life so much easier. Below, Tom Brennan explains in this breakdown of key energy storage + solar system sizing considerations.

Meeting customer expectations. There are two key components to meeting customer expectations in backup power applications. First, the stand-alone battery inverter power output must be enough to carry the peak instantaneous load. Second, the energy storage system (ESS) capacity must be adequate to reasonably support stand-alone operation. While the National Electrical Code (NEC) provides some minimal guidance regarding hybrid inverter sizing, ESS capacity (kWh) and power (kW) is outside the scope of codes and standards.

Inverter power. Section 710.15(A) of the NEC requires that the stand-alone power is “equal to or greater than the load posed by the largest single utilization equipment connected to the system.” Based on 10-second peak of 20-kW and 9-kW of continuous AC power ratings, a single Sol-Ark inverter has the power for all but the largest 240 V residential loads; multiple Sol-Ark HYBRID inverters can be stacked in parallel if additional power is required to carry an especially large load, such as a large heat pump.

Code requirements: As cautioned in NEC 90.1(B), it is important to recognize that meeting minimum code requirements does not necessarily guarantee adequacy or customer satisfaction. Multiple coincident loads could exceed the load posed by the largest single piece of equipment. In the absence of intelligent load management capabilities—which is the subject of a separate blog post—ESS designers may need to increase stand-alone battery inverter power in order to support the simultaneous operation of typical household appliances and mechanical systems.

ESS capacity. The nominal kilowatt-hour rating of a battery describes its ability to support loads over time. In the event of a power outage, homeowners may need or expect an ESS to support stand-alone battery backup operation for a period of hours or even days. The challenge for companies serving the residential ESS market is that most homeowners do not have a sophisticated understanding of how much electricity is required to support typical loads.

The ESS battery is the most expensive component of a grid-interactive battery backup energy storage system. As such, sales personnel are tempted to reduce ESS capacity in order to drive down first (initial) costs, reduce sticker shock, and increase sales. Reducing ESS capacity without properly considering customer loads and setting customer expectations is the number one cause of buyer’s remorse.

Whole-house energy storage backup power is fraught with challenges, primary among them being customer expectations. When customers spend more than $20,000 on a solar generator, they tend to have certain performance expectations for the ESS. These expectations may or may not be reasonable based on the loads in the home and the customers’ behavior.

Example of load analysis challenges

If an ESS is undersized or a battery inverter is overloaded, customer satisfaction suffers. Meanwhile, sizing an ESS based on an Excel spreadsheet is both complicated and error prone. Consider the challenges associated with accounting for only one ubiquitous appliance—the refrigerator.

Example load analysis. According to the label located inside the appliance, a refrigerator-freezer might have a nameplate current rating of 8 A, suggesting that it is a 960-watt load (8 A x 120 V). Since the client needs refrigeration all day, it appears that 23,040 watt-hours of energy storage capacity are required to support this load for 24 hours (960 W x 24 hours). Appearances can be deceiving.

In practice, this refrigerator—like many others—draws the peak rated power during the freezer’s daily 10-minute automatic defrost cycle. Under normal operation, the compressor draws only 1.5 amps, which represents a 180-watt load. This suggests that we need 4,320 watt-hours of storage capacity to run the refrigerator for a day (180 W x 24 hours). This result is also incorrect.

While a refrigerator keeps food cool all day, its compressor does not run all 24 hours but rather has roughly a 50% duty cycle. Accounting for the reduced runtime, we need 2,160 watt-hours of storage energy to run this refrigerator for a full day (180 W x 24 hours x 50% duty cycle).

This result is not at all what we might expect at first based on the product’s nameplate rating. Moreover, the nameplate rating may not provide any indication of the appliance’s instantaneous power surge requirements or cycling on/off. Using a data-logging power meter, we might discover that the compressor draws approximately 1,350 watts upon startup, roughly five to 10 times its run wattage.

Your Energy Storage Calculation Matters

Every home is different. Energy use will vary significantly depending on a home’s age and geographic location. Some homes are all-electric whereas others use natural gas or propane for heating, cooking, or drying clothes. Some homes have significant water pumping loads, associated with a well or swimming pool. These are consequential differences.

Every budget is different. While a few customers might not think twice about spending $80,000 to provide a whole-house backup hybrid energy storage system to an all-electric home, most prospective clients have budget constraints. A primary benefit of Sol-Ark’s solar battery calculator is that you can use it to provide customers with direct real-time feedback about how their loads and behaviors impact project pricing. This pre-sales exercise is an important part of setting realistic expectations on the front end of a solar generator project and ensuring customer satisfaction on the back end.