Stand-Alone System Design

Load analysis, equipment specification and lessons learned

Sizing and specifying a system for off-grid clients can be the hardest challenge a designer ever faces. Most stand-alone systems are inherently more complex, with more complicated interactions between components, than standard gridtied systems. Good stand-alone system design is based on careful interaction with the clients. Essential steps in creating an efficient and reliable system that meets the clients’ expectations include understanding their motivations and requirements, helping them determine their needs and desires, tailoring the design to match the hard realities of their site and budget and turning the results of this soul searching into hardware and components. Fifteen years ago, when my wife and I built a home in New Mexico and moved off the grid, these were our own challenges.

While the US solar industry has undergone amazing transformation in the past 15 years, the key steps to designing effective and durable stand-alone systems have remained consistent. In this article, I do not present a comprehensive design guide, but rather I introduce the critical decision points in the design and installation of high-quality stand-alone residential power systems.

The Art Of Load Analysis

Without drawing too fine a line, off-grid customers can be generally divided into two types: sailors and motorboaters. Sailors travel within the limits of what the wind provides, tacking and trimming their sails to best fit their intended path. Motorboaters expect to maintain their desired speed and direction, regardless of the weather. Neither mode of travel is right or wrong, but you need to take different system design approaches to match the lifestyle and needs of these disparate clients. When it comes to off-grid customers, sailors can usually live within the limits of a smaller, budget-constrained system, whereas systems designed for motorboaters should contain a higher level of automation and self-protection.

A comprehensive load analysis is the most important step in designing an off-grid system for three main reasons. First, determining the total electrical load is critical to choosing the correct battery bank and PV array. Second, this analysis also determines the inverter capacity required to efficiently power all connected loads. Third, by exploring the load analysis with the clients, you gain a clearer understanding of what energy demands they consider critical and what they consider expendable.

At its simplest level, a load analysis is a detailed examination of what household equipment the clients want to operate, how much they use it and how much energy it consumes. A critical need to one person may be a luxury to the next, so it is important to examine the clients’ requirements rather than apply a one-size-fits-all approach. One of the best ways to start a load analysis is to have the clients walk through their home and evaluate each object that consumes electricity to determine how much energy it draws and how often they use it.

While large systems can be challenging to design due to the complex level of interaction between the various components, the very smallest systems benefit the most from good design practices. In a large system, errors of omission or miscalculations are often glossed over by increased generator run-time. Large systems are sophisticated enough to self-protect, and the client may never be aware of any oversight in the design process. In a microsystem, however, there is no room for error. Any miscalculation or overlooked load can result in substandard performance and shortened battery life. Conversely, overestimating the load may needlessly increase PV array capacity and cost.

STAND-ALONE SYSTEM-SIZING TOOLS

Veteran integrators serving off-grid markets often develop in-house sizing tools. For those new to stand-alone systems, the available design resources range from simple worksheets, such as “Stand-Alone Sizing Worksheet” in Appendix D of SEI’s Photovoltaics: Design and Installation Manual, to powerful software tools capable of modeling annual system performance, including required generator input, using hourly weather data for specific locations. Maui Solar Energy Software, for example, offers PV-DesignPro-S for stand-alone system design and analysis as part of its $250 Solar Design Studio software suite. Somewhere between these options is “Simple Stand-Alone PV System Worksheet” available with this online version of this article (CLICK HERE). This Microsoft Excel spreadsheet was first developed by Windy Dankoff and is provided in its current format by Conergy. Whatever sizing tool you use, every load needs to be assigned an estimated energy consumption value.

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