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Publication Title | Doing a Load Analysis: The First Step in System Design

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Load Analysis

Doing a Load Analysis:

The First Step in System Design

Benjamin Root

It’s not that we really care about

electricity. We don’t even care

about the appliances that the electricity powers. Our wants and needs are even more basic than that. We want to read after dark, hear good music, and learn about what is happening in the world. We want water on demand and unspoiled food. We don’t need the electricity like we don’t need the drill. What we need is the hole.

Electricity is merely a tool used to meet our needs and wants. When planning a renewable energy (RE) system it is important not to lose sight of what our needs actually are. Only once our needs are defined can we then begin to design an RE system to meet them. We must analyze each need and determine how much energy it takes to meet that need. Long before we start comparing prices on photovoltaic modules we must first create a list of needs called a “load profile.” This article will first discuss some important considerations in choosing appliances to meet certain needs. Then we will go through a step by step discussion of the various elements in a load profile.

Why Do a Load Profile?

RE systems are expensive. Costs to produce one’s own electricity from renewable sources average between $0.25 and $1.15 per kilowatt hour (kWh). This is many times the price of buying power from the electric utility. Off grid, it is a waste of money to use more energy than we need to and a waste of money to produce energy that is not used.

If done correctly, your load profile’s average daily kWh figure can be quite accurate. Careful load analysis can assure that we size our RE system appropriately.

Which Loads are Appropriate Uses for Electricity?

Most of us need to eek out as much functionality from as little energy as possible. For example, electricity is an expensive way to produce thermal energy. The electricity needed to provide space heating is generally

38 Home Power #58 • April / May 1997

©1997 Home Power

cost prohibitive. Passive solar, wood heat, and propane furnaces are all much more practical. Domestic hot water heaters and cookstoves are also best powered by passive solar, wood, or gas.

Certain loads can be powered by electricity or by other sources. Refrigeration is a good example. Propane refrigerators are available but have their own set of pros and cons. In an energy efficient home the electric refrigerator (even the energy efficient kind) is usually the largest single load. Many RE systems use electric well pumps, but wind-powered mechanical pumps have effectively provided domestic water for generations. These choices are ours. Do we need a 1,200 watt hair dryer or will a towel do just as well? Is using candles or kerosene for light really a smart (or safe) alternative to compact fluorescents?

Some needs are surprisingly appropriate for use with renewable energy systems. Power tools, microwave ovens, toasters, and other kitchen appliances can draw a lot of power and are often mistakenly considered to be too much for an RE system. Actually, these appliances are used for short periods of time and the energy consumed is rather small.

Why Pay Extra for Efficiency?

It might sound like we must do without certain luxuries in order to live with a renewable energy system. This is not the case! RE systems can provide the same amenities that our city cousins enjoy. The trick is to carefully choose how these luxuries are implemented. The most cost effective way to produce one’s own energy is to first reduce one’s needs for that energy. Richard Perez has a saying that sums it up quite well, “Every watt not used is a watt that doesn’t have to be produced, processed, or stored.” When buying grid power we can dip into a limitless supply and pay as we go. But with RE systems the cost of the energy is the up front cost of expensive system components. Choosing energy efficient appliances is cheaper than renewable energy system components.

For example, compact fluorescent light bulbs have improved immensely. The light is natural colored, flicker free, and very efficient. A 15 watt compact fluorescent produces the same amount of light as a 60 watt incandescent bulb—at one fourth of the power consumption. They cost about $22 but last 10,000

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