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Chapter 6: Estimating Solar Requirements

The first step in estimating the unique requirements of any utility system is determining how much power is currently used. In general, most households in the United States use dramatically more power in the winter than in the summer. The major reason for this has to do with running a heater in the wintertime. Even in hotter regions of the US, most households are more likely to have and utilize a heater in the winter than to run air conditioning in the summer.

In addition to heat mechanisms, which can multiply a summer power bill by three to four fold in the winter, shorter winter hours also mean that lights are on for more hours of the day and more energy is required to heat water for showers and cleaning. Moreover, many people simply spend more time indoors during the winter than during the summer, so the use of lights, computers, television and stereos, and many other electrical devices in the home may be increased during the bad-weather months.

Unfortunately, when it comes to solar power, the availability of it is inversely related to the increased use of power in the winter. Thus, when designing a solar system for one’s home, it is necessary to consider the times of the year when power usage is highest. This may be the precise time when solar power availability is lowest. To estimate your energy requirements, do not merely look at one or two of your previous power bills. Instead, it is necessary to evaluate the kilowatt-hour usage for your home over the course of the entire year in order to identify the unique patterns of your household.

The fluctuation in power usage over the course of a year is one of the advantages of a grid-tied solar system. A household that produces its own power more autonomously in the summer months may find it helpful to fall back on additional reserves from the utility company during the winter months. The off-grid alternative requires the user to establish a system powerful enough to meet the home’s energy needs during the least efficient times of the year. This means that during other times, a surplus of energy is being produced beyond that needed by the home – and an off-grid system lacks the ability to sell this surplus back to the utility grid.

A household run in part or in whole by solar power benefits from a relative normalization of usage over the course of the year. To do this, it is necessary to evaluate which usage habits result in the largest discrepancies, and then take steps to correct them. Indeed, when evaluating a home’s usage for design of a solar system, one may find it is also a perfect time to address energy usage in general and take steps to cut back energy consumption when possible. The development of energy-saving habits now will also save money – or even make money – for the household in the long run once a solar utility system is in place.

Several calculator-based tools are available online to help homeowner’s determine the size and type of solar system that best meets the needs of a specific household. In addition, many companies offer free consultations with solar power experts to help you design and install the best system for your home and your needs. A very general rule of thumb for off-grid systems is that it will cost approximately $3-4 of equipment for every watt hour per day that must be supplied. On-grid, battery-free systems can be somewhat lower in set-up costs, at approximately $2 per watt hour per day, and on-grid battery-based systems can be estimated to cost approximately $3.50 of equipment per watt hour per day in initial system costs.