Living off the grid is a romantic ambition for some, a practical necessity for others. But whatever your motivation for off-grid living, cutting the electrical umbilical cord from the utility shouldn’t be taken lightly. Before you pull out the calculator, size up the realities and challenges of living off the grid. Then, once you’re convinced it’s the way for you, use this guide to design a successful stand-alone system.
Designing a stand-alone PV system differs substantially from designing a batteryless grid-direct system. Instead of meeting the home’s annual demand, a stand-alone system must be able to meet energy requirements every day of the year. The PV system must be able to keep the battery bank charged—or include a generator for backup—because once the last amp-hour is drawn, the lights go out (see “Backup Generators” sidebar).
Efficiency first! This long-standing mantra for PV system design still holds true and is especially important for off-grid systems. Using energy efficiently should always be a prerequisite to energy design and production. Every $1 spent on energy efficiency is estimated to save between $3 and $5 on PV system costs. As a system designer, it’s virtually impossible to mandate wise energy use by the end user, but we can specify efficient appliances, such as Energy Star refrigerators and clothes washers, and strategies, such as shifting loads to non-electric sources during times of low solar insolation. For more on efficiency and load-shifting, see “Toast, Pancakes & Waffles: Planning Wisely for Off-Grid Living” in HP133.
Energy Consumption and the Solar Resource. Carefully comparing the home’s daily and seasonal energy usage with the daily and seasonal availability of the sun will help prevent energy production shortages. This important step involves a careful analysis of the home’s changing seasonal load profile and the corresponding solar resource throughout the year. Paramount to this analysis is the presence or absence of a backup charging source, such as a generator. If a backup charging source is not incorporated, the designer should choose as the design target the time of year when energy consumption is expected to be highest and the solar resource at its lowest—usually during the depths of winter.
Without a backup generator, a PV system must produce every watt-hour required, at all times of the year. This is often a tall task during the winter months and typically results in a costly system that is oversized for the rest of the year. For this reason, stand-alone systems without a backup charging source are often limited to smaller, nonresidence applications, such as seasonal cabins.
For systems with a backup charging source, more design flexibility means designers can use average consumption numbers and peak sun-hour values. For example, they can choose to size the system at a time of year when energy consumption is not at its highest or lowest, but in the middle—say, a typical day in the fall or spring. In addition, they might use the specific location’s average solar resource. Using the average for both consumption and sun-hours will strike a good balance between an affordable array size and generator run time. If minimal generator run time is desired, the array and battery bank may need to be upsized based on more conservative consumption and sun-hour values.