Simplifying home solar + storage system sizing with Sol-Ark’s online calculator
Sol-Ark offers one of the most powerful solar hybrid inverters on the market. To help installers design systems using batteries, they have introduced a proprietary online battery calculation tool. Sizing a Solar + storage system is complicated (and missteps often lead to poor performance and customer complaints). Sol-Ark made your life so much easier with this calculator. Below, the company’s co-founder, Tom Brennan, explains the key considerations for storing energy and dimensioning the solar system.
Meet customer expectations. There are two key components to meeting customer expectations in backup power applications. First, the power of the stand-alone battery inverter must be sufficient to carry the current peak load. Second, the energy storage system (ESS) must have sufficient capacity to adequately support standalone operation. While the National Electrical Code (NEC) contains some minimal guidelines for sizing hybrid inverters, the ESS capacity (kWh) and power (kW) are outside the scope of the codes and standards.
Inverter power. According to Section 710.15 (A) of the NEC, standalone performance must be “equal to or greater than the load exerted by the largest single-use devices connected to the system”. Based on a 10-second spike of 20 kW and 9 kW of continuous AC power, a single Sol-Ark inverter can power all but the largest 240V residential loads. Multiple Sol-Ark HYBRID inverters can be stacked in parallel when additional power is required to carry an extra large load such as a large heat pump.
Code requirements: As noted in NEC 90.1 (B), it is important to recognize that meeting the minimum requirements for the code does not necessarily guarantee adequacy or customer satisfaction. Multiple coincident loads can exceed the load of the largest single device. In the absence of intelligent load management features, which are the subject of a separate blog post, ESS developers may need to increase the performance of the standalone battery inverter to support the simultaneous operation of typical home appliances and mechanical systems.
ESS capacity. The power rating of a battery in kilowatt hours describes its ability to carry loads over time. In the event of a power outage, homeowners may need or expect an ESS that can support self-contained battery backup operations for hours or even days. The challenge for companies serving the residential ESS market is that most homeowners do not know exactly how much electricity is needed to support typical loads.
The ESS battery is the most expensive component of a grid-interactive energy storage system. Therefore, the sales force is tempted to reduce the ESS capacity in order to lower the first (initial) costs, reduce the sticker shock and increase sales. Reducing ESS capacity without considering customer load and setting customer expectations is the main cause of buyer’s remorse.
Backup energy for energy storage throughout the home is fraught with challenges, most notably 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 due to household stresses and customer behavior.
Example of challenges in load analysis
If an ESS is too small or a battery inverter is overloaded, customer satisfaction suffers. In the meantime, sizing an ESS based on an Excel spreadsheet is both complicated and error-prone. Consider the challenges of considering only one ubiquitous device – the refrigerator.
Example of a load analysis. According to the label inside the device, a fridge / freezer may have a current rating of 8 A, which suggests it is a 960 watt (8 A x 120 V) load. Since the customer needs to be cooled throughout the day, it appears that 23,040 watt-hours of energy storage capacity are required to carry this load for 24 hours (960 W x 24 hours). Appearances are deceptive.
In practice, this refrigerator, like many others, consumes the maximum rated power during the daily automatic defrost cycle of the freezer of 10 minutes. In normal operation, the compressor only consumes 1.5 amps, which corresponds to a load of 180 watts. This suggests that we need 4,320 watt hours of storage capacity to run the refrigerator for a day (180 watts x 24 hours). This result is also wrong.
While a refrigerator keeps food cool all day, its compressor doesn’t run every 24 hours, but has a duty cycle of around 50%. Taking into account the reduced running time, we need 2,160 watt hours of storage energy to operate this refrigerator for a whole day (180 W x 24 hours x 50% duty cycle).
This result is not at all what we might initially expect based on the rating of the product label. Additionally, the nameplate rating may not provide an indication of the equipment’s instantaneous power surge or power on and off requirements. Using a data acquisition power meter, we can see that the compressor consumes approximately 1,350 watts at start-up, about five to ten times its operating power.
Your energy storage calculation is important
Every house is different. Energy consumption varies considerably depending on the age and geographic location of a house. Some homes are all-electric, while others use natural gas or propane to heat, cook, or dry clothes. Some homes have significant water pumps that are connected to a well or swimming pool. These are consequential differences.
Every budget is different. While some customers may not think twice about spending $ 80,000 to deploy an in-house backup hybrid energy storage system for an all-electric home, most potential customers have budget constraints. A key benefit of Sol-Ark’s Solar Battery Calculator is that it allows you to provide instant, real-time feedback to customers on how their loads and behaviors are affecting project prices. This pre-sale exercise is an important part of setting realistic expectations for the front end of a solar generator project and ensuring customer satisfaction on the back end.